International Journal of Environmental Research and Public Health

Article Individual Fit Testing of -Protection Devices Based on Microphones in Real Ears among Workers in Industries with High--Level Manufacturing

Chien-Chen Chiu 1,* and Terng-Jou Wan 2,* 1 Doctoral Program, Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, No. 123 University Road, Section 3, Douliou, Yunlin 64002, Taiwan 2 Department of Safety Health and Environmental Engineering, National Yunlin University of Science and Technology, No. 123 University Road, Section 3, Douliou, Yunlin 64002, Taiwan * Correspondence: [email protected] (C.-C.C.); [email protected] (T.-J.W.); Tel.: +886-5-534-2601 (ext. 4410, 4484) (T.-J.W.)  Received: 15 April 2020; Accepted: 4 May 2020; Published: 6 May 2020 

Abstract: Hearing-protection devices (HPDs) are particularly important in protecting the hearing of workers. The aim of this study was to prevent hearing damage in workplaces in Taiwan. It was conducted to determine the actual sound attenuation of the personal attenuation rating (PAR) values when wearing HPDs via measurements from field microphones in workers’ real ears (F-MIRE). Across 105 measurement trials for the Classic™ roll-down foam earplug HPDs worn by the workers, there were 23 cases of ineffective protection (including caution and fail); the proportion was 20% (including the first measurement and re-wear of HPDs after education and training). In addition, re-education and training in how to wear the HPDs was provided, improving wearing skills. A total of 29 testees wearing the Classic™ roll-down foam earplug HPDs failed to meet the pass standard for the first PAR test, and 6 of them improved and subsequently passed the PAR test. The improvement rate was 20%. These 23 testees switched to another HPD, namely Kneading-Free Push-Ins™ earplugs. From this group, 16 effective sound attenuation values were obtained, with an improvement rate of 70%. However, seven testees failed to pass the PAR test, and after education, training, and replacement of HPDs with different types, they still could not pass the PAR test. At that time, even if the UltraFit™ pre-molded earplugs were adopted again for wear and replacement, they were still unable to pass the PAR test. This HPD was eventually replaced with the PELTOR X4A Earmuff HPD and then tested again, with these HPDs finally passing the PAR test. In Taiwan, the use of fit testing has been increasing but it is not a common practice, and few studies on hearing-protection fit testing have been conducted in this country. The goal of this study was to gain more insight into the current hearing protection situation, including field attenuation of HPDs obtained by workers, the effects of training on improving the attenuation of HPDs after F-MIRE measurements, and the awareness of hearing health and motivation on the use of HPDs in a high-noise-level environment.

Keywords: hearing-protection device (HPD); noise exposure; personal attenuation rating (PAR); field microphone in real ear (F-MIRE); pre-workforce education

1. Introduction Noise-induced (NIHL) is one of the most common health hazards faced by workers in the workplace. According to the U.S. Bureau of Labor Statistics (BLS), from 2004 to 2010, the incidence rate of hearing loss in the manufacturing industry was 12.9%–16.9% per 10,000 workers, which makes it the most common nonfatal occupational injury [1]. In addition, in accordance with the reporting statistics for occupational diseases from the National Diagnosis and Treatment Network for Occupational

Int. J. Environ. Res. Public Health 2020, 17, 3242; doi:10.3390/ijerph17093242 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2020, 17, 3242 2 of 11

Diseases and Injuries, the Occupational Safety and Health Administration (OSHA) in Taiwan showed that the number of reported occupational diseases in 2018 (including prevention and treatment centers and network hospitals) was 2158. Among these figures, the highest rate was 813 events of occupational musculoskeletal diseases, accounting for 37.7%, followed by 609 events of occupational hearing loss, accounting for 28.2%; the third highest rate was 329 events of occupational skin disease, accounting for 15.2% [2]. Survey data from the Institute of Labor, Occupational Safety and Health (IOSH) in Taiwan show that the manufacturing industry in Taiwan includes various types of factories, and their noise ranges are recorded as follows: steel (80–110 dBA), textile (80–115 dBA), oil (80–100 dBA), machinery (80–110 dBA), and building (80–115 dBA) [3]. exposure limits are generally based on the assumption of employees working 8 hours a day for 5 days a week. The average sound pressure level for an 8 hour workday, i.e., the daily noise exposure level (or so-called Lex, 8 hours), is measured to determine whether it complies with the law [4]. The basic regulations set out in Taiwanese law are listed in Table1.

Table 1. The Taiwanese regulations on hearing protection.

Regulations Regulatory Requirements Employers shall have the necessary safety and health equipment and measures that comply with regulations to “prevent the risks of injuries posed by Occupational Safety and Health Act radiation, high temperature, low temperature, ultrasonic waves, noise, vibration, and abnormal ” [5]. States expressly that job site monitoring plans shall be Enforcement Rules of the Occupational Safety and formulated with the implementation of monitoring Health Act whether the job sites are emitting extreme noise [6]. Claims that 85 dBA is the action level, and the • hearing conservation program (HCP) must be implemented as required. In workplaces where the time-weighted average • sound pressure level for an 8 hour workday (8 hour TWA) exceeds 85 dBA or the exposure dose exceeds 50%, the employers shall have the following hearing protection measures taken, Occupational Safety and Health Facilities Regulation and an execution record shall be made and retained for three years: Noise monitoring and exposure assessment; • Noise hazard control; • Selection and wearing of hearing-protection • devices (HPDs); Hearing-protection education and training; • Management and examination of health; and • Effectiveness evaluation and improvement [7]. •

To date, comprehensive studies and reports have assessed hearing protection. Daniell et al. (2002) found that most industries pay little attention to noise control, instead relying on the use of HPDs. Increasing the wearing rate of HPDs depends on the effective HCP [8,9]. Miyauchi et al. (2000) also indicated that up to 29% of workers do not wear HPDs properly [10]. Heyer et al. (2011) showed that the mandatory use of HPDs can help to significantly reduce the hazard of hearing loss caused by noise [11]. Furthermore, there is a low proportion of people wearing HPDs in the industry, or no training provided on how to wear HPDs [12]. Four simplified indices (listed in Table2) are often used for hearing-protection performance evaluation of HPDs: (1) high, medium, low (HML); (2) single-number rating (SNR); (3) noise-reduction rating (NRR); and (4) octave band (OB) [13]. Int. J. Environ. Res. Public Health 2020, 17, 3242 3 of 11

Table 2. Hearing-protection performance evaluation of hearing-protection devices (HPDs).

