Coughing, talking, and breathing produce turbulent gas clouds with large and small droplets.  Those larger than 5 micrometers usually fall in seconds, within 2 meters.  But smaller droplets, called airborne aerosols, can float for hours, dry with viruses in them, and dissipate many meters, even into other rooms (Bourouiba; Brosseau; Stadnytskyi et al.).  When inhaled, they can float past the cilia and mucous membranes, and go into the lungs and infect the alveoli air sacs (Meselson). Scientists found SARS-CoV-2 in air in many samples near infected people.  In a Nebraska hospital, 63% of air samples in COVID-19 patients’ rooms had SARS-CoV-2 RNA, and 67% of air samples in hallways outside their rooms, even near patients who were not coughing.  But viral concentrations were so low that viable viruses could not be cultured, showing that it may or may not be possible to get infected from inhaling the air (Kimball et al.).

Initial reports from China indicated that droplets and surfaces were transmitting these viruses (World Health Organization “Modes…”).  Later, in Wuhan hospitals scientists found SARS-CoV-2 RNA in air samples in many rooms, with different kinds of patients, especially those having little ventilation (Guo et al.).  The rooms where medical staff changed into and out of personal protective equipment (PPE) clothing had the virus RNA in the air.  Maybe just moving the clothes and PPE around shook viruses into the air.  Scientists found the RNA in the air of bathrooms, which patients used only a few minutes at a time.  Perhaps the patients coughed the viruses into the air, and closed the bathroom door, leaving the tiny aerosol droplets to float for hours.  Scientists also found the RNA in the air outside a store and in a crowded area, where outpatients and others went, some of whom could be asymptomatic carriers (Liu et al.).  But the air in critical care unit (CCU), intensive care unit (ICU), ward room, and workstations had no or low concentrations of SARS-CoV-2 RNA, possibly because they exchanged air at high rates or were using negative pressure ventilation (Liu et al.; Ong et al.).

Other scientists conducted a lab experiment in which they nebulized SARS-CoV-2, took samples from the air three hours later, put these into living cells, and were able to culture viable viruses. These studies proved that airborne transmission is possible, but did not prove that it actually occurred (Brosseau; National Institutes of Allergies and Infectious Diseases; van Doremalen et al.).

Some experiments and epidemiology studies in different environments showed that SARS-CoV-2 lasted longer in cooler, drier air, and away from sunlight, but this might affect transmission less than non-environmental factors.  So, warm, sunny weather might decrease transmission a little (Department of Homeland Security; National Academies of Science “SARS-CoV-2 survival…”).