Adsorption Characteristics of Activated Carbon Fibers in Respirator Cartridges for Toluene

International Journal of Environmental Research and Public Health

Article Adsorption Characteristics of Activated Carbon Fibers in Respirator Cartridges for Toluene

Jo Anne G. Balanay 1,* and Jonghwa Oh 2

1 Environmental Health Sciences Program, Department of Health Education and Promotion, College of Health and Human Performance, East Carolina University, Greenville, NC 27858, USA 2 Department of Environmental Health Sciences, School of Public Health, University of Alabama at Birmingham, Birmingham, AL 35233, USA; [email protected] * Correspondence: [email protected]

Abstract: Respirator use has been shown to be associated with overall discomfort. Activated carbon fiber (ACF) has potential as an alternative adsorbent for developing thinner, lightweight, and efficient respirators due to its larger surface area, microporosity, and fabric form. The purpose of this pilot study was to determine the adsorption characteristics of commercially available ACF in respirator cartridges with varying ACF composition for toluene protection. Seven ACF types (one cloth, six felt) with varying properties were tested. Seven ACF cartridge configurations with varying ACF composition were challenged with five toluene concentrations (20–500 ppm) at constant air temperature (23 ◦C), relative humidity (50%), and air flow (32 LPM). Breakthrough curves were obtained using photoionization detectors. Breakthrough times (10%, 50%, and 5 ppm) and adsorption capacities were compared among ACF cartridge configurations to determine their suitable application in respiratory protection. Results showed that ACF cartridges containing the densest ACF felt types had the longest average breakthrough times (e.g., ~250–270 min to reach 5 ppm breakthrough time) Citation: Balanay, J.A.G.; Oh, J. and those containing ACF felt types with the highest specific surface areas had the highest average Adsorption Characteristics of adsorption capacity (~450–470 mg/g). The ACF cartridges demonstrated breakthrough times of <1 h Activated Carbon Fibers in Respirator for 500 ppm toluene and 8–16 h for 20 ppm toluene. The ACF cartridges are more reliable for use Cartridges for Toluene. Int. J. Environ. at low ambient toluene concentrations but still have potential for use at higher concentrations for Res. Public Health 2021, 18, 8505. short-term protection. ACF felt forms with appropriate properties (density of ~0.07 g/cm3; specific https://doi.org/10.3390/ surface area of ~2000 m2/g) have shown promising potential for the development of lighter and ijerph18168505 thinner respirators for protection against toluene.

Academic Editor: Emiliano Schena Keywords: activated carbon ﬁber; breakthrough; toluene; respiratory protection; service life; volatile

Received: 1 July 2021 organic compound Accepted: 10 August 2021 Published: 12 August 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in Millions of workers in various workplaces throughout the United States are required published maps and institutional affil- to use respirators for protection against airborne pollutants that may cause diseases or iations. death [1]. The proper use of respirators may prevent a significant number of deaths and illnesses annually. Granular activated carbon (GAC) is the current standard adsorbent in respirators against various gases and vapors, including volatile organic compounds (VOCs), because of its low cost, efficiency, and available technology. However, due to its Copyright: © 2021 by the authors. granular form, GAC needs containment (i.e., in a cartridge or canister), which contributes Licensee MDPI, Basel, Switzerland. to the weight and bulkiness of air-purifying respirators, in addition to the weight of the This article is an open access article GAC itself [2]. Respirator use has been shown to be associated with overall discomfort [3,4]. distributed under the terms and Lightweight respirators are significantly rated as more comfortable than heavy ones [5] and, conditions of the Creative Commons thus, are more likely to be worn. Therefore, it is essential to develop improved respirators Attribution (CC BY) license (https:// to address such discomfort-related and other issues, thereby improving user compliance creativecommons.org/licenses/by/ and protecting worker health. 4.0/).

Int. J. Environ. Res. Public Health 2021, 18, 8505. https://doi.org/10.3390/ijerph18168505 https://www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2021, 18, 8505 2 of 18

Activated carbon fibers (ACFs), which are obtained from the carbonization and activation of polymeric fibers from various precursors [6], have been considered as good alternative adsorbents for developing thinner, lightweight, and efficient respirators for VOC protection because of their larger surface area, higher adsorption capacities, lower critical bed depth, greater number of micropores, lighter weight, and self-containing fabric form compared to the GAC [7–10]. The small fiber diameter of ACFs allows homoge- neous fiber activation, leading to the ACF’s narrow pore size distribution in the micropore range [11,12]. Unlike the more complex porous network of GAC (i.e., macropores branch- ing to mesopores then to micropores), ACF has micropores that are directly accessible from the fiber surface by adsorbates, resulting in faster adsorption kinetics [6–8]. ACFs were demonstrated to effectively capture VOCs from gas streams at a broad range of concentrations [13–17]. Studies investigating the use of ACF in sorbent applications have been ongoing for a few decades, particularly for air pollution control of VOCs [18–25]. ACF has promising potential for short-term respiratory protection of workers and public health during accidental or intentional release of toxic air pollutants in a catastrophic event [9], and during routine work. However, the application of ACF as an adsorbent in respiratory protection remains understudied. Our previous ACF research studies have investigated the adsorption characteristics of ACF materials in two forms (i.e., woven cloth and non-woven felt) and compared them to GAC by challenging the carbon adsorbent samples with 500 ppm toluene, as a VOC representative, in a sample chamber [9]. Results have shown that ACF types with surface areas comparable to that of GAC have higher adsorption capacity and lower critical bed depth [9]. We further investigated the adsorption characteristics (i.e., adsorption capacity, critical bed depth, and breakthrough) of the same ACF materials by challenging the adsorbent samples with toluene at six different concentrations (50–500 ppm) in the same sample chamber, focusing on comparing the ACF cloth with felt [2,26]. Results showed that ACF cloth, due to its denser form, had a higher adsorption capacity, longer breakthrough time, and lower critical bed depth than the ACF felt with similar specific surface area, suggesting the potential application of ACF cloth for thinner and lighter respirators [2,26]. To further investigate the application of ACF in respirator components, various ACF materials (one cloth, six felt) were tested for pressure drop in a typical respirator cartridge to determine if they were acceptably breathable based on the National Institute for Occupational Safety and Health (NIOSH) requirement for respirator certification on breathing resistance (i.e., maximum initial inhalation resistance of 40 mm of water (mmH2O) for chemical-cartridge respirators for gases and/or vapors) [27]. Results showed the ACF felt was the more appropriate adsorbent material in ACF respirator cartridges with acceptable pressure resistance (<40 mmH2O) compared to ACF cloth, which exceeded two times the maximum limit. This may be attributable to the denser weaving of fibers. This indicates that using 100% ACF cloth in cartridges is not recommended and may result in respirators through which users may have difficulty in breathing [28]. Moreover, the ACF pressure drop study showed that combining two ACF types in a cartridge, including ACF cloth and felt combinations, resulted in acceptable pressure drop across these respirator components, demonstrating the possibility of incorporating dense ACF cloth with less dense ACF felt to achieve a compromise between adsorbent bulk density and permeability [28]. To further understand the potential application of ACFs in respiratory protection, it is important to investigate the adsorption characteristics of these ACF respirator cartridges, previously tested for acceptable breathing resistance, to determine their service lives for certain airborne pollutants. Personal protective equipment (PPE), such as respirators, plays an important role in worker protection, particularly in situations wherein engineering and administrative control strategies are not feasible or sufficient. Thus, it is essential that respirators are continually improved in terms of design, comfort, and efficiency. With ACF showing promise as an adsorbent in respirators, more information on its suitability and limitations as an alternative adsorbent is needed. The purpose of this pilot study was to determine the adsorption characteristics, particularly breakthrough times and adsorption capacity, Int. J. Environ. Res. Public Health 2021, 18, 8505 3 of 18

of commercially available ACFs of varying properties and compositions in respirator cartridges for toluene protection. It is hypothesized that the properties (e.g., density and surface area) and composition (e.g., single type and combination) of ACF in respirator cartridges will affect the adsorption characteristics of ACF for toluene. Findings of this study will be used to eventually achieve the ultimate goal of designing breathable and lighter ACF respirators that will provide sufﬁcient worker protection against airborne contaminants.

2. Materials and Methods 2.1. Materials Seven types of ACF materials (American Technical Trading, Inc., Pleasantville, NY, USA) with varying forms (1 cloth type (AC), 6 felt types (AF)), thickness, and density were tested. The ACFs were cut into 3 inch diameter discs and treated overnight prior to testing in a laboratory oven (Precision Compact Model 665, Thermo Scientific, Marietta, OH, USA) at 200 ◦C to desorb moisture and volatile impurities on the adsorbents. The ACF discs were placed in a desiccator to cool for a short period and then weighed (mg) using an analytical balance (Voyager Pro, Ohaus Corp., Parsippany, NJ, USA). The ACF discs were then placed and sealed in a typical respirator cartridge (7.62 cm (3 inch) internal diameter, 2.54 cm (1-inch) bed depth). Cartridges were filled with either 100% of each AF type or an AC/AF combination with pressure drop values of <40 mmH2O based on previous pilot study findings on pressure drop [28], resulting in 6 single AF cartridge (CS1-CS6) and 1 AC/AF combination cartridge (CC7) compositions (Table1). The number of layers per ACF composition that were placed to fill the cartridge ranged from 5 to 11 depending on the ACF layer thickness (Table1), and was determined in such a manner that the ACF materials were not excessively compressed in the cartridge, and thus maintained their natural density. The layer thickness for each ACF type was measured using a caliper. The volume of an ACF disc layer (cm3) was calculated by multiplying the disc area (45.6 cm2) by its layer thickness (cm). The density of each ACF type (g/cm3) was calculated by dividing the mass (g) of an ACF disc layer by its volume (cm3). The average ACF mass per cartridge type ranged from 3.96 ± 0.20 g (CS4 cartridge) to 8.01 ± 0.31 g (CS6 cartridge). There was a significant difference in average ACF mass (F(6,34) = 146.101, p < 0.01) between ACF cartridge types. Polypropylene sheets (Pall Life Sciences, Port Washington, NY, USA), cut into 7.62 cm (3 inch) diameter discs, were used to sandwich the bed of ACF discs in the cartridge, with the purpose of preventing the ACF material from potential shedding. Each ACF respirator cartridge was weighed again immediately before it was placed in the test chamber for chemical exposure to obtain the “pre-exposure” weight (mg).

