Quanta emission rate during speaking and coughing mediated by indoor temperature and humidity

In epidemiological prospective modelling, assessing the hypothetical infectious quanta emission rate (E_q) is critical for estimating airborne infection risk. Existing E_q models overlook environmental factors such as indoor relative humidity (RH) and temperature (T), despite their importance to droplet evaporation dynamics. Here we include these environmental factors in a prospective E_q model based on the airborne probability functions with emitted droplet distribution for speaking and coughing activities. Our results show relative humidity and temperature have substantial influence on E_q. Drier environments exhibit a notable increase in suspended droplets (cf. moist environments), with E_q having a 10-fold increase when RH decreases from 90 % to 20 % for coughing and a 2-fold increase for speaking at a representative summer indoor environment (T = 25° C). In warmer environments, E_q values are consistently higher (cf. colder), with increases of up to 22 % for coughing and 9 % for speaking. This indicates temperature has a smaller impact than humidity. We demonstrate that indoor environmental conditions are important when quantifying the quanta emission rate using a prospective method. This is essential for assessing airborne infection risk.