Table 1.

Four Critical Questions on Airborne Transmission and What We Know

Critical Question	What We Know
CQ1. What size particles are generated by people, and how do they spread in air?	• Individuals generate aerosols and droplets across a large range of sizes and concentrations. There is great variation among individuals and across activities. • Respiratory plumes, containing a continuum from small aerosols to large droplets, are an important determinant of exposure at short-range distances, up to about 1.5 m. • At longer-range distances (>1.5 m), smaller aerosols that can remain airborne for longer time periods dominate exposure.
CQ2. Which size particles are infectious and for how long?	• Humans infected with SARS-CoV-2 produce particles smaller than 5 μm that contain virus. • Viral half-life in aerosols is approximately 1 hour, but viruses can persist much longer.
CQ3. What behavioral and environmental factors determine personal exposure to SARS-CoV-2?	• Masks (face coverings) reduce the amount of virus emitted in aerosols and droplets and reduce the wearer’s exposure to them. • Masks (face coverings) reduce community transmission. • Ventilation can reduce room-based exposure and filtration is an effective supplement to ventilation for reducing aerosol concentrations indoors.
CQ4. What do we know about the infectious dose and disease relationship for COVID-19?	• Human and animal studies on different coronaviruses have demonstrated that viral infectivity is dependent on host and environmental factors. • The role of infectious dose remains to be characterized while individual characteristics are important determinants of SARS-CoV-2 disease severity.

Abbreviations: COVID-19, coronavirus disease 2019; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.