Method Illustration/Formula Referring to ISO 4869-2 (1992), three values are provided to calculate the sound attenuation values of high frequency (H), intermediate frequency (M), and low frequency (L).

high, medium, low (HML) P4 P4 (1) Hx = 0.25 i = 1 PNRxi 0.48 i = 1(di.PNRxi) P8 − P8 (2) Mx = 0.25 i = 5 PNRxi 0.16 i = 5(di.PNRxi) P8 − P8 (3) Lx = 0.25 PNR 0.23 (di.PNR ) i = 5 xi − i = 51 xi

Referring to ISO 4869-2 (1992) for a single index of sound attenuation value. It is the value measured under the pink single-number rating (SNR) background noise of 100 dB (C). P SNR = 100dBC 101 log 8 100.1 ( LAf(K) APVf(k)) , dB(A) xi − K = 1 − Referring to ANSI S12.6 (1984) for a single index of sound attenuation value. It is the value obtained by testing under the pink noise-reduction rating (NRR) background noise (total energy 107.9 dBC), in which the energy of each frequency band in the eight band noise is 100 dB (C).

P8 0.1 ( LA ( ) APV ( ) ) NRR = 107.9dBC 101 log 10 f k − f k 3dB(A) − K = 1 − Referring to ISO 4869-2 (1992), after calculating the sound attenuation value of the eight note frequency band separately, the octave band (OB) sound attenuation values of the HPDs can be obtained.

P8 0.1 ( Lf(k)+Af(k) APVf(k) ) L0A = 101 log K = 1 10 − dB(A)

For business entities in Taiwan, most adopt the NRR value of ANSI S12.6 (1984) to assess the hearing-protection performance of HPDs as a basis for selecting HPDs. The NRR, which is a single index of the sound attenuation value, is a tested value measured under pink background noise (total energy is 107.9 dBC) with 100 dBC in each band of energy among the octave noise [14]. There are two international standards for the noise-reduction value of HPDs, SNR and NRR: SNR is the single-value noise-reduction value detected according to the international standard ISO 4969-2; NRR is a single-valued noise-reduction value detected according to the American standard ANSI S3.19-1974. In comparisons of two kinds of HPDs, SNR will be above 3 dB higher than NRR. NRR considers the individual differences of the wearer. Thus, the calculated protection average is subtracted by two standard deviations to determine the point at which 97% of wearers can wear this HPD and achieve the desired NRR value. Based on the consideration of protecting workers’ hearing from damage due to exposure, institutions should choose HPDs marked with the tested sound-attenuation value according to the conditions of the workplace [15]. Although the NRR aims to achieve a theoretical noise reduction for 98% of workers if they follow the HPD instructions, according to the research, fewer than 5% of workers actually reach the protection level predicted by the NRR [16,17]. At present, there are many systems available for measuring the attenuation performance of HPDs worn by workers in workplaces, including the following: real-ear attenuation at threshold (REAT) (see Method B of ANSI S12.6-1997 and SA/SNZ 1270), which is particularly intended for continuous usage conditions and can provide estimates of field performance [15], field microphone in real ear (F-MIRE), and acoustic text fixture (ATF). In this study, one of these test methods was specifically introduced, the F-MIRE [18] system, which combines a single miniature two-element microphone and relevant professional technologies [19–21]. In Taiwan, the labor noise exposure in many industries still exceeds the regulation of 90 dBA for TWA (the time-weighted average sound pressure level during an 8 hour working day with noise exposure). Even if employers provide and require the wearing of HPDs for tasks involving noise, there are still laborers who are classified as “health management level 4” in health management, the Int. J. Environ. Res. Public Health 2020, 17, 3242 4 of 11 cause of which is worthy of in-depth discussion [22,23]. Therefore, this study was conducted through onsite visits in the high-noise manufacturing industry to two steel plants (Factories A and B) and a textile factory (Factory C). The 3M™ EA-RfitTM (3M, St. Paul, MN, USA) HPD adaptability evaluation system is shown in Figure1. It was adopted to measure the personal attenuation rating (PAR) of laborers while wearing HPDs in these three noisy workplaces. In this study, we also analyzed the Int. J. Environ. Res. Public Health 2020, 17, x FOR PEER REVIEW 5 of 12 spectral characteristics for the high-noise work type and implemented an external sound pressure level test after HPDs were worn. We expected to establish an HPD-fit test method and to determine whether it metInt. J. the Environ. noise Res. exposure Public Health standards. 2020, 17, x WeFOR provide PEER REVIEW an eff ective evaluation of hearing-protection devices 5 of 12 as references.

Figure 1. Schematic diagram for the measurement location in the evaluation of the attenuation of HPDs via the F-MIRE test method.

✽Remarks: Figureinsertion 1. Theloss equipment (IL) = A – used A’ ≌ in REAT; the F-MIRE noise test reduction method (NR) in this =study. B’ – A’; TFOE is measuredFigure 1. Schematic with respect diagram to Point for the B, measurementwhich is generally location the in head-centerthe evaluation location of the withattenuation the of 2. Methods headHPDs absent. via the NRF-MIRE = IL test– TFOE. method.