Table 1. Activated carbon ﬁber (ACF) composition, number, and mass of layers by cartridge type.

ACF Composition a Number of ACF Average ACF Mass Cartridge Type by % Weight Layers (g) b CS1 100% AF1 11 4.97 ± 0.06 CS2 100% AF2 9 7.76 ± 0.64 CS3 100% AF3 8 5.31 ± 0.46 CS4 100% AF4 10 3.96 ± 0.20 CS5 100% AF5 8 4.43 ± 0.16 CS6 100% AF6 5 8.01 ± 0.31 CC7 71% AF1/29% AC1 8 (AF1)/3 (AC1) 5.04 ± 0.06 a AF1–AF6 (ACF felt types); AC1 (ACF cloth type). b Pre-exposure weight.

2.2. Characterization of ACF Structural Properties The structural properties of the ACF materials, including surface area and porosity, were determined by nitrogen adsorption at 77 K using a Micromeritics ASAP 2020 auto- matic physisorption analyzer (Micromeritics Corp., Norcross, GA, USA), using a coverage factor, k = 2. The speciﬁc surface area was determined using the Brunauer–Emmett–Teller Int. J. Environ. Res. Public Health 2021, 18, 8505 4 of 18

(BET) method based on the Rouquerol criterion, wherein a linear fit between 0.01–<0.2 relative pressure (P/P0) was obtained. Micropore volume was determined using the t-plot, and pore size distribution was obtained using the Barrett–Joyner–Halenda (BJH) method. Each ACF type was analyzed in triplicate (n = 3), resulting in a total of 21 physisorption analyses. The fiber organization and morphology of the ACF materials were also examined under scanning electron microscopy (SEM, FEI Quanta 200, Hillsboro, OR, USA). SEM images of each ACF type were obtained at 200× magnification to visualize the fiber organization in the ACF samples.

2.3. Breakthrough Determination ACF respirator cartridges were challenged with different concentrations of toluene in a customized cylindrical PTFE test chamber, with a 15.24 cm (6 inch) internal diameter. Laboratory-grade toluene (≥99.5%, Sigma-Aldrich, St. Louis, MO, USA) was used as the adsorbate without further purification. Toluene was used as the representative VOC in this study as similarly undertaken in several adsorption studies that tested ACF materials [15,18–20,22]. Toluene is among the widely used VOCs in occupational settings and a major organic vapor in indoor spaces [29]. Five toluene challenge concentrations (20, 100, 200, 300, and 500 ppm) were used, most of which represent various occupational exposure limits for toluene (from TLV-TWA (Threshold Limit Value − Time Weighted Average) = 20 ppm to IDLH (Immediately Dangerous to Life and Health) concentration = 500 ppm). Using a programmable syringe pump (Aladdin-1000, World Precision Instruments, Sarasota, FL, USA), liquid toluene was continuously injected at a specific rate into pre- conditioned air at constant temperature (23 ◦C), relative humidity (50%), and air flow (32 LPM) to create the desired toluene challenge concentration. The constant temperature and relative humidity used in this study were also used in previous ACF adsorption studies and in NIOSH service life testing of organic vapor respirators as standard experimental conditions [19,22,30]. This study tested a respirator cartridge type that is typically utilized in pairs in a dual-cartridge respirator, wherein each cartridge receives half of the total airflow. Given that one cartridge was tested at a time, the airflow used in the breakthrough tests was half (32 LPM) of the constant airflow used by NIOSH (64 LPM) in service life testing of respirator chemical cartridges (condition as received) for organic vapors for certification [30]. Dry, oil-free air was pre-conditioned and flow-controlled using a Miller- Nelson Model HCS-401 instrument (Assay Technology, Inc., Livermore, CA, USA). The air flow was measured downstream of the experimental setup before and after each day the breakthrough experiments were conducted using a Gilibrator 2 primary flow calibrator (Sensidyne, St. Petersburg, FL, USA). The residence time, RT = 217 msec, was calculated using RT = D*A/Q, wherein D (2.54 cm) is the cartridge bed depth, A (45.6 cm2) is the cartridge cross-section area, and Q (0.53 cm3/msec) is the flow rate [31]. Monitoring of the temperature and relative humidity in the test chamber was conducted using a temperature and relative humidity datalogger (HOBO Model U14-002, Onset Computer Corp., Pocasset, MA, USA). Breakthrough curves of toluene were obtained for each ACF cartridge configuration at each toluene concentration by continuous monitoring of the effluent (i.e., downstream of the ACF cartridge) using a photoionization detector (PID) (VOC-TRAQ II, MOCON Baseline Series, Lyons, CO, USA). The effluent was monitored continuously until 100% breakthrough occurred. The influent (i.e., upstream of the ACF cartridge) was also monitored continuously in the same manner using another VOC-TRAQ II PID to confirm the influent gas concentration. The exposure system was calibrated at the desired challenge concentration before and after every exposure run while using the PID units in the same manner as in the breakthrough experiments. The schematic diagram of the experimental setup for breakthrough determination is shown in Figure1. Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 5 of 19

Int. J. Environ. Res. Public Health 2021, 18, 8505 5 of 18

Air Water Purifier Compressor

Air Filtering Units

Airflow/Temperature/ Humidity Controller

Syringe Pump Photoionization Temperature/ Test Detector Humidity Datalogger Chamber

ACF Exhaust cartridge Photoionization Detector

Figure 1. Experimental setup for breakthrough determination for toluene across an ACF respira- Figuretor 1. cartridge.Experimental setup for breakthrough determination for toluene across an ACF respirator cartridge. The time in minutes when Cx/C0 = 0.1 (i.e., 10% breakthrough time) and Cx/C0 = 0.5 (i.e.,The time 50% breakthroughin minutes when time), Cx/C wherein0 = 0.1 (i.e., Cx = 10% effluent breakthrough or exit concentration time) and C andx/C0 C= 00.5= inlet (i.e., 50%or initial breakthrough challenge time), concentration, wherein C werex = effluent determined or exit concentration for each breakthrough and C0 = inlet curve, or and initialcompared challenge for concentration, each ACF cartridge were determined tested. The for breakthrough each breakthrough times untilcurve, C xand= 5 com- ppm (i.e., pared5 for ppm each breakthrough ACF cartridge time) tested. were The also breakthrough determined andtimes used until in C thisx = 5 studyppm (i.e., to estimate 5 ppm the breakthroughservice life time) of thewere ACF also respirator determined cartridges, and used which in this isstudy based to onestimate the NIOSH the service standard life oftesting the ACF procedure respirator for cartridges, organic vapor which cartridge is based serviceon the NIOSH life [30]. standard Duplicates testing were pro- run on cedure20% for of organic the total vapor breakthrough cartridge service tests. life Immediately [30]. Duplicates after thewere toluene run onexposure, 20% of the each total ACF breakthroughrespirator tests. cartridge Immediately was weighed after the using tolu anene analytical exposure, balance each ACF to obtain respirator the “post-exposure” cartridge was weighedweight (mg). using an analytical balance to obtain the “post-exposure” weight (mg). The massThe mass transfer transfer zone zone(MTZ) (MTZ) length length (LT) at (L Tfive) at toluene five toluene concentrations concentrations for all for ACF all ACF cartridgecartridge types types was wascalculated calculated to examine to examine the diffusional the diffusional constrai constraintsnts within within each eachACF ACF cartridgecartridge using using Equation Equation (1): (1): tf − tr LTt= −t × L (1) L = tf×L (1) t where tf is the total equilibrium time (minutes) when Cx/C0 = 1, tr is the bed saturation wheretime tf is(minutes) the total equilibrium when Cx/C time0 = 0.05, (minutes) and L when is the bedCx/C length0 = 1, tr (cm).is the bed saturation time (minutes) when Cx/C0 = 0.05, and L is the bed length (cm). 2.4. Determination of Adsorption Capacity 2.4. DeterminationAdsorption of Adsorption capacity Capacity is defined in this study as the amount of toluene adsorbed perAdsorption mass of capacity ACF material. is defined The in mass this study of toluene as the adsorbed amount of on toluene each ACF adsorbed cartridge per (mg) masswas of ACF obtained material. by subtracting The mass theof toluene ACF pre-exposure adsorbed on weight each ACF (W1, cartridge mg) from (mg) the ACF was post- obtainedexposure by subtracting weight (W the2, mg).ACF Thepre-exposure adsorption weight capacity (W1, (mg/g) mg) from of eachthe ACF ACF post-expo- cartridge was sure calculatedweight (W by2, mg). determining The adsorption the mass capacity ratio of (mg/g) adsorbed of each chemical ACF (mg) cartridge to the was ACF calcu- materials lated(i.e., by determining W1, g) in the the cartridge mass ratio as follows: of adsorbed chemical (mg) to the ACF materials (i.e., W1, g) in the cartridge as follows: mg W (mg) − W (mg) Adsorption capacity = 2 1 (2) mg W(mg) − W((mg)) Adsorption capacity =g W1 g (2) g W(g)

Int. J. Environ. Res. Public Health 2021, 18, 8505 6 of 18

2.5. Comparison of Adsorption Characteristics by ACF Type The breakthrough times and adsorption capacities were compared by ACF cartridge types using the analysis of covariance (ANCOVA), which was conducted to determine signiﬁcant differences in breakthrough times and adsorption capacities among the cartridge types when adjusted for toluene concentration. The breakthrough time (10%, 50%, or 5 ppm) for all toluene concentrations were pooled together for each cartridge type to calculate the adjusted mean breakthrough times, which were compared among the cartridge types. The same analysis was conducted to compare the adsorption capacity among the cartridge types. Data analysis was conducted using SPSS software version 25 (IBM, Armonk, NY, USA). p < 0.05 was considered as statistically signiﬁcant.

