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Using thermodynamic and kinetic data for respiratory fluids as input to the biophysical respiratory aerosol model ResAM (Luo et al., 2023; 2025), we show that exhaled particles rapidly desorb H$_2$O and CO$_2$, becoming alkaline, and then absorb gaseous nitric acid (HNO$_3$) and other acidic substances from the ambient air, becoming acidic. The rate of these processes depends primarily on particle size. Large droplets (> 50 µm) remain alkaline for days due to the loss of bicarbonate, whereas submicron particles turn acidic in less than 1 minute at typical concentrations of gaseous acids and bases in indoor air (Nazaroff and Weschler, 2020), with alkaline ammonia playing only a secondary role. Subsequently, we use virus inactivation measurements in bulk solutions of synthetic lung fluid acidified to different pH values (from acidic pH2 to alkaline pH11) to calculate the inactivation times of Influenza A Virus (IAV) in exhaled particles of different sizes (Klein et al., 2022). The results compare well with IAV inactivation measurements in an aerosol chamber (Motos et al., 2024; 2025) at different relative humidities (RH) and HNO$_3$ concentrations, provided that certain assumptions are made about the diffusivities of molecules and ions in the liquid phase. By weighting the results with size distributions characteristic of aerosol in exhaled breath (Pöhlker et al., 2023), changes in relative transmission risk as a function of RH and pH can be estimated and applied to typical indoor conditions in single-family homes and public spaces, such as schools, libraries, and museums, where acidic gases could be partially removed by air filtration. In doing so, the distribution of viruses in particles of different sizes with a preference for small particles (< 5 µm) must be factored in (Milton et al., 2013; Yan et al., 2018). Our results suggest that achieving indoor concentrations of gaseous acids at levels found outdoors leads to rapid inactivation of IAV. Conversely, indoor concentrations, as we typically encounter them in winter, delay inactivation. This effect is enhanced by the seasonality in RH, which can slow or even inhibit the uptake of acidic gases under dry winter conditions. Therefore, indoor air acidity and humidity are important factors in understanding seasonal influenza. We discuss the implications of these results for risk assessment of airborne transmission and possible public health measures.
References:
Klein et al., Expiratory aerosol pH is determined by indoor room trace gases and particle size, PNAS, 119 (39), DOI10.1073/pnas.2212140119, 2022.
Luo et al., Expiratory Aerosol pH: The Overlooked Driver of Airborne Virus Inactivation, Environ. Sci. Technol., 57 (1), 486-497, DOI10.1021/acs.est.2c05777, 2023.
Luo et al., Biophysical Respiratory Aerosol Model (ResAM): composition and pH of exhaled aerosol and application of airborne virus inactivation, contribution to this conference, 2025.
Milton et al., Influenza virus aerosols in human exhaled breath: particle size, culturability, and effect of surgical masks, PLoS Pathog. 9, DOI10.1371/journal.ppat.1003205, 2013.
Motos et al., Dependence of aerosol-borne influenza A virus infectivity on relative humidity and aerosol composition, Frontiers Microbiol., 15, DOI10.3389/fmicb.2024.1484992, 2024.
Motos et al., Infectivity Of Airborne Influenza A virus: The Effect Of Matrix And Air Composition, contribution to this conference, 2025.
Pöhlker et al., Respiratory aerosols and droplets in the transmission of infectious diseases, Revs. Mod. Phys., 95, DOI: 10.1103/RevModPhys.95.045001, 2023.
Nazaroff and Weschler, Indoor acids and bases, Indoor Air, 30, 559–644, DOI: 10.1111/ina.12670, 2020.
Yan et al., Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community, PNAS, 115 (5), DOI10.1073/pnas.1716561115, 2018.