2.1. TestThe✽ FacilityRemarks: 3M™ and EA-RfitTM insertion Equipment lossHPD (IL) adaptability = A – A’ ≌evaluati REAT;on noise system reduction is composed (NR) = B’of –software A’; TFOE and is hardware.Themeasured attenuation Figure with 2 provides of respect HPDs anto implemented Pointimage B,of whichthe viamicrophone is the generally F-MIRE and the testprobed head-center method, earplug which locationtips. The examines with F-MIRE the the system consists of a sound source that can generate high levels of wideband random noise at the relationshiphead betweenabsent. NR the = insertion IL – TFOE. loss (IL) and the noise reduction (NR), where TFOE is the transfer testee’s ear, a dual-element microphone that measures in a quotable location both the sound present function of the open ear, is shown in Figure2. When the HPD (earplugs or earmu ffs) is worn, at theThe outside 3M™ of theEA-RfitTM earplug and HPD the adaptabilitysound present evaluati in the earon canal system after is having composed passed of through software the and the exposure volume in the ear is A’, and the noise exposure volume at the position of the external earplug,hardware. a probed Figure earplug 2 provides to act an as imagea substitute of the for microphone the actual earplug and probed that subjects earplug will tips. wear, The and F-MIRE a reference point wearing the HPD is B’. The difference between A’ and B’, i.e., B’ – A’, represents the robustsystem analysisconsists systemof a sound installed source on thata PC can laptop generate that highcan rapidly levels oftake wideband accurate randomand repeatable noise at the noisemeasurementstestee’s reduction ear, a (NR).dual-element in 10 However,s.Int. The J. Environ. sound microphoneInt. regardless Res.J.levels Environ. Public used, of thatRes.Health the dependingPublic measures type2020 Health, 1 of7, x HPD, 20FORinupon20 a, PEERquotable1 while 7the, x FOR REVIEWlevel subjective PEER location of attenuation REVIEW sensory both the provided testing sound by bypresent the 5 of 13 5 of 13 humantheat the earplug, ear outside is used, were of thethe up earplug ditoff erence90 dBA. and between theThe sound testee’s the present sound nose was pressurein the positioned ear level canal without40 after cm havingfrom and withthe passed front the HPD,throughof the i.e., the A –loudspeaker.earplug, A’, is called a probed Most the insertion earplug3M™ HPDs lossto act (IL),can as a whichbe substitute measured is equivalent for using the actual tothis the evaluation earplug difference that system in subjects the hearingto obtainwill thresholdwear, their and a measuredattenuation.robust analysis whether The actualsystem the HPD measurement installed is worn on or only a not, PC takes i.e., laptop REAT.approximately that Therefore, can rapidly10 s, when which take the can accurate NR provide and ILand the values sevenrepeatable are comparedstandardmeasurements with test frequencies each in 10 other, s. The (from no sound correction 125 Hzlevels to will 8 used, KHz) result depending and in the an errorindividual upon of 5–10 the binaural dBlevel in of thePAR attenuation measured values. valueprovided [22]. by the earplug, were up to 90 dBA. The testee’s nose was positioned 40 cm from the front of the loudspeaker. Most 3M™ HPDs can be measured using this evaluation system to obtain their attenuation. The actual measurement only takes approximately 10 s, which can provide the seven standard test frequencies (from 125 Hz to 8 KHz) and the individual binaural PAR values.

Figure 2. FigureSchematic 2. Schematic diagram fordiagram the measurem for the measurement locationent inlocation the evaluation in the evaluation of the attenuation of the attenuation of of

HPDs viaHPDs the F- MIREvia the test F-MIRE method. test method. Figure 2. SchematicFigure diagram 2. The equipment for the measurement used in the F-MIRE location test in method the evaluation in this study. of the attenuation of ✽ Remarks: insertion loss (IL) = A – A’ ≌ REAT; noise reduction (NR) = B’ – A’; HPDs via the F-MIRE test method. ✽ Remarks: insertion insertion loss loss (IL) (IL)= A = AA0 – A’REAT; ≌ noise reductionREAT; noise reduction (NR) = B’ – A’; In this study, 50 male workers from noisy work areas of steel mills− (A, B) and textile factories (NR) = B0 A0; TFOE is measuredTFOE is with TFOE measured respect is measured to with Point respect B, with which to respect is P generallyoint to B, P which theoint head-center B, is which generally location is generally the head the-cent header location-center location (C) served as− the testees for testing. We fully explained the research to all testees, and the research with the head absent. NRwith= IL theTFOE. withhead the absent. head NR absent. = IL –NR TFOE = IL. – TFOE. was approved by the occupational− safety and health management entities of the three participating The 3M™ EA-RfitTM HPD adaptability evaluation system is composed of software and factories and was conductedFigure 2. The with equipment theThe consent 3M™ used of EAinall the -testees.RfitTM F-MIRE HPD test method adaptability in this study. evaluation system is composed of software and hardware.hardware. Figure 1 Figureprovides 1 provides an image an of image the microphone of the microphone and probed and earplug probed earplugtips. The tips. F-MIRE The F-MIRE 2.2. TypesIn this of HPDsstudy, 50system male workers consistssystem fromconsistsof a soundnoisy of workasource sound areas that source ofcan steel thatgenerate mills can (A, generatehigh B) levelsand high textile of levelswideband factories of wideband random randomnoise at noisethe at the testee’s ear, a dual-element microphone that measures in a quotable location both the sound present (C) servedVarious as types the testeesof HPDs for were testing.testee prepared’s Weear, fullya in dual the expl- factorieselementained themicrophoneparticipating research ttoinhat thisall measures testees, research and in so a thatthequotable researchthe location both the sound present

workerswas approved could selectby the theat occupational the appropriatoutsideat the e ofoutsidesafety type. the earplug Aand oftotal thehealth of andearplug four managementthe differen soundand thet presenttypes sound entities of in presentHPD ofthe the earwere inthree canal the adopted participatingear after canal inhaving after passed having through passed throughthe the factories and was conductedearplug, withearplug,a probed the consenta earplug probed of toearplug all act testees. as toa substituteact as a substitute for the actual for the earplug actual thatearplug subjects that willsubjects wear, will and wear, a and a robust analysisrobust analysis system installed system installed on a PC onlaptop a PC thatlaptop can that rapidly can take rapidly accurate take accurateand repeatable and repeatable 2.2. Types of HPDs measurementsmeasurements in 10 s. The in 10 sound s. The levels sound used, levels depending used, depending upon the upon level theof attenuationlevel of attenuation provided provided by by the earplug,the earplug,were up were to 90 up dBA. to 90 The dBA. testee’s The nosetestee’s was nose posi wastioned posi 40tioned cm from 40 cm the from front the of front the of the Various types of HPDs were prepared in the factories participating in this research so that the loudspeaker.loudspeaker. Most 3M™ Most HPDs 3M™ can HPDs be measured can be measured using this using evaluation this evaluation system to system obtain to their obtain their workers could select the appropriate type. A total of four different types of HPD were adopted in attenuation.attenuation. The actual The measurement actual measurement only takes only approximately takes approximately 10 s, which 10 scan, which provide can providethe seven the seven standardstandard test frequencies test frequencies (from 125 (from Hz to 125 8 KHz) Hz to and 8 KHz) the individualand the individual binaural binauralPAR values. PAR values. In this study,In this 50 study, male workers50 male workersfrom noisy from work noisy areas work of areassteel millsof steel (A, mills B) and (A, textile B) and factories textile factories (C) served(C) as served the testees as the for testees testing. for Wetesting. fully We explained fully explained the research the researchto all testees, to all and testees, the researchand the research was approvedwas approved by the occupational by the occupational safety and safety health and management health management entities of entities the three of the participating three participating factories factoriesand was andconducted was conducted with the withconsent the ofconsent all testees. of all testees.