3. Results and Discussion 3.1. ACF Structural Properties The structural properties of the ACF materials are summarized in Table2, including weave type, thickness, density, BET surface area, and microporosity. The non-woven AF types are 2- to 4.5-fold thicker than the woven AC type but are less dense (0.04–0.07 g/cm3) than the AC type (0.11 g/cm3), giving the AF types a spongy appearance. The BET surface area of the ACF materials ranged from 1204 to 2028 m2/g, and the micropore area ranged from 1122 to 1757 m2/g. The average % micropore by area ranged from 83.9 to 93.2% and the average pore size ranged from 1.58 to 1.69 nm (<2 nm), demonstrating that the ACF materials are mainly microporous. The BET surface area had a strong positive correlation with the micropore area (r = 0.997) but had a strong negative correlation with % micropore by area (r = −0.982). The organization of fibers in the ACF materials is shown in the SEM images. At 200× magnification, the AF types (AF1–AF6) show loose non-woven fibers that are randomly arranged, whereas the AC type (AC1) shows tightly woven fibers that give a denser appearance (Figure2).

Table 2. Activated carbon ﬁber (ACF) structural properties by ACF type.

BET Surface Micropore % Micropore ACF Thickness Density Pore Size Weave Type Area Area Micropore Volume Type (cm) a (g/cm3) a (nm) b (m2/g) b (m2/g) b by Area b (cm3/g) b 1203.63 ± 1122.14 ± 93.23 ± AF1 Non-woven 0.23 0.043 0.43 ± 0.00 1.66 ± 0.00 2.61 1.66 0.10 1265.48 ± 1172.93 ± 92.68 ± AF2 Non-woven 0.29 0.069 0.45 ± 0.01 1.58 ± 0.00 34.76 35.85 0.29 1707.28 ± 1529.05 ± 89.56 ± AF3 Non-woven 0.29 0.057 0.59 ± 0.01 1.59 ± 0.04 19.45 14.38 0.18 2091.13 ± 1757.11 ± 84.03 ± AF4 Non-woven 0.24 0.043 0.69 ± 0.00 1.62 ± 0.00 9.28 5.11 0.19 2028.08 ± 1701.54 ± 83.90 ± AF5 Non-woven 0.30 0.041 0.67 ± 0.00 1.69 ± 0.00 12.65 9.49 0.06 1474.60 ± 1350.12 ± 91.55 ± AF6 Non-woven 0.50 0.070 0.51 ± 0.01 1.67 ± 0.00 26.56 33.55 0.62 1903.45 ± 1637.87 ± 86.05 ± AC1 Woven 0.11 0.101 0.64 ± 0.00 1.59 ± 0.00 0.56 0.15 0.03 a average of 2 samples (n = 2) [28]. b average of 3 samples (n = 3). Int. J. J. Environ. Environ. Res. Res. Public Public Health Health 20212021,, 1818,, x 8505 FOR PEER REVIEW 7 of 19 18

Figure 2. SEMSEM images images of of activated activated carbon carbon fibers ﬁbers (ACF) (ACF) at at200× 200 magn× magniﬁcation.ification. ACF ACF felt types felt types (AF1 (AF1 to AF6); to AF6); ACF ACFcloth clothtype (AC1).type (AC1).

3.2.3.2. Breakthrough Breakthrough Characteri Characteristicsstics by Cartridge Type AA total of 42 breakthrough curves were obtained in this study. Figure 33 showsshows thethe breakthroughbreakthrough curves curves for for each each cartridge type type at five ﬁve challenge concentrations of of toluene, demonstratingdemonstrating varying breakthrough times. CS2 and CS6 cartridges that contain AF2 and AF6 (i.e.,(i.e., thethe twotwo densest densest ACF ACF felt felt types), types), respectively, respectively, have have the longestthe longest breakthrough breakthrough times times(e.g., 10%(e.g., breakthrough 10% breakthrough times times of 809 of and 809 873and min, 873 min, respectively, respectively, at 20 at ppm, 20 ppm, and 44and and 44 and43 min, 43 min, respectively, respectively, at 500 at ppm). 500 ppm). In contrast, In contrast, the CS1 the cartridge CS1 cartridge that contains that contains AF1, which AF1, is whichthe least is densethe least among dense the among ACF types,the ACF has types, the shortest has the breakthrough shortest breakthrough time (e.g., 22time min (e.g., for 2210% min breakthrough for 10% breakthrough at 500 ppm). at Moreover, 500 ppm). CS2 Moreover, and CS6 cartridgesCS2 and CS6 also cartridges have the highestalso have av- theerage highest ACFmass average (i.e., ACF 7.76 andmass 8.01 (i.e., g, respectively).7.76 and 8.01 Theg, respectively). 10%, 50%, and The 5 ppm 10%, breakthrough 50%, and 5 ppmtimes breakthrough have strong positive times have correlations strong positive with ACF correlations density ( rwith = 0.953 ACF, 0.948, density and (r 0.960, = 0.953, re- 0.948,spectively) and 0.960, and totalrespectively) ACF mass and (r total = 0.885, ACF 0.911, mass and(r = 0.885, 0.903, 0.911, respectively). and 0.903, The respectively). increase in Thetoluene increase concentration in toluene for concentration the CS6 cartridge for the occurs CS6 cartridge at a slower occurs rate (e.g., at a 22.3slower ppm/min rate (e.g., at 22.3500 ppm)ppm/min from at the 500 breakpoint ppm) from (i.e., the point breakpoi of thent breakthrough (i.e., point of curve the breakthrough wherein the efﬂuent curve whereinstarts rising) the effluent compared starts to the rising) other compared cartridge typesto the (e.g., other 40.8–53.7 cartridge ppm/min types (e.g., at 50040.8–53.7 ppm), ppm/mindemonstrating at 500 a ppm), less steep demonstrating slope of its a breakthrough less steep slope curve of its (Figure breakthrough3 ). Compared curve to(Figure other 3).ACF Compared properties, to 10%,other 50%,ACF andproperties, 5 ppm breakthrough10%, 50%, and times 5 ppm have breakthrough weak correlations times have with weakBET surface correlations area (r with = 0.286, BET 0.331, surface and area 0.328, (r respectively),= 0.286, 0.331, micropore and 0.328, area respectively), (r = 0.237, 0.283, microporeand 0.280, area respectively), (r = 0.237, 0.283, % microporeand 0.280, respectively), by area (r = 0.461, % micropore 0.495, and by area 0.495, (r = respectively), 0.461, 0.495, andmicropore 0.495, respectively), volume (r = 0.273, micropore 0.323, andvolume 0.316, (r respectively)= 0.273, 0.323, and and pore 0.316, size respectively) (r = 0.273, 0.161, and poreand 0.266,size (r respectively). = 0.273, 0.161, and 0.266, respectively).

Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 8 of 19 Int. J. Environ. Res. Public Health 2021, 18, 8505 8 of 18

(a) (b)

1.0 1.0

0.8 0.8

CS1 CS1 0.6 0.6 CS2 CS2 0 0 CS3 CS3 /C /C x x

C 0.4 CS4 C 0.4 CS4 CS5 CS5 0.2 CS6 0.2 CS6 CC7 CC7

0.0 0.0 0 200 400 600 800 1000 1200 1400 0 50 100 150 200 250 300 350 400 Elapsed Time (min) Elapsed Time (min)

1.0 1.0

0.8 0.8

CS1 CS1 0.6 0.6 CS2 CS2 0 0 CS3 CS3 /C /C x x

C 0.4 CS4 C 0.4 CS4 CS5 CS5 0.2 CS6 0.2 CS6 CC7 CC7

0.0 0.0 0 50 100 150 200 0 50 100 150 200 Elapsed Time (min) Elapsed Time (min)

(e)

1.0

0.8

CS1 0.6 CS2 0 CS3 /C x

C 0.4 CS4 CS5 0.2 CS6 CC7

0.0 0 20406080100 Elapsed Time (min)

Figure 3. Breakthrough curves by cartridge type at 5 toluene concentrations: (a) 20 ppm, (b) 100 ppm, (c) 200 ppm, (dFigure) 300 ppm, 3. Breakthrough(e) 500 ppm. curves Cartridge by cartridge type (ACF ty composition):pe at 5 toluene CS1 concentrations: (100% AF1); CS2(a) 20 (100% ppm, AF2); (b) 100 CS3 ppm, (100% (c) AF3); 200 ppm, CS4 (100%(d) 300 AF4);ppm, CS5 (e) 500 (100% ppm. AF5); Cartridge CS6 (100% type AF6); (ACFCC7 composition): (71% AF1/29% CS1 (100% AC1). AF1); CS2 (100% AF2); CS3 (100% AF3); CS4 (100% AF4); CS5 (100% AF5); CS6 (100% AF6); CC7 (71% AF1/29% AC1). The CC7 cartridge that contains the AF1/AC1 combination has the 2nd shortest breakthroughThe CC7 time, cartridge following that CS1contains (Figure the3 ).AF1/AC1 The purpose combination of incorporating has the a2nd few shortest layers ofbreakthrough AC1 between time, the AF1following layers CS1 in the (Figure CC7 cartridge 3). The purpose type is toof increaseincorporating the breakthrough a few layers timeof AC1 compared between to the using AF1 100% layers AF1 in the in theCC7 CS1 cart cartridgeridge type type. is to Because increase CC7 the breakthrough has a longer breakthroughtime compared time to (e.g.,using 6 100% minmore AF1 in 10%the CS breakthrough1 cartridge type. time atBecause 500 ppm CC7 toluene) has a longer than CS1,breakthrough combining time the (e.g., ACF 6 cloth min andmore felt in may10% havebreakthrough improved time toluene at 500 adsorption ppm toluene) but than not signiﬁcantlyCS1, combining enough the toACF be comparablecloth and felt to may cartridge have typesimproved with toluene dense ACF adsorption felt types but (i.e., not CS2 and CS6).

Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 9 of 19

significantly enough to be comparable to cartridge types with dense ACF felt types (i.e., Int. J. Environ. Res. Public HealthCS22021 ,and18, 8505 CS6). 9 of 18

3.3. Effects of Toluene Concentration on Breakthrough Times 3.3.The Effects effect of of Toluene toluene Concentration concentration on Breakthrough on the Times breakthrough curves was investigated in this study.The Figure effect of 4 toluenedemonstrates concentration the breakthr on the breakthroughough curves curves at five was investigatedtoluene concentrations in as representedthis study. Figure by the4 demonstrates CS1 and CS6 the cartridges. breakthrough As curves the toluene at ﬁve toluene concentration concentrations decreases, the as represented by the CS1 and CS6 cartridges. As the toluene concentration decreases, breakthroughthe breakthrough time of time toluene of toluene through through the the AC ACFF cartridge increases, increases, regardless regardless of the of the dif- ferencesdifferences in ACF in density ACF density in the in cartridges the cartridges (Figur (Figuree 4).4). Such Such a aconcentration concentration effect effect is is expected becauseexpected lower because toluene lower concentration toluene concentration means fe meanswer toluene fewer toluene molecules molecules adsorb adsorb onto the ACF and moveonto the through ACF and the move ACF through bed thedepth, ACF thus bed depth, taking thus longer taking for longer breakthrough for breakthrough to occur. to occur.

(a) (b)

1.0 1.0

0.8 0.8

0.6 0.6 0 0 /C x /C C 20 ppm x 20 ppm 0.4 C 0.4 100 ppm 100 ppm

200 ppm 200 ppm 0.2 0.2 300 ppm 300 ppm

500 ppm 500 ppm 0.0 0.0 0 100 200 300 400 500 600 700 800 900 0 200 400 600 800 1000 1200 Elapsed Time (min) Elapsed Time (min)

Figure 4. Breakthrough curves for (a) CS1 (100% AF1) and (b) CS6 (100% AF6) cartridge types by toluene concentration. Figure 4. Breakthrough curves for (a) CS1 (100% AF1) and (b) CS6 (100% AF6) cartridge types by toluene concentration. Such an increase in breakthrough time, however, is not linear with the toluene concentration.Such an increase The breakthrough in breakthrough time increase time, at lower however, toluene concentrationsis not linear waswith observed the toluene concentration.to be greater The breakthrough than that at higher time concentrations, increase at even lower at the toluene same proportional concentrations change was in observed toluene concentration. For example, the breakthrough time increase from 100 to 300 ppm to betoluene greater is than larger that compared at higher to the concentratio time increase fromns, even 300 to at 500 the ppm same toluene proportional for CS1 and change in tolueneCS6 concentration. cartridge types, despite For example, the sameconcentration the breakthrough increment time of 200 increase ppm (Figure from4). 100 Such to 300 ppm toluenea non-linear is larger relationship compared between to the breakthrough time increase times from (10%, 300 50%, to and 500 5 ppm ppm) andtoluene toluene for CS1 and CS6 cartridgeconcentration types, was founddespite for th alle cartridge same concentration types, as shown increment in Figure5. Similarof 200 resultsppm (Figure were 4). Such found in a previous ACF study that involved the testing of ACF samples against toluene a non-linearfor respirator relationship application between [26]. This breakthrough finding indicates times that the (10%, ACF cartridges 50%, and may 5 ppm) be more and toluene concentrationefficient in was providing found a longerfor all service cartridge life attypes, lower as toluene shown concentrations, in Figure 5. andSimilar may beresults were foundattributed in a previous to the microporosity ACF study ofthat the involved ACF adsorbents, the testing because of micropores ACF samples in activated against toluene for respiratorcarbons are application mainly responsible [26]. This for thefinding enhanced indicates adsorption that the of vapors ACF cartridges at low ambient may be more efficientconcentrations in providing and are a longer favorably service filled at life low at concentrations lower toluene due toconcentrations, the overlapping and of may be attractive forces associated with the close proximity of the opposite pore walls [32–34]. attributedA study to by theFuertes microporosity et al. [35] found of thatthe theACF adsorbate adsorbents, amount because at low concentrations micropores de-in activated carbonspends are on mainly the pore responsible size distribution for ofthe the enhanced adsorbent, adsorption specifically, its of microporosity, vapors at low which ambient con- centrationsallows higher and are adsorption favorably capacities filled at low concentrations.concentrations Evidence due to hasthe shown overlapping the im- of attractive forcesportance associated of narrow microporositywith the close in ACFproximit materialsy of in the optimizing opposite VOC pore adsorption walls [32–34]. at low A study concentrations [35]. by Fuertes et al. [35] found that the adsorbate amount at low concentrations depends on the pore size distribution of the adsorbent, specifically, its microporosity, which allows higher adsorption capacities at low concentrations. Evidence has shown the importance of narrow microporosity in ACF materials in optimizing VOC adsorption at low concentrations [35].

Int. J. J. Environ. Environ. Res. Res. Public Public Health Health 20212021,, 1818,, x 8505 FOR PEER REVIEW 10 of 19 18

Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 10 of 19 10% Breakthrough Time 50% Breakthrough Time 5 ppm Breakthrough TimeTime 1200

CS1CS1 CS2CS2 1000 CS3CS3 CS4CS4 10% Breakthrough Time 50% Breakthrough Time 5 ppm Breakthrough TimeTime CS5CS5 CS6CS6 1200 800 CC7CC7 CS1CS1 CS2CS2 1000 CS3CS3 CS4CS4 600 CS5CS5 CS6CS6

800 CC7CC7 400

Breakthrough Time(min) 600

200 400 Breakthrough Time(min) 0 200 0 100 200 300 400 500 0 100 200 300 400 500 00 100 200 200 300 300 400 400 500 500 Toluene concentration (ppm) 0 0 100 200 300 400 500 0 100 200 300 400 500 00 100 200 200 300 300 400 400 500 500 Figure 5. 5. BreakthroughBreakthrough times times as as a function a function of toluene of tolueneToluene concentration concentration concentration (ppm) by cartridge by cartridge type. type. Cartridge Cartridge type (ACF type composition): (ACF compo-

CS1sition): (100% CS1 AF1); (100% CS2 AF1); (100% CS2 AF2); (100% CS3 AF2);(100% CS3AF3); (100% CS4 (100% AF3); AF4); CS4 (100%CS5 (100% AF4); AF5); CS5 CS6 (100% (100% AF5); AF6); CS6 CC7 (100% (71% AF6); AF1/29% CC7 Figure 5. Breakthrough times as a function of toluene concentration by cartridge type. Cartridge type (ACF composition): (71%AC1). AF1/29% AC1). CS1 (100% AF1); CS2 (100% AF2); CS3 (100% AF3); CS4 (100% AF4); CS5 (100% AF5); CS6 (100% AF6); CC7 (71% AF1/29% AC1). 3.4.3.4. Breakthrough Breakthrough Times by Cartridge Type 3.4. BreakthroughTheThe 10%,10%, Times 50%,50%, andby and Cartridge 5 ppm5 ppm breakthroughType breakthrough times times were were determined determined and compared and compared among amongcartridgeThe cartridge10%, types, 50%, withtypes,and 5 the withppm aim thebreakthrough of aim determining of determining times the were potential thedetermined potential respiratory and respiratory compared protection protection each amongeachcartridge cartridge cartridge type typetypes, may may with provide provide the aim at different atof differentdetermining concentrations co ncentrationsthe potential of respiratoryof toluene. toluene. Figureprotection Figure6 6shows shows the the eachaverageaverage cartridge breakthrough breakthrough type may provide times, at which different were concentrations obtained by of pooling toluene. the Figure breakthrough 6 shows the times for averageallall toluene breakthrough concentrations, times, which for for all allwere cartridge obtained types. types. by pooling The The CS6 CS6the breakthroughca cartridgertridge has has times the the highestfor highest 10%, 10%, all toluene concentrations, for all cartridge types. The CS6 cartridge has the highest 10%, 50%,50%, and 55 ppmppm breakthroughbreakthrough times times (267, (267, 321, 321, and and 272 272 min, min, respectively), respectively), followed followed by CS2 by 50%, and 5 ppm breakthrough times (267, 321, and 272 min, respectively), followed by (244, 271, and 251 min, respectively). The ACF types in the CS6 and CS2 cartridges (AF6 CS2CS2 (244, (244, 271, 271, and and 251 251 min, min, respectively). respectively). The ACF The types ACF in types the CS6 in theand CS6 CS2 andcartridges CS2 cartridges (AF6(AF6and and AF2, and AF2, respectively)AF2, respectively) respectively) are are the theare densest densest the densestamong among among the felt the types, types, felt which whichtypes, may maywhich have have may con- contributed have con- tributedtributedto the to high tothe the breakthroughhigh high breakthrough breakthrough times. times. times. However, However, However, there was wasthere no no wassignificant signiﬁcant no significant difference difference difference in in 10% in 10%10%(F(6,27) (F(6,27) (F(6,27) = = 0.308, 0.308, = 0.308,p p == 0.927),p 0.927), = 0.927), 50% 50% 50%(F(6,27) (F(6,27) (F(6,27) = 0.357, = 0.357,= 0.357,p = 0.899),p p= = 0.899), and0.899), 5 ppm and and (F(6,27)5 5 ppmppm = (F(6,27) (F(6,27)0.284, = 0.284, 0.284, pp p= =0.939) 0.939) 0.939 breakthrough) breakthrough times times between between ACF caACF rtridge cartridgecartridge types, whiletypes, adjusting while adjusting for toluene for toluene concentration.concentration.concentration. In practicalIn In practical practical applications, applications, applications, however, however, however, a 100 min a a100 difference 100 min min difference differencein the duration in inthe the ofduration duration of workerworkerof worker protection protection protection time time still time stillmatters. still matters. matters. For exam For Forple,exam example,theple, use the of theuse the CS6 useof thecartridge of CS6 the CS6cartridge would cartridge still would would still bebestill recommended recommended be recommended over over the overtheuse ofuse the the of use CS1 the of cart CS1 theridge CS1cart dueridge cartridge to duethe approximately dueto the to approximately the approximately 130 min dif- 130 min 130 mindif- ference in the average 5 ppm breakthrough times. ferencedifference in the inthe average average 5 ppm 5 ppm breakthrough breakthrough times. times.

