  The effect of evaporation on droplet sedimentation times is crucial for
estimating the risk of infection from virus-containing airborne droplets. For
droplet radii in the range 100 nm < R < 60 {\mu}m, evaporation can be described
in the stagnant-flow approximation and is diffusion limited. Analytical
equations are presented for the droplet evaporation rate, the time-dependent
droplet size and the sedimentation time, including the significant effect of
evaporation cooling. Evaporation makes the time for large droplets to sediment
much longer and thus significantly increases the viral air load. Using recent
estimates for SARS-CoV-2 concentrations in sputum and droplet production rates
while speaking, a single infected person that constantly speaks without a mouth
cover produces a total air load of more than 10^4 virions. In a mid-size closed
room, this leads to a viral inhalation frequency of at least 2.5 per minute.
Low relative humidity, as encountered inside buildings in winter and in
airliners, speeds evaporation and thus keeps initially larger droplets
suspended in air.