2.2. Types2.2. of HPDsTypes of HPDs Various typesVarious of typesHPDs of were HPDs prepared were prepared in the factories in the factoriesparticipating participating in this researc in thish researcso thath the so that the workers workerscould select could the select appropriate the appropriate type. A totaltype. of A fourtotal different of four different types of typesHPD wereof HPD adopted were adoptedin in this studythis (as study listed (as in listed Table in3 ): Table 3M™ 3 ):Classic™ 3M™ Classic™ (3M, St.Paul (3M, Minnesota, St.Paul Minnesota, USA) roll USA)-down roll foam-down foam earplugs,earplugs, 3M™ Push3M™-Ins™ Push (3M,-Ins™ St.Paul (3M, Minnesota,St.Paul Minnesota, USA) stemmed USA) stemmed-style pod-style plugs, pod 3M™ plugs, 3M™ UltraFit™UltraFit™ (3M, St.Paul (3M, Minnesota, St.Paul Minnesota, USA) pre USA)-molded pre -molded earplugs, earplugs, and 3M™ and PELTOR 3M™ PELTOR X4A Earmuff X4A Earmuff (3M, St.Paul(3M, Minnesota, St.Paul Minnesota, USA). In USA) addition,. In addition, the number the ofnumber experimental of experimental trials using trials these using four these types four types of HPD wasof HPD 111, was23, 7 1, 11and, 23, 7, 7respectiv, and 7, respectively. ely.

Table 3. VariousTable 3. types Various and types appearances and appearances of the HPD of samplesthe HPD tested samples in thistested study. in this study.

Classic™Classic™ Push-Ins™Push -Ins™ UltraFit™UltraFit™ PELTORPELTOR X4A X4A Type. Type.roll -downroll foam-down foamstemmed stemmed-style pod-style podpre -moldedpre- molded Earmuff Earmuff earplugsearplugs plugs plugs earplugsearplugs

Int. J. Environ. Res. Public Health 2020, 17, 3242 5 of 11

The 3M™ EA-RfitTM HPD adaptability evaluation system is composed of software and hardware. Figure1 provides an image of the microphone and probed earplug tips. The F-MIRE system consists of a sound source that can generate high levels of wideband random noise at the testee’s ear, a dual-element microphone that measures in a quotable location both the sound present at the outside of the earplug and the sound present in the ear canal after having passed through the earplug, a probed earplug to act as a substitute for the actual earplug that subjects will wear, and a robust analysis system installed on a PC laptop that can rapidly take accurate and repeatable measurements in 10 s. The sound levels used, depending upon the level of attenuation provided by the earplug, were up to 90 dBA. The testee’s nose was positioned 40 cm from the front of the loudspeaker. Most 3M™ HPDs can be measured using this evaluation system to obtain their attenuation. The actual measurement only takes approximately 10 s, which can provide the seven standard test frequencies (from 125 Hz to 8 KHz) and the individual binaural PAR values. In this study, 50 male workers from noisy work areas of steel mills (A, B) and textile factories (C) served as the testees for testing. We fully explained the research to all testees, and the research was approved by the occupational safety and health management entities of the three participating factories and was conducted with the consent of all testees.

Int.Int.Int.2.2. J.J. Int.J. Environ.Environ. Environ. TypesJ. Environ. Res.Res. ofRes. HPDs PublicRes.Public Public Public HealthHealth Health Health 20202020 2020 2020,, ,1717 17,, ,xx x17 FORFOR FOR, x FOR PEERPEER PEER PEER REVIEWREVIEW REVIEW REVIEW 6 6 6 ofof of 12 612 12 of 12 Various types of HPDs were prepared in the factories participating in this research so that the thisthisthisthis studystudystudy study (as(as(as (as listedlistedlisted listed ininin TableTableinTable Table 3):3):3): 3M™3):3M™3M™ 3M™ Classic™Classic™Classic™ Classic™ (3M,(3M,(3M, (3M, St.PaulSt.PaulSt.Paul St.Paul Minnesota,Minnesota,Minnesota, Minnesota, USA)USA)USA) USA) roll-downroll-downroll-down roll-down foamfoamfoam foam workers could select the appropriate type. A total of four different types of HPD were adopted in earplugs,earplugs,earplugs,earplugs, 3M™3M™3M™ 3M™ Push-Ins™Push-Ins™Push-Ins™ Push-Ins™ (3M,(3M,(3M, (3M, St.PaulSt.PaulSt.Paul St.Paul Minnesota,Minnesota,Minnesota, Minnesota, USA)USA)USA) USA) stemmed-stylestemmed-stylestemmed-style stemmed-style podpodpod pod plugs,plugs,plugs, plugs, 3M™3M™3M™ 3M™ this study (as listed in Table3): 3M ™ Classic™ (3M, St. Paul, MN, USA) roll-down foam earplugs, UltraFit™UltraFit™UltraFit™UltraFit™ (3M,(3M,(3M, (3M, St.PaulSt.PaulSt.Paul St.Paul Minnesota,Minnesota,Minnesota, Minnesota, USA)USA)USA) USA) pre-mopre-mopre-mo pre-moldedldedldedlded earplugs,earplugs,earplugs, earplugs, andandand and 3M™3M™3M™ 3M™ PELTORPELTORPELTOR PELTOR X4AX4AX4A X4A EarmuffEarmuffEarmuff Earmuff 3M™ Push-Ins™ (3M, St. Paul MI, USA) stemmed-style pod plugs, 3M™ UltraFit™ (3M, St. Paul, MN, (3M,(3M,(3M,(3M, St.Paul St.PaulSt.Paul St.Paul Minnesota, Minnesota,Minnesota, Minnesota, USA). USA).USA). USA). In InIn addition, addition,addition,In addition, the thethe thenumber numbernumber number of ofof experimental experimental experimentalof experimental trials trialstrials trials using usingusing using these thesethese these four fourfour four types typestypes types USA) pre-molded earplugs, and 3M™ PELTOR X4A Earmuff (3M, St. Paul, MN, USA). In addition, the ofofof HPD HPDHPDof HPD was waswas was 111, 111,111, 111, 23, 23,23, 7,23, 7,7, and and and7, and 7, 7,7, respectively. respectively. respectively.7, respectively. number of experimental trials using these four types of HPD was 111, 23, 7, and 7, respectively.

TableTableTableTable 3. 3.3. Various VariousVarious 3. Various types typestypes types and andand and appearances appearancesappearances appearances of ofof the the theof HPDthe HPDHPD HPD samples samplessamples samples tested testedtested tested in inin this this thisin this study. study.study. study. Table 3. Various types and appearances of the HPD samples tested in this study.

