HUMIDIFICATION Fights Against Bioaerosol Infectivity

HUMIDIFICATION fights against bioaerosol infectivity

Relative humidity between 40% and 60% is ideal

• To reduce bioaerosol droplet evaporation, • To reduce infection transmission, • To reduce bioaerosol infectivity.

BARRIER GESTURES

Everybody knows the barrier gestures: No face touching, hand washing, disposable tissue using, mask wearing, cough/sneeze in the elbow. Mask wearing is very important when people are in public places or supermarkets. But it is not always possible to wear a mask at home in family, in offices. Controlled relative humidity between 40% and 60% is ideal to reduce the contamination by bioaerosols.

SURGICAL FACE MASK EFFICIENCY (1)

When indoor humidity ratio is around 7 - 10 g/kg (relative humidity 40%-60%) (2)(3), the respiratory infection effects decrease.

OPTIMUM ZONE Bacteria

Virus

Fungi

Respiratory infections

Asthma

0 10 20 30 40 50 60 70 80 90 100 From Scofiel - Sterling diagram(2)(3) RELATIVE HUMIDITY (%) Bioaerosol transmission

At this time, evidences accepted scientifically suggest that a controlled relative humidity can reduce the transmission of infectious agents by bioaerosols. (5)

What are bioaerosols?

Bioaerosols are airborne biological particles derived from virus, bacteria, fungi, protozoa, algae, mites, plants, insects and their by-products, fragments and cell mass components (11).

AIRBORNE PARTICULE SIZES

Exhaled bioaerosols

They fall 1.5 -2.0 m height:

Exhalation - in less of 6 seconds if diameter > 1mm, V < 1 m/s o - in ~ 6 seconds if diameter ~ 100 μm, - in ~ 8 minutes if diameter ~ 10 μm, - in ~ 1.5 hours if diameter ~ 3 μm, 1.5 - 2.0 m - in ~ 12 hours if diameter ~ 1 μm, - in ~ 41 hours if diameter ~ 0.5 μm. (4) (5)(7) Large aerosols > 100 μm 1.5m Note: 87% of exhaled bioaerosols are < 1 μm. (6)

Bioaerosols emited by coughing or sneezing

Droplets are swept by air motion and fall due to gravity. Their fall is resisted by drag force. Air flow Drag force

Droplets evaporated Gravity become droplet nuclei droplet nuclei or bioaerosols < 10 μm Vo = 50 m/s If inhaled : sneeze if >5 μm, trapped in V =10 m/s o upper respiratory cough Large aerosols tract > 100 μm if ≤ 5 μm, trapped in lower respiratory tract. Large aerosols > 100 μm > 2m > 6 m Dry particles ejected into the air (1)(5)(6)(10) Correct relative humidity reduces bioaerosol transmission

Maintaining indoor humidity ratio around 7 - 10 g/kg (relative humidity 40%-60%) helps To reduce the droplet evaporation process (5): large droplets are less rapidly changed into droplet nuclei. To reduce the transmission efficiency of virus airbornes:

“At 20ºC (dashed line), transmission efficiency is highest at low RH, when influenza virions in an aerosol are relatively stable, and desiccation of exhaled respiratory droplets produces droplet nuclei. Transmission is diminished at intermediate RH when virus particles are relatively unstable, but improves in parallel with influenza virus stability at higher humidities. At high RH, evaporation from exhaled particles is limited, respiratory droplets settle out of the air, and transmission is blocked” (8)

50% Variation of transmission efficiency with relative humidity To reduce the bioaerosols infectivity:

100 100 Total aerosols fractions 1 - 4 μm fraction

50 50 % INFECTIVITY % INFECTIVITY

0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80

RELATIVE HUMIDITY (%) RELATIVE HUMIDITY (%)

100 100 > 4 μm fraction < 1 μm fraction

50 50 % INFECTIVITY % INFECTIVITY

0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 RELATIVE HUMIDITY (%) RELATIVE HUMIDITY (%) “At constant temperature (20ºC) high humidity reduces the infectivity of influenza.”(9) Sources: (1) COVID-19: Why we should ware all mask, Sui Huang. (2) C. Mike Scofiel, P.E., and Elia Sterling. Dry Climate evaporative cooling with refrigeration backup, ASHRAE Journal 1992 (3) 2016 ASHRAE HVAC Systems and Equipment Handbook, Chap 22. Humidfiers (4) Wells, W. F., On Air-borne Infection. Study II. Droplets and Droplet Nuclei. Journal article: American Journal of Hygiene 1934 Vol.20 pp.611-18 (5) ASHRAE Position Document on Airborne Infectious Diseases Approved by ASHRAE Board of Directors April 14 , 2020 (6)Fabian P, McDevitt JJ, DeHaan WH, Fung ROP, Cowling BJ, et al. (2008) Influenza Virus in Human Exhaled Breath: An Observational Study. PLoS ONE 3(7):e2691. doi:10.1371/journal.pone.0002691 (7) Natural Ventilation for Infection Control in Health-Care Settings Editors: James Atkinson, Yves Chartier, Carmen Lúcia Pessoa-Silva, Paul Jensen, Yuguo Li, and Wing-Hong Seto. Geneva: World Health Organization; 2009. (8) Lowen AC, Mubareka S, Steel J, Palese P (2007) Influenza virus transmission is dependent on relative humidity and temperature. PLoS Pathog 3(10): e151. doi:10. 1371/journal.ppat.0030151 (9) Noti JD et. al, High Humidity Leads to Loss of Infections Influenza Virus from Simulated Coughs, PLoS ONE 8(2): e57485, 2013 (10) Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". (11) 2017 ASHRAE Fundamentals Handbook, Chap 10. Indoor environment health Author: P.E. David, Pharmacist and HVAC Specialist at Armstrong International

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