Environmental temperature and relative humidity shape post-emission aerosol fate and airborne influenza transmission

Airborne transmission is a major route of influenza virus spread, yet how environmental conditions shape the persistence and downrange transport of infectious exhaled virions is not fully understood. Using a physiologically relevant swine model infected with A/California/04/2009 (H1N1), we investigated how temperature and relative humidity (T/RH) influence airborne influenza emission, persistence, and transmission under two environmental conditions: 20°C/50% RH (ambient indoor), and 7°C/73% RH (cold/high-humidity). Donor pigs shed comparable nasal viral loads across conditions, but naïve sentinel pigs housed 4 m away became infected 1 day earlier under ambient conditions. Breath and environmental air sampling showed that cold/high-humidity conditions transiently increased viral RNA in exhaled aerosols at 1 day post-infection (dpi), whereas ambient conditions supported greater and more persistent airborne viral burdens at 2-3 dpi, particularly at downrange locations. Controlled aerosol generation experiments further showed that ambient conditions enabled substantially greater recovery of infectious virus with distance, even though RNA-containing particles were transported under both T/RH states. Together, these results demonstrate that, under the tested environmental conditions, infectious influenza aerosols persisted longer and transmitted farther under the ambient indoor environment than in the cold/high-humidity environment. These findings establish that environmental temperature-humidity conditions shape post-emission aerosol fate, and thereby constrain the airborne transmission range of the influenza virus. IMPORTANCE: Influenza viruses spread efficiently through the air, yet the environmental conditions that determine whether exhaled virions remain infectious long enough to initiate new infections remain poorly defined. Using a swine model that closely replicates human expiratory aerosol output, we identify environmental temperature-humidity conditions as a critical determinant of airborne infectious range. Cold/high-humidity conditions increased early viral RNA levels near the host but failed to sustain infectious particles at a distance. In contrast, ambient conditions supported prolonged airborne suspension and rapid transmission to distant recipients. Controlled aerosolization experiments showed that infectious virus is transported far more effectively under ambient indoor conditions than in cold/high-humidity air despite similar RNA dispersal. These results reveal post-emission aerosol fate as the critical bottleneck in determining airborne influenza transmissibility. This mechanistic insight is essential for refining predictive models of influenza spread and developing environmental and public health strategies that more effectively limit airborne infection.