Classic™Classic™Classic™Classic™Classic ™ Push-Ins™Push-Ins™Push-Ins™Push-InsPush-Ins™™ UltraFit™UltraFit™UltraFit™UltraFitUltraFit™™ PELTORPELTORPELTORPELTOR X4A X4AX4A X4A Type.Type.Type.Type.Type. Roll-DownRoll-DownRoll-DownRoll-DownRoll-Down Foam FoamFoam FoamFoam Stemmed-StyleStemmed-StyleStemmed-StyleStemmed-StyleStemmed-Style Pod PodPod Pod Pre-MoldedPre-MoldedPre-MoldedPre-Molded EarmuffEarmuffEarmuEarmuffEarmuffff EarplugsEarplugsEarplugsEarplugsEarplugs PlugsPlugsPlugsPlugsPlugs EarplugsEarplugsEarplugsEarplugs

AppearanceAppearanceAppearanceAppearanceAppearance

2.3.2.3.2.3.2.3. 2.3. Experimental ExperimentalExperimental Experimental Experimental Procedure ProcedureProcedure Procedure Procedure The research trials were conducted in the office areas of each participating factory in this TheTheTheThe researchresearchresearch research trialstrialstrials werewere were conducted conductedconducted inininin the thethethe o officeffiofficeofficece areas areasareasareas of ofof eachof eacheacheach participating participatingparticipatingparticipating factory factoryfactoryfactory in this ininin this study.thisthis study. The background noise in the office was less than 80 dBA, complying with the regulations of study.study.study.The background The TheThe background backgroundbackground noise noise noise innoise the in in oin ffi the thethece office wasofficeoffice less was waswas than less lessless 80 than thanthan dBA, 80 8080 complying dBA, dBA,dBA, complying complyingcomplying with the with withwith regulations the thethe regulations regulationsregulations of the 3M of ofof ™ the 3M™ EA-RfitTM HPD adaptability evaluation system. The evaluation process is shown in thethetheEA-RfitTM 3M™3M™3M™ EA-RfitTMEA-RfitTMEA-RfitTM HPD adaptability HPDHPDHPD adaptabilityadaptabilityadaptability evaluation evaluationevaluationevaluation system. The system.system.system. evaluation TheTheThe processevaluationevaluationevaluation is shown processprocessprocess in Figure isisis shownshownshown3; at in theinin Figure 3; at the beginning, during the untaught condition, researchers observed the testees’ FigureFigureFigurebeginning, 3;3;3; atatat during thethethe beginning,beginning,beginning, the untaught duringduringduring condition, thethethe untaughtuntaughtuntaught researchers condition,condition,condition, observed researchersresearchers theresearchers testees’ HPD-wearingobservedobservedobserved thethethe skills testees’testees’testees’ from HPD-wearingHPD-wearing skills skills from from the theside side and and used used these these results results as the as thebenchmark benchmark for thefor thewearability wearability of the of the HPD-wearingHPD-wearingthe side and usedskillsskills these fromfrom results thethe sideside as andand the used benchmarkused thesethese rere forsultssults the asas wearability thethe benchmarkbenchmark of the for HPD.for thethe If wearabilitywearability the PAR value ofof thethe did HPD.HPD. If the If thePAR PAR value value did didnot notmeet meet the thepass pass stan standard,dard, i.e., i.e.,did didNOT NOT meet meet the therequirement requirement (Figure (Figure HPD.HPD.not meet IfIf thethe the PARPAR pass valuevalue standard, diddid notnot i.e., meetmeet did NOT thethe pass meetpass stanstan the requirementdard,dard, i.e.,i.e., diddid (Figure NOTNOT 5 meetmeet), or if thethe an requirement inappropriaterequirement (Figure(Figure manner 4), or4), ifor an if inappropriatean inappropriate manner manner of weof arwe wasar was observed, observed, even even if the if thetestee’s testee’s PAR PAR value value met met the the 4),4),of oror wear ifif anan was inappropriateinappropriate observed, even mannermanner if the testee’sofof wewearar PAR waswas value observed,observed, met the eveneven passing ifif thethe standard, testee’stestee’s thePARPAR researchers valuevalue metmet taught thethe passingpassing standard, standard, the theresearchers researchers taught taught the thetestee testee how how to properlyto properly wear wear the theHPDs HPDs and and then then passingpassingthe testee standard,standard, how to properlythethe researchersresearchers wear the taughttaught HPDs the andthe te thentesteestee repeated howhow toto theproperlyproperly adaptability wearwear test.thethe HPDs AfterHPDs the andand teaching thenthen repeatedrepeated the theadaptability adaptability test. test. After After the theteaching teaching component, component, if the if thePAR PAR value value still still indicated indicated a fail a failor or repeatedrepeatedcomponent, thethe adaptabilityadaptability if the PAR value test.test. After stillAfter indicated thethe teachingteaching a fail component,component, or showed no ifif thethe signs PARPAR of improvement,valuevalue stillstill indicatedindicated a different aa failfail type oror showedshowed no nosigns signs of improvement,of improvement, a differenta different type ty peof HPDof HPD and and additional additional instructions instructions were were showedshowedof HPD no andno signssigns additional ofof improvement,improvement, instructions were aa differentdifferent provided, tyty andpepe of theof HPDHPD adaptability andand additionaladditional was tested instructionsinstructions again with the werewere new provided,provided, and and the theadaptability adaptability was was tested tested again again with with the thenew new HPD HPD until until the thePAR PAR value value met met the the provided,provided,HPD until andand the thethe PAR adaptabilityadaptability value met the waswas “pass” testedtested standard againagain withwith (Figure thethe 6 new).new The HPDHPD evaluation untiluntil thethe steps PARPAR were valuevalue as follows: metmet thethe “pass”“pass”“pass”“pass” standard standardstandard standard (Figure (Figure(Figure (Figure 5). 5).5). The The5).The The evaluation evaluationevaluation evaluation steps stepssteps steps were werewere were as asas follows: follows:follows:as follows: ••• • AfterAfterAfterAfterAfter each eacheach each each testee testeetestee testee testee entered enteredentered entered entered the thethe the thetest testtest test test site, site,site, site, site, the thethe the theengi engiengi engineers engineersneersneersneers first firstfirst first first explained explainedexplained explained explained the thethe the thePAR PARPAR PAR PAR test testtest test test procedure; procedure;procedure; procedure; procedure; • ••• • TheTheTheTheThe testee testeetestee testee testee wore worewore wore wore the thethe the theearplugs earplugsearplugs earplugs earplugs with withwith with with probes probesprobes probes probes (Classic™ (Classic™(Classic™ (Classic (Classic™™ roll-down roll-downroll-downroll-down roll-down foam foamfoam foam foam earplugs); earplugs);earplugs); earplugs); earplugs); • ••• • TheTheTheThe researchers researchersresearchers researchers connected connectedconnected connected the thethe thetwo-element two-elementtwo-element two-element miniature miniatureminiature miniature microphone microphonemicrophone microphone to toto the thetheto theleft leftleft left and andand and right rightright right probes probesprobes probes ininin the thethein thetestee’s testee’stestee’s testee’s HPD; HPD;HPD; HPD; ••• • TheTheTheThe testee testeetestee testee faced facedfaced faced the thethe theloudspeaker loudspeakerloudspeaker loudspeaker at atat the thetheat thedesignat designatdesignat designatededed position, position,edposition, position, keeping keepingkeeping keeping the thethe theear earear earand andand and loudspeaker loudspeakerloudspeaker loudspeaker atatat the thetheat thesame samesame same height, height,height, height, and andand and the thethe thedistance distancedistance distance from fromfrom from the thethe thenose nosenose nose to toto the thetothe theclip clipclip clip on onon the onthethe theloudspeaker loudspeakerloudspeaker loudspeaker was waswas was 40 4040 cm cm40cm cm (Figure(Figure(Figure(Figure 6); 6);6); 6); ••• • TheTheTheThe testee’s testee’stestee’s testee’s PAR PARPAR PAR value valuevalue value was waswas was obtained obtainedobtained obtained using usingusing using the thethe theF-MIRE F-MIREF-MIRE F-MIRE test testtest test method; method;method; method; ••• • TheTheTheThe binauralbinauralbinaural binaural PARPARPAR PAR dBdBdB dB waswaswas was thethethe the differencedifferencedifference difference betweenbetweenbetween between thethethe the binauralbinauralbinaural binaural PARPARPAR PAR dBdBdB dB andandand and thethethe the binauralbinauralbinaural binaural variability;variability;variability;variability; and andand and ••• • IfIfIf the thetheIf thesum sumsum sum of ofof the thetheof thebinaural binauralbinaural binaural PAR PARPAR PAR dB dBdB anddB andand and the thethe thestatut statutstatut statutoryoryory ory noise noisenoise noise value valuevalue value in inin the thethein theworkplace workplaceworkplace workplace exceeded exceededexceeded exceeded the thethe the settingsettingsettingsetting standard standardstandard standard for forfor participatingfor participatingparticipating participating research researchresearch research factories, factories,factories, factories, the thethe theresult resultresult result was waswas was a aa pass; pass;pass; a pass; otherwise, otherwise,otherwise, otherwise, the thethe theresult resultresult result waswaswaswas a aa fail. fail.fail. a fail.