350 350 10% BT 300 50% BT10% BT 300 5 ppm BT50% BT 250 5 ppm BT 250 200 200 150

150 100

50100 Average Breakthrough Time(min) 050 CS1 CS2 CS3 CS4 CS5 CS6 CC7 Average Breakthrough Time(min) Cartridge Type 0

Figure 6. AverageCS1 breakthrough CS2 times CS3by cartridge CS4 type. Cartridge CS5 type (ACF CS6 composition): CC7 CS1 (100% AF1); CS2 (100% AF2); CS3 (100% AF3);Cartridge CS4 (100% Type AF4); CS5 (100% AF5); CS6 (100% AF6); CC7 (71% AF1/29% AC1).

Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 11 of 19

Int. J. Environ. Res. Public Health 2021, 18, 8505 11 of 18 Figure 6. Average breakthrough times by cartridge type. Cartridge type (ACF composition): CS1 (100% AF1); CS2 (100% AF2); CS3 (100% AF3); CS4 (100% AF4); CS5 (100% AF5); CS6 (100% AF6); CC7 (71% AF1/29% AC1). 3.5. Estimation of ACF Cartridge Service Lives 3.5. EstimationThe breakthrough of ACF Cartridge time Service to reach Lives 5 ppm toluene (i.e., 5 ppm breakthrough time) was usedThe in breakthrough this study to time estimate to reach the 5 ppm service toluene lives (i.e., of 5 the ppm ACF breakthrough cartridges. time) This was is based on usedthe NIOSHin this study standard to estimate testing the procedureservice lives for of organicthe ACF vaporcartridges. chemical This is cartridgesbased on the challenged NIOSH standard testing procedure for organic vapor chemical cartridges challenged with with a 1000 ppm carbon tetrachloride, wherein the maximum allowable breakthrough is a 1000 ppm carbon tetrachloride, wherein the maximum allowable breakthrough is set at set at 5 ppm with a minimum service life requirement of 50 min [30]. Figure7 shows the 5 ppm with a minimum service life requirement of 50 min [30]. Figure 7 shows the 5 ppm breakthrough5 ppm breakthrough times for all times toluene for challenge all toluene concentrations challenge concentrations by cartridge type. by For cartridge exam- type. For ple,example, the ACF the cartridges ACF cartridges will take will21–44 take min 21–44to reach min 5 ppm to reach breakthrough 5 ppm breakthrough at 500 ppm tol- at 500 ppm uenetoluene exposure exposure (i.e., IDLH (i.e., value IDLH for value toluene) for and toluene) will take and 51 min–1.5 will take h at 51 200 min–1.5 ppm toluene h at 200 ppm exposuretoluene (i.e., exposure OSHA (i.e.,PEL-TWA OSHA for PEL-TWA toluene) (Figure for toluene) 7). At the ( Figurelowest 7toluene). At concentra- the lowest toluene tionconcentration of 20 ppm, 5 ofppm 20 breakthrough ppm, 5 ppm time breakthrough for the ACF time cartridges for the ranges ACF from cartridges 8.5 to 16 ranges h from (Figure8.5 to 7). 16 It h is (Figure important7 ). Itto isnote important that these to breakthrough note that these times breakthrough are obtained by times challeng- are obtained ingby one challenging cartridge oneonly. cartridge Thus, theonly. use of Thus, a typical the usedual-cartridge of a typical respirator dual-cartridge may provide respirator may twiceprovide the protection twice the time protection observed time in this observed study. in this study.

1000

900 CS1 800 CS2

700 CS3

600 CS4 CS5 500 CS6 400 CC7 300

200

5 ppm Breakthrough Time(min) 100

0 20 100 200 300 500 Toluene Concentration (ppm)

FigureFigure 7. 7.TolueneToluene 5 ppm 5 ppm breakthrough breakthrough time by time cartridg by cartridgee type and type toluene and concentration. toluene concentration. Cartridge Cartridge typetype (ACF (ACF composition): composition): CS1 CS1(100% (100% AF1);AF1); CS2 (100% CS2 (100%AF2); CS3 AF2); (100% CS3 AF3); (100% CS4 AF3); (100% CS4 AF4); (100% CS5 AF4); CS5 (100%(100% AF5); AF5); CS6 CS6 (100% (100% AF6); AF6); CC7 CC7(71% (71%AF1/29% AF1/29% AC1). AC1).

TheThe service service life lifeof an of organic an organic vapor vapor cartridge cartridge at other at conditions other conditions (e.g., different (e.g., chal- different chal- lengelenge concentrations) concentrations) may maybe estimated be estimated by various by variousmethods, methods,such as breakthrough such as breakthrough exper- ex- imentalperimental tests using tests full-size using full-size cartridges cartridges or modified or containers modified [36–38] containers and predictive [36–38] andmod- predictive elingmodeling [39,40]. [ 39However,,40]. However, one rule oneof thumb rule offor thumb estimating for estimatingsuch service such life states service that life re- states that ducing the challenge concentration by a factor of 10 will increase the service life of a car- reducing the challenge concentration by a factor of 10 will increase the service life of a tridge by a factor of five [41]. Using this rule of thumb, three concentration values (1000 ppmcartridge as the byreference a factor challenge of five concentration, [41]. Using this100 ruleppm, of and thumb, 10 ppm) three and concentration their corre- values sponding(1000 ppm minimum as the service reference life requirements challenge concentration, (50, 250, and 1250 100 min, ppm, respectively) and 10 ppm) were and their usedcorresponding to determine minimumthe best-fitting service equation, life requirementsy = 6250x−0.699, to calculate (50, 250, the and minimum 1250 min, service respectively) −0.699 lifewere requirements used to determine (y) for the five the toluene best-fitting concen equation,trations (x) y used = 6250x in this study., to The calculate calcu- the mini- latedmum minimum servicelife service requirements life requirements (y) for thewere five then toluene compared concentrations to the range (x) of used experi- in this study. mentalThe calculated 5 ppm breakthrough minimum times service obtained life requirements for all ACF cartridges were then (Table compared 3). Among to the range of experimental 5 ppm breakthrough times obtained for all ACF cartridges (Table3). Among the toluene challenge concentrations, only 20 ppm had values within its range of 5 ppm breakthrough times (i.e., 770–960 min) that met the minimum service life requirement (770 min). Specifically, the CS6 and CS2 cartridges met the minimum service life requirement, with breakthrough times at 960 and 857 min, respectively. The remainder of the concentrations had maximum 5 ppm breakthrough times that were below the minimum service life requirements. This finding again supports the earlier discussion that indicates the better performance and suitability of the ACF cartridges in providing longer respiratory protection at lower toluene concentrations [26]. Int. J. Environ. Res. Public Health 2021, 18, 8505 12 of 18

Table 3. Minimum service life requirement and experimental 5 ppm breakthrough times per organic vapor cartridge by toluene concentration.

Toluene Concentration Minimum Service Life 5-ppm Breakthrough Times (ppm) Requirement (Minutes) a (Minutes) b 500 81 21–38 300 116 36–70 200 154 51–99 100 250 107–191 20 770 510–960 a Calculated based on the rule of thumb stating that reduction in concentration by a factor of 10 increases service life by a factor of 5 [41]. b Range obtained for all ACF cartridges tested.

