![]() The distance from a smoker at which one smells cigarette smoke indicates the distance in those surroundings at which one could inhale infectious aerosols. Assuming SARS-CoV-2 virions are contained in submicron aerosols, as is the case for influenza virus, a good comparison is exhaled cigarette smoke, which also contains submicron particles and will likely follow comparable flows and dilution patterns. Given how little is known about the production and airborne behavior of infectious respiratory droplets, it is difficult to define a safe distance for social distancing. Because respiratory viruses can remain airborne for prolonged periods before being inhaled by a potential host, studies are needed to characterize the factors leading to loss of infectivity over time in a variety of outdoor environments over a range of conditions Moreover, people living in areas with higher concentrations of air pollution have been shown to have higher severity of COVID-19 ( 11). Viruses can attach to other particles such as dust and pollution, which can modify the aerodynamic characteristics and increase dispersion. Additionally, SARS-CoV-2 can be inactivated by ultraviolet radiation in sunlight, and it is likely sensitive to ambient temperature and relative humidity, as well as the presence of atmospheric aerosols that occur in highly polluted areas. Viral concentrations will be more rapidly diluted outdoors, but few studies have been carried out on outdoor transmission of SARS-CoV-2. Asymptomatic individuals who are speaking while exercising can release infectious aerosols that can be picked up by airstreams ( 10). Breezes and winds often occur and can transport infectious droplets and aerosols long distances. In outdoor environments, numerous factors will determine the concentrations and distance traveled, and whether respiratory viruses remain infectious in aerosols. Increasing evidence for SARS-CoV-2 suggests the 6 feet CDC recommendation is likely not enough under many indoor conditions, where aerosols can remain airborne for hours, accumulate over time, and follow airflows over distances further than 6 feet ( 5, 10). Measurements now show that intense coughs and sneezes that propel larger droplets more than 20 feet can also create thousands of aerosols that can travel even further ( 1). As a comparison, calculations predict that in still air, a 100-µm droplet will settle to the ground from 8 feet in 4.6 s, whereas a 1-µm aerosol particle will take 12.4 hours ( 4). ![]() However, when these studies were conducted, the technology did not exist for detecting submicron aerosols. These studies showed that large, ∼100 µm droplets produced in coughs and sneezes quickly underwent gravitational settling ( 1). Centers for Disease Control and Prevention (CDC) recommendations for social distancing of 6 feet and hand washing to reduce the spread of SARS-CoV-2 are based on studies of respiratory droplets carried out in the 1930s.
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