Int. J. Environ. Res. Public Health 2020, 17, 3242 6 of 11

The researchers connected the two-element miniature microphone to the left and right probes in • the testee’s HPD; The testee faced the loudspeaker at the designated position, keeping the ear and loudspeaker • at the same height, and the distance from the nose to the clip on the loudspeaker was 40 cm (Figure4); The testee’s PAR value was obtained using the F-MIRE test method; • The binaural PAR dB was the difference between the binaural PAR dB and the binaural • variability; and If the sum of the binaural PAR dB and the statutory noise value in the workplace exceeded the • setting standard for participating research factories, the result was a pass; otherwise, the result Int.was J. Environ. a fail. Res. Public Health 2020, 17, x FOR PEER REVIEW 7 of 12

First Tested with Classic™ roll-down foam earplugs For the Testees

Testing Results Requirement met Personal Attenuation Rating Pass (PAR) (PAR)

Int. J. Environ. Res. Public Health 2020, 17, x FOR FailPEER REVIEW 8 of 12

Requirement NOT met (PAR)

Fail Pass

Adjust Testees’ HPD Wearing Method

Fail

Replaced Testees’ HPD

Figure 5. An example where the test result of the PAR value was “pass” after additional instructions FigureandFigure a different 3. The3. The flowchart type flowchart of ofHPD the of evaluationwere the provided,evaluation process followedprocess for the for 3Mby ™thea re-testEA-RfitTM 3M™ of EA-RfitTM the HPD adaptability adaptability HPD adaptabilitywith evaluation the new systemHPD.evaluation applied system in this applied study. in this study.

FigureFigure 4. An 4. exampleThe distance where from the the test testee’s result noseof the to PAR the clip value on thewasloudspeaker “fail” in the was first 40 test cm. in the Figure 6. The distance from the testee’s nose to the clip on the loudspeaker was 40 cm. untaught situation of the testee. 3. Results In this study, a total of 148 PAR tests were conducted for Factories A, B, and C. Of these, 52 were held in Factory A, where the testees wore the HPDs sequentially as follows: the Classic ™ roll-down foam earplugs 36 times, accounting for 69%; the Push-Ins™ stemmed-style pod plugs 12 times, accounting for 23%; the UltraFit™ pre-molded earplugs 2 times, accounting for 4%; and the PELTOR X4A Earmuff 2 times, accounting for 4%. Factory B followed with 23 testees, accounting for 16%. The testees wore the HPDs sequentially as follows: the Classic ™ roll-down foam earplugs 18 times, accounting for 78%; the Push-Ins™ stemmed-style pod plugs 4 times, accounting for 17%; and the UltraFit™ pre-molded earplugs 1 time, accounting for 5%. In addition, Factory C was the site of 73 tests, accounting for 49%. The testees wore the HPDs sequentially as follows: the Classic ™ roll-down foam earplugs 57 times, accounting for 78%; the Push-Ins™ stemmed-style pod plugs 7 times, accounting for 10%; the UltraFit™ pre-molded earplugs 4 times, accounting for 5%; and the PELTOR X4A Earmuff 5 times, accounting for 7%. Statistical data for the frequencies and percentages of the abovementioned four various types of HPD for all 148 PAR tests across the three different factories are shown in Table 4. In addition, of the PAR tests for the four HPDs used in this study, the largest number was conducted for the Classic ™ roll-down foam earplugs (up to 105 times, accounting for 71%). The Push-Ins™ stemmed-style pod plugs followed with 23 times, accounting for 16%. The UltraFit™ pre-molded earplugs were tested 7 times, accounting for 5%, and the PELTOR X4A Earmuff was tested 7 times, accounting for 5% (Table 4).

