Meteorological factors and domestic new cases of coronavirus disease (COVID-19) in nine Asian cities: A time-series analysis

Zonglin He; Yiqiao Chin; Jian Huang; Yi He; Babatunde O. Akinwunmi; Shinning Yu; Casper J.P. Zhang; Wai-kit Ming

Humidity can influence aerosol transmission via altering the proportion of respiratory droplets undergoing aerosolization, and influencing the stability and viability of the virus within these aerosols. Respiratory droplets are generated in the high humidity of the respiratory tract. On entering an environment with low humidity, respiratory droplets reduce in size within seconds due to evaporation. At higher environmental humidity, respiratory droplets evaporate more slowly, and hence are larger and settle faster, and less aerosol nuclei are produced (39, 40). Previous studies have shown that influenza transmission between mice reduced as relative humidity increased from 47% to 70% (13, 41) . Moreover, humidity can also influence indirect transmission by changing the mass of respiratory droplets accumulating on surfaces, and affecting the survival of the virus on surfaces. While increased humidity reduces the number of droplet nuclei formed, the same mechanisms (reduced droplet evaporation and faster droplet settling) result in a greater mass of respiratory droplets on surfaces (39, 40). Area with relatively low temperature and humidity has a higher infection rate comparing with tropical areas because the cold and dry weather is suitable for virus to survive and transmit (42). Viability of influenza virus appears greater at lower humidity, and progressively reduced influenza survival with increasing relative humidity over the range from 27% to 84%, with an increase in survival at 99% relative humidity, the mechanisms underlying which may be due to the low evaporation from droplets at high relative humidity maintain the solute concentration thus protecting the virus (27, 43) . Alongside, 30-50 o N zone has become a zone for transmitting COVID-19 with similar average temperature of 5-11 ℃ and 47-79% humidity, which may be influenced by transoceanic migration of virus, but the underlying mechanism is still not understood (44).