Aerosol Transmission of SARS-CoV-2: Physical Principles and Implications

Abstract

Evidence has emerged that SARS-CoV-2, the coronavirus that causes COVID-19, can be transmitted airborne in aerosol particles as well as in larger droplets or by surface deposits. This minireview outlines the underlying aerosol science, making links to aerosol research in other disciplines. SARS-CoV-2 is emitted in aerosol form during normal breathing by both asymptomatic and symptomatic people, remaining viable with a half-life of up to about an hour during which air movement can carry it considerable distances, although it simultaneously disperses. The proportion of the droplet size distribution within the aerosol range depends on the sites of origin within the respiratory tract and on whether the distribution is presented on a number or volume basis. Evaporation and fragmentation reduce the size of the droplets, whereas coalescence increases the mean droplet size. Aerosol particles containing SARS-CoV-2 can also coalesce with pollution particulates, and infection rates correlate with pollution. The operation of ventilation systems in public buildings and transportation can create infection hazards via aerosols, but provides opportunities for reducing the risk of transmission in ways as simple as switching from recirculated to outside air. There are also opportunities to inactivate SARS-CoV-2 in aerosol form with sunlight or UV lamps. The efficiency of masks for blocking aerosol transmission depends strongly on how well they fit. Research areas that urgently need further experimentation include the basis for variation in droplet size distribution and viral load, including droplets emitted by “superspreader” individuals; the evolution of droplet sizes after emission, their interaction with pollutant aerosols and their dispersal by turbulence, which gives a different basis for social distancing.

SARS-CoV-2 Survival on Surfaces and the Effect of UV-C Light

“…SARS-CoV-2 transmission via speech-generated respiratory droplets is responsible for most of the viral transmission and the environmental disinfection without ventilation improvement is not enough to reduce the viral spread in indoor spaces [29]. In fact, another important route of virus transmission is aerosolization during human speech or normal breathing [30]. In this setting, SARS-CoV-2 particles emitted in aerosols could remain viable for hours and be carried over longer distances.…”

UV-C irradiation is highly effective in inactivating SARS-CoV-2 replication

“…The COVID-19 pandemic caused by SARS-CoV-2 virus 1 has had an enormous, as yet barely understood, impact on health and economic outlook at the global level 2 . The identification of effective microbicide approaches is of paramount importance in order to limit further viral spread, as the virus can be transmitted via aerosol 3,4 and can survive for hours outside the body 5–7 . Non-contact disinfection technologies are highly desirable, and UV radiation, in particular UV-C (200 – 280 nm) has been suggested to be able to inactivate different viruses, including SARS-CoV 8–12 .…”

Effects of combined oxygen and surgical masks on inspired fraction of oxygen: relevance to COVID-19-induced respiratory failure

“…2 Severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2), the causative agent of COVID-19, can be transmitted by aerosol inhalation or contact with fomites. 3 The risk of aerosol transmission has been the largest concern among healthcare workers who care for patients with COVID-19 and acute hypoxaemic respiratory failure. The use of a surgical mask is recommended for patients receiving supportive oxygen therapy.…”