3.6. Mass Transfer Zone (MTZ) Length by Cartridge Type

Table4 shows the 5% breakthrough time (t r), the total equilibrium time (tf), the mass transfer zone (MTZ) length (LT), and LT/L. The CS3 cartridge showed the lowest to the second lowest LT (0.19–0.25) or LT/L (0.07–0.10) throughout the five concentration conditions, indicating a more effective cartridge among the tested cartridges [42,43]. On the other hand, some of the highest LT and LT/L values were shown in the CS5 (0.81–1.11 and 0.32–0.44 cm, respectively) and CS6 (0.45–0.89 and 0.18–0.35 cm, respectively) cartridges throughout most of the five toluene concentrations. The higher LT means that the diffusion of toluene molecules will be more restricted within the adsorbent, which is more likely to result in a decrease in the breakthrough adsorption capacity [42,43]. However, our findings showed a weak correlation between the LT and adsorption capacity (r = 0.010 to 0.474), regardless of the toluene concentration.

Table 4. Mass transfer zone (MTZ) properties from breakthrough curves by cartridge type and toluene concentration (bed length (L) = 2.54 cm, ﬂow rate = 32 LPM).

MTZ Properties Cartridge Type C0 = 20 ppm C0 = 100 ppm C0 = 200 ppm C0 = 200 ppm C0 = 500 ppm CS1 0.42 0.31 1.15 0.30 0.30 CS2 0.34 0.29 0.30 0.33 0.21 Length of mass CS3 0.25 0.19 0.20 0.22 0.23 CS4 0.63 0.27 0.35 0.77 0.40 transfer zone, LT (cm) CS5 0.28 0.81 0.85 0.25 1.11 CS6 0.89 0.61 0.38 0.50 0.45 CC7 0.32 0.33 0.37 0.42 0.46 CS1 0.17 0.12 0.45 0.12 0.12 CS2 0.14 0.12 0.12 0.13 0.08 CS3 0.10 0.07 0.08 0.09 0.09 LT/L CS4 0.25 0.10 0.14 0.30 0.16 CS5 0.11 0.32 0.34 0.10 0.44 CS6 0.35 0.24 0.15 0.20 0.18 CC7 0.13 0.13 0.15 0.17 0.18 CS1 561 122 95 42 25 CS2 903 225 102 78 49 Total equilibrium CS3 779 187 91 70 45 CS4 648 153 94 80.5 38 time, tf (minute) CS5 604 225 119 62 58.5 CS6 1315 252 120 91 51 CC7 609 152 75 54 33 CS1 468 107 52 37 22 CS2 781 199 90 68 45 5% breakthrough CS3 701 173 84 64 41 CS4 488 137 81 56 32 time, tr (minutes) CS5 537 153 79 56 33 CS6 852 191 102 73 42 CC7 532 132 64 45 27 Int. J. Environ. Res. Public Health 2021, 18, 8505 13 of 18

3.7. Adsorption Capacity by Cartridge Type Figure8 shows the average adsorption capacity values, which were obtained by pooling the adsorption capacities for all toluene concentrations, for all cartridge types. The CS4 cartridge has the highest adsorption capacity (467 mg/g), followed by the CS5 cartridge (450 mg/g). The ACF types in the CS4 and CS5 cartridges (AF4 and AF5, respectively) have the highest measured BET surface areas among all ACF types, which may be attributed to the high adsorption capacities. The CS1 cartridge has the lowest adsorption capacity, which may be due to its ACF type, AF1, which has the lowest surface area. Adsorption capacity has a strong positive correlation with BET surface area (r = 0.980), micropore area (r = 0.990), and micropore volume (r = 0.986). Increasing surface area of either felt or cloth forms of ACF has been shown to increase adsorption capacity for toluene, which is attributed to the increasing amount of available adsorption sites for toluene molecules at a given ACF mass [2]. Previous studies on ACF and other carbon adsorbents challenged with other organic compounds have shown a positive linear relationship between the adsorption capacity and the surface area, independent of the adsorbent type. For example, the adsorption capacity of ACF increased for nonafluorobutyl methyl ether (NFE) as the ACF specific surface area increased from 918 to 1754 m2/g [44]. Similarly, on a mass basis, the adsorption capacities of ACF and GAC (with BET surface area of 949 and 706 m2/g, respectively) for aromatic organic compounds were found to be higher than those of carbon nanotubes (with BET surface area from 164 to 537 m2/g) [45]. Moreover, there was a significant difference in adsorption capacity (F(6,27) = 13.736, p < 0.01) between Int. J. Environ. Res. Public Health 2021, 18ACF, x FOR cartridge PEER REVIEW types, when adjusted for toluene concentration. Compared to other14 of ACF 19

properties, adsorption capacity has a strong negative correlation with % micropore by area (r = −0.905) but has weak correlations with density (r = 0.369) and pore size (r = 0.045).

600 C C

500 BC

AB 400 A AB

A 300

200

100

Average Adsorption Capacity (mg/g) 0 CS1CS2CS3CS4CS5CS6CC7 Cartridge Type

Figure 8. Average adsorption capacity by cartridge type. Shown with error bars. The letters above Figure 8. Average adsorption capacity by cartridge type. Shown with error bars. The letters above each bar represent differences or similarities between cartridge types. Cartridge types with the same each bar represent differences or similarities between cartridge types. Cartridge types with the same letter are not signiﬁcantly different by average comparison. Cartridge types with a different letter letter are not significantly different by average comparison. Cartridge types with a different letter areare significantly signiﬁcantly different different by by av averageerage comparison. comparison. Cartridge Cartridge type type (ACF (ACF composition): composition): CS1 CS1 (100% (100% AF1);AF1); CS2 CS2 (100% (100% AF2); AF2); CS3 CS3 (100% (100% AF3); CS4CS4 (100%(100% AF4); AF4); CS5 CS5 (100% (100% AF5); AF5); CS6 CS6 (100% (100% AF6); AF6); CC7 CC7 (71% (71%AF1/29% AF1/29% AC1). AC1).

PairwisePairwise comparison comparison of of the the average average adsorption adsorption capacity capacity between between all all 21 21 possible possible pairs pairs of cartridge types was conducted. Ten of the 21 pairwise comparisons (47.6%) showed of cartridge types was conducted. Ten of the 21 pairwise comparisons (47.6%) showed signiﬁcance differences, with p-values ranging from <0.001 to 0.047. Figure8 shows whether significance differences, with p-values ranging from <0.001 to 0.047. Figure 8 shows or not there is signiﬁcant difference in adsorption capacity between each cartridge pair. whether or not there is significant difference in adsorption capacity between each cartridge pair. Figure 9 shows the adsorption capacities of the ACF cartridge types by toluene concentration, ranging from 252 to 532 mg/g. The adsorption capacity shows an increasing trend as the challenge toluene concentration increases for all cartridge types (Figure 9). However, the rate of increase varies by cartridge type. The cartridges filled with ACF types with the highest surface areas (i.e., CS4 and CS5 cartridges) have the highest increase rate as demonstrated by their steepest slopes, whereas the cartridge filled with ACF with the lowest surface area (i.e., CS1 cartridge) has the lowest increase rate. These results are similar to those in our previous ACF study, which involved testing of ACF samples that are not in cartridges [2].

Int. J. Environ. Res. Public Health 2021, 18, 8505 14 of 18

Figure9 shows the adsorption capacities of the ACF cartridge types by toluene concentration, ranging from 252 to 532 mg/g. The adsorption capacity shows an increasing trend as the challenge toluene concentration increases for all cartridge types (Figure9). However, the rate of increase varies by cartridge type. The cartridges ﬁlled with ACF types with the highest surface areas (i.e., CS4 and CS5 cartridges) have the highest increase rate as demonstrated by their steepest slopes, whereas the cartridge ﬁlled with ACF with Int. J. Environ. Res. Public Health 2021, the18, x lowest FOR PEER surface REVIEW area (i.e., CS1 cartridge) has the lowest increase rate. These results15 of are 19

similar to those in our previous ACF study, which involved testing of ACF samples that are not in cartridges [2].

600 CS1 CS2 CS3 CS4 550 CS5 CS6 CC7 500

450

400

350

300 Adsorption Capacity (mg/g) 250

200 0 100 200 300 400 500 Toluene Concentration (ppm)

Figure 9. Adsorption capacity by toluene concentration and cartridge type. Cartridge type (ACF Figure 9. Adsorption capacity by toluene concentration and cartridge type. Cartridge type (ACF composition): CS1 (100% AF1); CS2 (100% AF2); CS3 (100% AF3); CS4 (100% AF4); CS5 (100% AF5); composition): CS1 (100% AF1); CS2 (100% AF2); CS3 (100% AF3); CS4 (100% AF4); CS5 (100% AF5); CS6CS6 (100% (100% AF6); AF6); CC7 (71% AF1/29% AC1). AC1). The adsorption isotherms of toluene by ACF cartridge type were derived using the The adsorption isotherms of toluene by ACF cartridge type were derived using the calculated adsorption capacities and the five toluene concentrations used in this study. calculated adsorption capacities and the five toluene concentrations used in this study. Each toluene concentration was converted into relative pressure (P/P0), where saturation Each toluene concentration was◦ converted into relative pressure (P/P0), where saturation pressure, P0 = 3.5 kPa at 23 C. Figure 10 shows the adsorption isotherms of toluene at pressure, P0 = 3.5 kPa at 23 °C. Figure 10 shows the adsorption isotherms of toluene at 23 23 ◦C for each ACF cartridge type over the range of relative pressure from 0.000 to 0.015. °C for each ACF cartridge type over the range of relative pressure from 0.000 to 0.015. At At this narrow relative pressure range, the shape of the adsorption isotherms for all ACF this narrow relative pressure range, the shape of the adsorption isotherms for all ACF cartridge types demonstrates Type I or the Langmuir type of isotherms according to the cartridgeIUPAC classification, types demonstrates indicating Type that I or the the ACF Langmuir materials type are of essentially isotherms microporous according to [the32]. IUPACThis is supportedclassification, by indicating the study that findings the AC thatF materials 83.90–93.23% are essentially of the ACF microporous surface area [32]. are Thiscomprised is supported of micropores by the (Tablestudy2 findings) and previous that 83.90–93.23% ACF adsorption of the studies ACF [surface7–10]. area are comprised of micropores (Table 2) and previous ACF adsorption studies [7–10].

Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 16 of 19 Int. J. Environ. Res. Public Health 2021, 18, 8505 15 of 18

600

500

400

300

200

100 CS1 CS2 CS3 CS4 Adsorption Capacity Adsorption Capacity (mg/g) CS5 CS6 CC7 0 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014

Relative Pressure, P/P0

Figure 10. Toluene adsorption isotherms at 23 ◦C by cartridge type over the relative pressure (P/P0) Figure 10. Toluene adsorption isotherms at 23 °C by cartridge type over the relative pressure (P/P0) range 0.000–0.015. Cartridge type (ACF composition): CS1 (100% AF1); CS2 (100% AF2); CS3 (100% range 0.000–0.015. Cartridge type (ACF composition): CS1 (100% AF1); CS2 (100% AF2); CS3 (100% AF3);AF3); CS4 CS4 (100% (100% AF4); AF4); CS5 (100% AF5); CS6 (100% AF6); CC7 (71% AF1/29% AC1). AC1). 4. Conclusions 4. Conclusions Activated carbon fiber is considered to be an alternative adsorbent that may be used to developActivated thinner, carbon lightweight, fiber is considered and efficient to be respirators, an alternative but itsadsorbent adsorption that characteristics may be used toneed develop to be thinner, investigated lightweight, further. and The efficient findings respirators, of this study but its show adsorption the extent characteristics of respira- needtory protectionto be investigated that ACF further. respirator The cartridgesfindings of may this providestudy show against the airborneextent of toluene respiratory and, protectionpotentially, that other ACF VOCs. respirator Cartridges cartridges containing may provide the densest against ACF airborne felt types toluene (i.e., AF2and, andpo- tentially,AF6) have other the VOCs. longest Ca breakthroughrtridges containing times the (e.g., densest ~250–270 ACF minfelt types to 5 ppm (i.e., AF2 breakthrough and AF6) havetime), the and longest thus provide breakthrough the longest times protection (e.g., ~250–270 time against min to toluene. 5 ppm Cartridges breakthrough containing time), andACF thus felt typesprovide with the the longest highest protection specific surfacetime against areas havetoluene. the highestCartridges adsorption containing capacity ACF felt(~450–470 types with mg/g). the Although highest specific the ACF surface cloth type areas was have previously the highest shown adsorption to have promising capacity (~450–470adsorption mg/g). characteristics Although (i.e.,the ACF high cloth adsorption type was capacity previously and longshown breakthrough to have promising times), adsorptionincorporating characteristics ACF cloth with (i.e., ACF high felt adsorption did not significantly capacity and increase long thebreakthrough breakthrough times), time. incorporatingThus, there is ACF no need cloth to with use ACF ACF cloth felt did in ACF not cartridgessignificantly to improveincrease adsorption,the breakthrough given time.one of Thus, its disadvantages there is no need ishigh to use pressure ACF clot drop.h in UsingACF cartridges the appropriate to improve ACF adsorption, felt type in giventerms one of density of its disadvantages (~0.07 g/cm3) is and high specific pressu surfacere drop. area Using (~2000 the appropriate m2/g) will be ACF sufficient felt type in inACF terms respirator of density cartridges. (~0.07 g/cm3) and specific surface area (~2000 m2/g) will be sufficient in ACFThe respirator breakthrough cartridges. times of toluene across the ACF cartridges decreased with increasing tolueneThe challenge breakthrough concentration, times of toluene but this across relationship the ACF was cartridges shown to decreased be non-linear. with The increas- ACF ingcartridge toluene may challenge provide concentration, respiratory protection but this relationship for <1 h for was 500 shown pm toluene, to be non-linear. and for 8–16 The h ACFfor 20 cartridge ppm toluene. may provide At its current respiratory configuration, protection the for ACF <1 h cartridges for 500 pm may toluene, be more and reliable for 8– 16for h use for at20 low ppm ambient toluene. toluene At its current concentrations, configuration, which the may ACF be attributable cartridges may to the beACF’s more reliablemicroporosity. for use Nonetheless,at low ambient findings toluene indicate concen thattrations, ACF feltwhich types may still be have attributable potential to to the be ACF’sused as microporosity. short-term protection Nonetheless, at high findings toluene indicate concentration, that ACF suchfelt types as during still have emergency poten- tialescape to be or used evacuation as short-term (e.g., protection high-volume at hi spillsgh toluene resulting concentration, in high ambient such as concentration), during emer- gencybut the escape design or of evacuation the ACF respirator (e.g., high-volum or respiratore spills component resulting needs in high to ambient be modified. concentra- Given tion),its non-granular but the design and of light the ACF fabric respirator form, ACF or respirator may be utilized component in respirators needs to be without modified. the Givenneed for its hardnon-granular containment and (e.g., light cartridge) fabric form, and ACF be developed may be utilized into a lightin respirators N95-type without filtering thefacepiece need for respirator hard containment (FFR) that may(e.g., be cartridge) potentially and reusable. be developed If used into as such, a light the N95-type ACF felt filteringmay be utilizedfacepiece with respirator a contact (FFR) surface that areamay thatbe potentially is larger than reusable. that of If theused ACF as such, cartridge the 2 ACFtested felt in may this studybe utilized (~46 cmwith), whicha contact is possiblesurface area given that a typical is larger N95 than FFR that surface of the area ACF of 2 cartridgeapproximately tested 165in this cm study. However, (~46 cm increasing2), which theis possible current ACFgiven bed a typical depth N95 (1 in) FFR to increasesurface protection time is not recommended because this may result in unacceptable breathing

Int. J. Environ. Res. Public Health 2021, 18, 8505 16 of 18

resistance. The ACF cartridges in this study have been previously tested and shown to have acceptable pressure drop based on NIOSH criteria for respirator use approval, but increasing the bed depth may compromise the breathability of the respirator. Further studies on the adsorption characteristics of improved ACF respirator and/or respirator components for toluene and other VOCs at various environmental (e.g., temperature and relative humidity) and worker (e.g., breathing pattern and rate) conditions, in addition to comparison studies with currently available respirator cartridges (i.e., using GAC as adsorbent), are warranted to better understand the strengths and limitations of ACF as an adsorbent for respiratory protection.

Author Contributions: Conceptualization, J.A.G.B.; methodology, J.A.G.B. and J.O.; software, J.A.G.B.; validation, J.A.G.B. and J.O.; formal analysis, J.A.G.B.; investigation, J.A.G.B. and J.O.; resources, J.A.G.B. and J.O.; data curation, J.A.G.B.; writing—original draft preparation, J.A.G.B.; writing— review and editing, J.A.G.B. and J.O.; visualization, J.A.G.B.; supervision, J.A.G.B.; project administration, J.A.G.B.; funding acquisition, J.A.G.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Institute for Occupational Safety and Health (NIOSH), U.S.A. through the North Carolina Occupational Safety and Health Education and Research Center (NC OSHERC). Its contents, including any opinions and/or conclusions expressed, are solely the responsibility of the authors and do not necessarily represent the official views of NIOSH or NC OSHERC. Acknowledgments: The authors would like to thank Gene Oakley of East Carolina University Department of Physics for the design and construction of the test chamber used in the study, the Deep South Center for Occupational Health and Safety for support on the physisorption analyses of the ACF samples, and three anonymous reviewers for providing their helpful suggestions and comments to improve this manuscript. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References 1. Occupational Safety and Health Administration. Safety and Health Topics: Respiratory Protection; U.S. Department of Labor: Washington, DC, USA. Available online: https://www.osha.gov/SLTC/respiratoryprotection/ (accessed on 23 June 2021). 2. Balanay, J.G.; Bartolucci, A.A.; Lungu, C.T. Adsorption characteristics of activated carbon fibers (ACFs) for toluene: Application in respiratory protection. J. Occup. Environ. Hyg. 2014, 11, 133–143. [CrossRef] 3. Radonovich, L.J.; Cheng, J.; Shenal, B.V.; Hodgson, M.; Bender, B.S. Respirator tolerance in health care workers. JAMA 2009, 30, 36–38. [CrossRef] 4. Baig, A.S.; Knapp, C.; Eagan, A.E.; Radonovich, L.J. Health care workers’ views about respirator use and features that should be included in the next generation of respirators. Am. J. Infect. Control 2010, 38, 18–25. [CrossRef][PubMed] 5. Hooper, A.J.; Crawford, J.O.; Thomas, D. An evaluation of physiological demands and comfort between the use of conventional and lightweight self-contained breathing apparatus. Appl. Ergon. 2001, 32, 399–406. [CrossRef] 6. Suzuki, M. Activated carbon fiber: Fundamentals and applications. Carbon 1994, 32, 577–586. [CrossRef] 7. Lu, A.H.; Zheng, J.T. Study of microstructure of high-surface-area polyacrylonitrile activated carbon fibers. J. Colloid Interface Sci. 2001, 236, 369–374. [CrossRef][PubMed] 8. Huang, Z.H.; Kang, F.; Zheng, Y.P.; Yang, J.B.; Liang, K.M. Adsorption of trace polar methy-ethyl-ketone and non-polar benzene vapors on viscose rayon-based activated carbon fibers. Carbon 2002, 40, 1363–1367. [CrossRef] 9. Balanay, J.G.; Crawford, S.A.; Lungu, C.T. Comparison of toluene adsorption among granular activated carbon and different types of activated carbon fibers (ACFs). J. Occup. Environ. Hyg. 2011, 8, 573–579. [CrossRef] 10. Balanay, J.G.; Lungu, C.T. Morphologic and surface characterization of different types of activated carbon fibres. Adsorpt. Sci. Technol. 2012, 30, 355–367. [CrossRef] 11. Lu, A.H.; Zheng, J.T. Microstructures of PAN-ACF modified by catalytic benzene deposition. Carbon 2002, 40, 1353–1361. [CrossRef] 12. Feng, W.; Kwon, S.; Borguet, E.; Vidic, R. Adsorption of hydrogen sulfide onto activated carbon fibers: Effect of pore structure and surface chemistry. Environ. Sci. Technol. 2005, 39, 9744–9749. [CrossRef][PubMed] 13. Ramirez, D.; Sullivan, P.D.; Rood, M.J.; James, K.J. Equilibrium adsorption of organic vapors on phenol-, tire- and coal-derived activated carbons. J. Environ. Eng. 2004, 130, 231–241. [CrossRef] Int. J. Environ. Res. Public Health 2021, 18, 8505 17 of 18