Int. J. Environ. Res. Public Health 2020, 17, x FOR PEER REVIEW 7 of 12

First Tested with Classic™ roll-down foam earplugs For the Testees

Testing Results Requirement met Personal Attenuation Rating Pass (PAR) (PAR)

Fail

Requirement NOT met (PAR)

Fail Pass

Adjust Testees’ HPD Wearing Method

Fail

Replaced Testees’ HPD

Figure 3. The flowchart of the evaluation process for the 3M™ EA-RfitTM HPD adaptability evaluation system applied in this study. Int. J. Environ. Res. Public Health 2020, 17, 3242 7 of 11

Int. J. Environ. Res. Public Health 2020, 17, x FOR PEER REVIEW 8 of 12 FigureFigure 5. An4. An example example where where the testthe resulttest result of the of PAR the value PAR was value “fail” was in the“fail” first in test the in first the untaughttest in the situationuntaught of thesituation testee. of the testee.

FigureFigure 6. An5. An example example where where the the test test result result of of the the PAR PAR value value was was “pass” “pass” after after additional additional instructions instructions andand a di aff differenterent type type of HPD of HPD were were provided, provided, followed followed by a re-test by a re-test of the adaptabilityof the adaptability with the with new the HPD. new 3. ResultsHPD.

In this study, a total of 148 PAR tests were conducted for Factories A, B, and C. Of these, 52 were held in Factory A, where the testees wore the HPDs sequentially as follows: the Classic ™ roll-down foam earplugs 36 times, accounting for 69%; the Push-Ins™ stemmed-style pod plugs 12 times, accounting for 23%; the UltraFit™ pre-molded earplugs 2 times, accounting for 4%; and the PELTOR X4A Earmuff 2 times, accounting for 4%. Factory B followed with 23 testees, accounting for 16%. The testees wore the HPDs sequentially as follows: the Classic ™ roll-down foam earplugs 18 times, accounting for 78%; the Push-Ins™ stemmed-style pod plugs 4 times, accounting for 17%; and the UltraFit™ pre-molded earplugs 1 time, accounting for 5%. In addition, Factory C was the site of 73 tests, accounting for 49%. The testees wore the HPDs sequentially as follows: the Classic ™ roll-down foam earplugs 57 times, accounting for 78%; the Push-Ins™ stemmed-style pod plugs 7 times, accounting for 10%; the UltraFit™ pre-molded earplugs 4 times, accounting for 5%; and the PELTOR X4A Earmuff 5 times, accounting for 7%. Statistical data for the frequencies and percentages of the abovementioned Figure 6. The distance from the testee’s nose to the clip on the loudspeaker was 40 cm. four various types of HPD for all 148 PAR tests across the three different factories are shown in Table4. 3. Results Table 4. Distributions of the 148 PAR tests and percentages of the four types of HPDs at each factory. In this study, a total of 148 PAR tests were conducted for Factories A, B, and C. Of these, 52 were held in Factory A, whereClassic the™ testees worePush-Ins the™ HPDs UltraFitsequentially™ PELTORas follows: the Classic ™ Roll-Down Foam Stemmed-Style Pre-Molded X4A Total roll-down foam earplugs 36Earplugs times, accountingPod for Plugs 69%; the Push-Ins™Earplugs stemmed-styleEarmuff pod plugs 12 times, accounting for 23%; the UltraFit™ pre-molded earplugs 2 times, accounting for 4%; and the A 36 (69%) 12 (23%) 2 (4%) 2 (4%) 52 PELTORFactory X4A EarmuffB 2 times,18 (78%) accounting for 4 (17%)4%. Factory B 1followed (5%) with 0 (0%) 23 testees, 23 accounting for 16%. The testeesC wore the57 (78%)HPDs sequentially 7 (10%) as follows: the 4 (5%)Classic ™ roll-down 5 (7%) foam 73 earplugs 18 times, accountingTotal for 78%; 111the Push-Ins™ ste 23mmed-style pod 7plugs 4 times, 7 accounting 148 for 17%; and the UltraFit™ pre-molded earplugs 1 time, accounting for 5%. In addition, Factory C was the site of 73 tests, accounting for 49%. The testees wore the HPDs sequentially as follows: the Classic ™ roll-down foam earplugs 57 times, accounting for 78%; the Push-Ins™ stemmed-style pod plugs 7 times, accounting for 10%; the UltraFit™ pre-molded earplugs 4 times, accounting for 5%; and the PELTOR X4A Earmuff 5 times, accounting for 7%. Statistical data for the frequencies and percentages of the abovementioned four various types of HPD for all 148 PAR tests across the three different factories are shown in Table 4. In addition, of the PAR tests for the four HPDs used in this study, the largest number was conducted for the Classic ™ roll-down foam earplugs (up to 105 times, accounting for 71%). The Push-Ins™ stemmed-style pod plugs followed with 23 times, accounting for 16%. The UltraFit™ pre-molded earplugs were tested 7 times, accounting for 5%, and the PELTOR X4A Earmuff was tested 7 times, accounting for 5% (Table 4).

Int. J. Environ. Res. Public Health 2020, 17, 3242 8 of 11

In addition, of the PAR tests for the four HPDs used in this study, the largest number was conducted for the Classic ™ roll-down foam earplugs (up to 105 times, accounting for 71%). The Push-Ins™ stemmed-style pod plugs followed with 23 times, accounting for 16%. The UltraFit™ pre-molded earplugs were tested 7 times, accounting for 5%, and the PELTOR X4A Earmuff was tested 7 times, accounting for 5% (Table4). The different environments of Factories A, B, and C may have affected the PAR values, as summarized in Table5, which lists the relevant statistical analysis and test data for the PAR values from the three participating factories in this study. In Factory A, the mean (M) value of the left PAR dB was 25.5, and the standard deviation (SD) value was 8.3. The M value of the right PAR dB was 25.9 (SD = 6.9). For the binaural PAR dB, the M value was 22.7 (SD = 7.3). In Factory B, the M value of the left PAR dB was 24.7 (SD = 8.8); the M value of the right PAR dB was 22.2 (SD = 10.6); and the M value of the binaural PAR dB was 19.6 (SD = 9.9). In Factory C, the M value for the left PAR dB was 27.4 (SD = 8.4); the M value of the right PAR dB was 25.2 (SD = 9.3); and the M value of the binaural PAR dB was 22.7 (SD = 9.3). Furthermore, one-way ANOVA was used in the statistical test approach for the three participating factories in this study, yielding the following results for the left PAR dB: F = 4.192, p = 0.018 < 0.05. These findings indicated that the F-value for left PAR dB was 4.192, and the p-value was 0.018, which was less than 0.05 (the level of significance). Similarly, the other findings were as follows: right PAR dB: F = 4.120, p = 0.019 < 0.05; binaural PAR dB: F = 3.625, p = 0.030 < 0.05. Thus, all three variables showed significant differences. Another post hoc test via Scheffe’s method showed that for the left PAR dB, C > A, indicating that Factory C had a higher value than Factory A. Similarly, for the right PAR dB, C > B (Factory C was higher than Factory B), and for the binaural PAR dB, C > B (Factory C was higher than Factory B).

