Evaporation and hygroscopic growth
High level of certainty:
- Aerosols (typically defined as respirable with <10 Mµm diameter) and droplets (as large as >100 Mµm) are generated under physiological conditions of high relative humidity (often assumed to be >99 % RH) and temperature (37°C) and, thus, are dominated by their water content. The majority (~99%) are of respirable size.
- Sneezes generate ~104 droplets at, typically, 20 m/s, coughs ~10²-10³ at ~10 m/s. Speaking can generate 50 particles/s at ~5 m/s.
- Once generated by an expiratory event into an environment at, typically, lower RH and temperature, the droplets and aerosols lose moisture moving towards a state of equilibrium water content. This decrease in size occurs over a timescale <1 s to >10 s depending on size, RH and temperature.
- The evaporation of the droplets and aerosols is coupled with the forward momentum of the jet and with sedimentation. The interplay of these, governs the transmission distance of the aerosols/droplets and the fraction that remain airborne. A 100 Mµm droplet sediments 1 m in 3 s, a 1 mm particle takes 8 hours.
Low level of certainty:
- The hygroscopic response (moisture content with variation in RH) and evaporation kinetics of droplets/aerosols created from expiratory events, e.g. saliva, deep lung fluid etc. Most models assume the aerosols/droplets behave as pure water or salt solution.
- The impact of phase behavior (e.g. crystallization) and the slow release of water from viscous partially dried particles on aerodynamic size, sedimentation rate and if the virus remains infectious.
- The size distributions and compositions of aerosols/droplets from individuals with COVID 19 and differences to a healthy individual.
- The impact of other airborne pollutants (e.g. (semi-)volatile organics, oxidants) on the airborne particle properties.
The statements above are intended to be reviewed regularly as more information and new references emerge. If you have queries about the content of the above and wish to discuss these please contact the editors.