14. Sullivan, P.D.; Rood, M.J.; Dombrowski, K.D.; James, K.J. Capture and recovery of organic compounds using adsorption and electrothermal regeneration. J. Environ. Eng. 2004, 130, 258–267. [CrossRef] 15. Fournel, L.; Mocho, P.; Fanlo, J.L.; Le Cloirec, P. External capillary condensation and adsorption of VOCs onto activated carbon fiber cloth and felt. Environ. Technol. 2005, 26, 1277–1287. [CrossRef] 16. Mallouk, K.E.; Johnsen, D.L.; Rood, M.J. Capture and recovery of isobutane by electrothermal swing adsorption with post- desorption post-desorption liquefaction. Environ. Sci. Technol. 2010, 44, 7070–7075. [CrossRef][PubMed] 17. Song, M.; Yu, L.; Song, B.; Meng, F.; Tang, X. Alkali promoted the adsorption of toluene by adjusting the surface properties of lignin-derived carbon fibers. Environ. Sci. Pollut. Res. 2019, 26, 22284–22294. [CrossRef] 18. Brasquet, C.; Le Cloirec, P. Adsorption onto activated carbon fibers: Application to water and air treatments. Carbon 1997, 35, 1307–1313. [CrossRef] 19. Cheng, T.; Jiang, Y.; Zhang, Y.; Liu, S. Prediction of breakthrough curves for adsorption on activated carbon fibers in a fixed bed. Carbon 2004, 42, 3081–3085. [CrossRef] 20. Das, D.; Gaur, V.; Verma, N. Removal of volatile organic compound by activated carbon fiber. Carbon 2004, 42, 2949–2963. [CrossRef] 21. Hashisho, Z.; Rood, M. Microwave-swing adsorption to capture and recover vapors from air streams with activated carbon fiber cloth. Environ. Sci. Technol. 2005, 39, 6851–6859. [CrossRef][PubMed] 22. Lorimier, C.; Subrenat, A.; Le Coq, L.; Le Cloirec, P. Adsorption of toluene onto activated carbon fibre cloths and felts: Application to indoor air treatment. Environ. Technol. 2005, 26, 1217–1230. [CrossRef] 23. Sidheswaran, M.A.; Destaillats, H.; Sullivan, D.P.; Cohn, S.; Fisk, W.J. Energy efficient indoor VOC air cleaning with activated carbon fiber (ACF) filters. Build. Environ. 2012, 47, 357–367. [CrossRef] 24. Son, H.K.; Sivakumar, S.; Rood, M.J.; Kim, B.J. Electrothermal adsorption and desorption of volatile organic compounds on activated carbon fiber cloth. J. Hazard. Mater. 2016, 301, 27–34. [CrossRef][PubMed] 25. Roegiers, J.; Denys, S. CFD-modelling of activated carbon fibers for indoor air purification. Chem. Eng. J. 2019, 365, 80–87. [CrossRef] 26. Balanay, J.G.; Floyd, E.L.; Lungu, C.T. Breakthrough curves for toluene adsorption of different types of activated carbon fibers: Application in respiratory protection. Ann. Occup. Hyg. 2015, 59, 481–490. [CrossRef][PubMed] 27. National Institute for Occupational Safety and Health. Determination of Inhalation Resistance Test, Air-purifying Respirators Standard Testing Procedure (STP); Procedure No. TEB-APR-STP-0007, Revision 2.3; U.S. Department of Health and Human Services: Pittsburgh, PA, USA, 2019. Available online: https://www.cdc.gov/niosh/npptl/stps/pdfs/TEB-APR-STP-0007-508.pdf (accessed on 19 June 2021). 28. Balanay, J.G.; Lungu, C.T. Determination of pressure drop across activated carbon fiber respirator cartridges. J. Occup. Environ. Hyg. 2016, 13, 141–147. [CrossRef][PubMed] 29. Agency for Toxic Substances and Disease Registry. Toxicological Profile for Toluene; U.S. Department of Health and Human Services: Atlanta, GA, USA, 2017. Available online: https://www.atsdr.cdc.gov/toxprofiles/tp56.pdf (accessed on 27 July 2021). 30. National Institute for Occupational Safety and Health. Determination of Organic Vapor (Carbon Tetrachloride) Service Life Test, Air-purifying Respirators with Cartridges Standard Testing Procedure (STP); Procedure No. TEB-APR-STP-0046A, Revision 2.0; U.S. Department of Health and Human Services: Pittsburgh, PA, USA, 2006. Available online: https://www.cdc.gov/niosh/npptl/ stps/pdfs/TEB-APR-STP-0046A-508.pdf (accessed on 19 June 2021). 31. Amid, H.; Mazé, B.; Flickinger, M.C.; Pourdeyhimi, B. Dynamic adsorption of ammonia: Apparatus, testing conditions, and adsorption capacities. Meas. Sci. Technol. 2017, 28, 055901. [CrossRef] 32. Gregg, S.J.; Sing, K.S.W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, UK, 1982; ISBN 0-12-300956-1. 33. Foster, K.L.; Fuerman, R.G.; Economy, J.; Larson, S.M.; Rood, M.J. Adsorption characteristics of trace volatile organic compounds in gas streams onto activated carbon fibres. Chem. Mater. 1992, 4, 1068–1073. [CrossRef] 34. Bradley, R.H.; Rand, B. On the physical adsorption of vapors by microporous carbons. J. Colloid Interface Sci. 1995, 169, 168–176. [CrossRef] 35. Fuertes, A.B.; Marbán, G.; Nevskaia, D.M. Adsorption of volatile organic compounds by means of activated carbon fibre-based monoliths. Carbon 2003, 41, 87–96. [CrossRef] 36. Cohen, H.J.; Garrison, R.P. Development of a field method for evaluating the service life of organic vapor cartridges: Results of laboratory testing using carbon tetrachloride. Am. Ind. Hyg. Assoc. J. 1989, 50, 486–495. [CrossRef] 37. Janvier, F.; Tuduri, L.; Cossement, D.; Drolet, D.; Lara, J. Micropore characterization of activated carbons of respirator cartridges with argon, carbon dioxide, and organic vapors of different vapor pressures. Carbon 2015, 94, 781–791. [CrossRef] 38. Janvier, F.; Tuduri, L.; Cossement, D.; Drolet, D.; Lara, J. Systematic evaluation of the adsorption of organic vapors onto a miniaturized cartridge device using breakthrough tests in parallel experiment with a full size respirator cartridge. Adsorpt. Sci. Technol. 2016, 34, 287–306. [CrossRef] 39. Wood, G.O. Estimating service lives of organic vapor cartridges. Am. Ind. Hyg. Assoc. J. 1994, 55, 11–15. [CrossRef] 40. Yoon, Y.H.; Nelson, J.H.; Lara, J. Respirator cartridge service-life: Exposure to mixtures. Am. Ind. Hyg. Assoc. J. 1996, 57, 809–819. [CrossRef] 41. Wood, G. Correlating and extrapolating air purifying respirator cartridge breakthrough times: A review. J. Int. Soc. Respir. Prot. 2015, 32, 23–36. Int. J. Environ. Res. Public Health 2021, 18, 8505 18 of 18

42. Lopez, J.M.; Navarro, M.V.; Garcia, T.; Murillo, R.; Mastral, A.M.; Varela-Gandia, F.J.; Lozano-Castello, D.; Bueno-Lopez, A.; Cazorla-Amoros, D. Screening of different zeolites and silicoaluminophosphates for the retention of propene under cold start conditions. Microporous Mesoporous Mater. 2010, 130, 239–247. [CrossRef] 43. Chebbi, M.; Azambre, B.; Cantrel, L.; Huvé, M.; Albiol, T. Influence of structural, textural and chemical parameters of silver zeolites on the retention of methyl iodide. Microporous Mesoporous Mater. 2017, 244, 137–150. [CrossRef] 44. Tanada, S.; Kawasaki, N.; Nakamura, T.; Araki, T.; Tachibana, Y. Characteristics of nonafluorobutyl methyl ether (NFE) adsorption onto activated carbon fibers and different-size-activated carbon particles. J. Colloid Interface Sci. 2000, 228, 220–225. [CrossRef] 45. Zhang, S.; Shao, T.; Kose, H.S.; Karanfil, T. Adsorption of aromatic compounds by carbonaceous adsorbents: A comparative study on granular activated carbon, activated carbon fiber, and carbon nanotubes. Environ. Sci. Technol. 2010, 44, 6377–6383. [CrossRef]