Reducing transmission of SARS-CoV-2
**!!!!*** "Airborne spread from undiagnosed infections will continuously undermine the effectiveness of even the most vigorous testing, tracing, and social distancing programs." "Aerosols can accumulate, remain infectious in indoor air for hours, and be easily inhaled deep into the lungs.
A competition between droplet size, inertia, gravity, and evaporation determines how far emitted droplets and aerosols will travel in air. Larger respiratory droplets will undergo gravitational settling faster than they evaporate, contaminating surfaces and leading to contact transmission. Smaller droplets and aerosols will evaporate faster than they can settle, are buoyant, and thus can be affected by air currents, which can transport them over longer distances.
...Respiratory droplet size has been shown to affect the severity of disease. For example, influenza virus is more commonly contained in aerosols with sizes below 1 µm (submicron), which lead to more severe infection. In the case of (SARS-CoV-2), it is possible that submicron virus-containing aerosols are being transferred deep into the alveolar region of the lungs, where immune responses seem to be temporarily bypassed. SARS-CoV-2 has been shown to replicate three times faster than SARS-CoV-1 and thus can rapidly spread to the pharynx, from which it can be shed before the innate immune response becomes activated and produces symptoms. By the time symptoms occur, the patient has transmitted the virus without knowing....
The US CDC recommendations... are based on studies of respiratory droplets carried out in the 1930s.
In outdoor environments, numerous factors will determine the concentrations and distance traveled, and whether respiratory viruses remain infectious in aerosols.
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 airflow will all modulate viral transmission pathways and exposure. Universal masking is highly important."