Table 5. Relevant results of statistical analysis and tests of the PAR values from the three participating factories in this study.

Significance Scheffe’s Factory A B C F value (p value) Method Mean (M) 25.5 24.7 27.4 Left PAR dB Standard 4.192 0.018 * C > A 8.3 8.8 8.4 deviation (SD) Mean (M) 25.9 22.2 25.2 Right PAR dB Standard 4.120 0.019 * C > B 6.9 10.6 9.3 deviation (SD) Mean (M) 22.7 19.6 22.7 Binaural PAR dB Standard 3.625 0.030 * C > B 7.3 9.9 9.3 deviation (SD) * p < 0.05.

Additionally, across the 111 measurement trials for the Classic™ roll-down foam earplugs worn by the workers (Table6), there were 29 cases of ine ffective protection (including caution and fail), a proportion of 22% (including the first measurement and re-wear of HPD after education and training). In addition, via the re-education and training of the wearing method to improve wearing skills, 23 testees wearing the Classic™ roll-down foam earplug HPDs failed to meet the pass standard for the first PAR test, and 6 of them improved and subsequently passed the PAR test. The improvement rate was 20%. These 23 testees then switched to another HPD, namely Kneading-Free Push-Ins™ earplugs; at this time, 16 effective sound attenuation values were obtained, and the improvement rate was 70%. However, seven testees failed to pass the PAR test after education training and the replacement of different types of HPD and could not pass the PAR test. In these cases, even if the UltraFit™ pre-molded earplugs were worn and replaced again, they were still unable to pass the PAR test. They were eventually replaced with the PELTOR X4A Earmuff HPDand then tested again; Int. J. Environ. Res. Public Health 2020, 17, 3242 9 of 11 this HPD finally passed the PAR test. The analysis results revealed that wearing Classic™ roll-down foam earplugs was the most convenient, lowest cost, and protective choice for 78% of the testees. Among these, four testees did not achieve hearing protection because of ear canal structural problems, and could not be protected until their HPDs were replaced with earmuffs.

Table 6. Distributions of the PAR test results and percentages of the four types of HPD.

Push-Ins™ UltraFit™ Classic™ PELTOR X4A Stemmed-Style Pre-Molded Roll-Down Foam Earplugs Earmuff Pod Plugs Earplugs 1st test 2nd test Caution 10 (4%) 4 (14%) - 4 (57%) - Fail 19 (18%) 19 (66%) 7 (30%) 3 (43%) - Pass 82 (78%) 6 (20%) 16 (70%) - 7 (100%) Total 111 (100%) 29 (100%) 23 (100%) 7 (100%) 7 (100%)

4. Discussion In this study, three high-noise job site visits were completed, and PAR measurement evaluations were conducted 140 person-times. During the process of the onsite visits, it was also found that after factors such as economy, convenience, and personnel acceptance were considered, most of the business entities still used mainly earplug-type HPDs. Only a few workers in specific working environments used earmuff-type HPDs. The general conclusions are summarized as follows: The highest usage rate of the earplug-type HPDs was the Classic ™ roll-down foam earplugs. • For employers, this was because of their relatively low price and convenience for workers to wear. The F-MIRE test method could separately measure the sound pressure inside and outside the ear • after HPDs were worn, which is intuitive and convenient. However, the test result was easily affected by incorrect posture and contact with the inner wall of the ear canal during wear. For the same type of HPD at different work sites, the measured average PAR value would also be different (Table4). The test result under professional instructions and re-wear of HPD for the testees could indeed • show improved protective efficiency of HPD for the wearers (excluding those with different ear canals, which could increase by ca. 35%). This confirmed the effectiveness of education and training. At present, the commercially available HPD adaptability test and evaluation system is mainly • based on the individual test applications of earplugs and earmuffs. Herein, the F-MIRE test method, among the systems adopted in this study, was convenient to implement at noisy work sites, and the evaluation results could be quickly obtained. This study was limited by the fact that it could only be performed during the production period • of each plant, and the experimental sampling needed to be completed within a limited time. This study is the first to use the commercially available hearing protectors with the highest usage rate in the current industry. During this period, 105 samples were taken.

5. Conclusions Based on the results and conclusions of related literature and field analysis, the following suggestions are proposed for future research: The F-MIRE test method could be utilized as an educational training tool to help workers to • wear and select HPDs, as it can directly evaluate the protective efficiency of HPDs for wearers. However, attention should be paid to the ambient background noise when the test is conducted. The current F-MIRE test method uses a miniature microphone and its extended ear canal probe • to measure the sound pressure inside the ear in the external ear canal after earmuffs are worn. Int. J. Environ. Res. Public Health 2020, 17, 3242 10 of 11

This method can directly evaluate the protective efficiency as individuals wear the HPD, but its ear canal probe tube easily touches the inner wall of the ear canal which affects the measurement results. Relevant improvements in measurement methods could be further explored in the future. The sound attenuation value, or NRR value, which is marked on the HPD, did not truly reflect • the effects of workers wearing HPDs at noisy work sites, and its value was not higher. The best HPD was suitable for the wearers; they could communicate without any obstacles when worn and it could be used effectively for a long time, i.e., it could effectively perform its function and be used continuously. This study found that some test subjects had smaller ear canals and the polymeric partially • compressible foam earplugs were not easy to insert, making the workers’ earplugs likely to fall off when worn for a long time. Thus, these workers should consider replacing their earbuds with different styles. In future work, we will assign different forms of hearing protection to the same tester to verify the • best type and performance of personal hearing protection.

Author Contributions: Conceptualization, C.-C.C. and T.-J.W.; methodology, C.-C.C.; investigation, C.-C.C.; data curation, C.-C.C. and T.-J.W.; writing—original draft preparation, C.-C.C.; writing—review and editing, C.-C.C. and T.-J.W.; visualization, C.-C.C.; supervision, T.-J.W. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to thank Derry Yo (Customer Service Manager for the 3M™ Occupational Safety Protection Products) for assisting in data collection and occupational safety. In addition, we are grateful to Kuo-Ming Cheng, Senior Technical Specialist at Central Center of Occupational Safety and Health, Occupational Safety and Health Administration, Ministry of Labor. The authors would also like to thank the Republic of China (Taiwan) for its support of this study. Conflicts of Interest: The authors declare no conflict of interest.

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