The Importance of Masks in Reducing the Risk of COVID-19

An article in the journal Science details the importance of masks in reducing transmission of the SARS-CoV-2 virus.  Social distancing may not be sufficient by itself, especially in indoor conditions., so masks help to reduce aerosols, the leading method of spread, especially from asymptomatic or pre-symptomatic individuals.  Even the act of speaking can emit aerosols.  And any properly fitting mask will help reduce the risk.   (Of course, wearing a mask down around your chin, with mouth and nose exposed, does nothing).

Science
https://science.sciencemag.org/content/early/2020/05/27/science.abc6197
Reducing transmission of SARS-CoV-2

Kimberly A. Prather1, Chia C. Wang,2,3 Robert T. Schooley4 1Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92037, USA. 2Department of Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan 804, Republic of China. 3Aerosol Science Research Center, National Sun Yat-Sen University, Kaohsiung, Taiwan 804, Republic of China. 4Department of Medicine, Division of Infectious Diseases and Global Public Health, School of Medicine, University of California San Diego, La Jolla, CA 92093, USA. Email: kprather@ucsd.edu 

Masks and testing are necessary to combat asymptomatic spread in aerosols and droplets

Excerpts:

"Recent studies have shown that in addition to droplets, SARS-CoV-2 may also be transmitted through aerosols. A study in hospitals in Wuhan, China, found SARS-CoV-2 in aerosols further than 6 ft from patients with higher concentrations detected in more crowded areas (8). Estimates using an average sputum viral load for SARS-CoV-2 indicate that 1 min of loud speaking could generate >1000 virion-containing aerosols (9). Assuming viral titers for infected super-emitters (with 100fold higher viral load than average) yields an increase to more than 100,000 virions in emitted droplets per minute of speaking."

"The World Health Organization (WHO) recommendations for social distancing of 6 ft and hand washing to reduce the spread of SARS-CoV-2 are based on studies of respiratory droplets carried out in the 1930s. These studies showed that large, ~100 μm droplets produced in coughs and sneezes quickly underwent gravitational settling (1). However, when these studies were conducted, the technology did not exist for detecting submicron aerosols. As a comparison, calculations predict that in still air, a 100-μm droplet will settle to the ground from 8 ft in 4.6 s whereas a 1-μm aerosol particle will take 12.4 hours (4). Measurements now show that intense coughs and sneezes that propel larger droplets more than 20 ft can also create thousands of aerosols that can travel even further (1). Increasing evidence for SARS-CoV-2 suggests the 6 ft WHO recommendation is likely not enough under many indoor conditions where aerosols can remain airborne for hours, accumulate over time, and follow air flows over distances further than 6 ft (5, 10)."

"Overall, the probability of becoming infected indoors will depend on the total amount of SARS-CoV-2 inhaled. Ultimately, the amount of ventilation, number of people, how long one visits an indoor facility, and activities that affect air flow will all modulate viral transmission pathways and exposure (10). For these reasons, it is important to wear properly fitted masks indoors even when 6 ft apart. Airborne transmission could account, in part, for the high secondary transmission rates to medical staff, as well as major outbreaks in nursing facilities."

"After evidence revealed that airborne transmission by asymptomatic individuals might be a key driver in the global spread of COVID-19, the WHO recommended universal use of face masks. Masks provide a critical barrier, reducing the number of infectious viruses in exhaled breath, especially of asymptomatic people and those with mild symptoms (12) (see the figure). Surgical mask material reduces the likelihood and severity of COVID-19 by substantially reducing airborne viral concentrations (13). Masks also protect uninfected individuals from SARS-CoV-2 aerosols (12, 13). Thus, it is particularly important to wear masks in locations with conditions that can accumulate high concentrations of viruses, such as health care settings, airplanes, restaurants, and other crowded places with reduced ventilation. The aerosol filtering efficiency of different materials, thicknesses, and layers used in properly fitted homemade masks was recently found to be similar to that of the medical masks that were tested (14). Thus, the option of universal masking is no longer held back by shortages."

"From epidemiological data, countries that have been most effective in reducing the spread of COVID-19 have implemented universal masking, including Taiwan, Hong Kong, Singapore, and South Korea."