Upon exhalation, virus-laden respiratory droplets experience rapid changes in environmental conditions that lead to chemical and physical alterations that can affect virus infectivity. By manipulating the concentration of gaseous carbon dioxide (CO2) surrounding sessile saliva droplets, we altered their chemistry and then assessed the impacts of these changes on the infectivity of influenza A virus at relative humidities of 30, 50, and 80%. For virus exposed to low CO2 (2 in N2) vs high CO2 (4.3-5% CO2 in N2), differences in inactivation were small except at 80% RH, where the virus decayed less (i.e., maintained greater infectivity) in low CO2 than in high CO2. The difference exceeded 1log10 at 2 h. For comparison, virus inactivation in ambient air (0.04% CO2) varied across conditions, sometimes exceeding and sometimes falling below that observed under high- and low-CO2 atmospheres. Collectively, these results suggest that the driving factors for virus inactivation vary with RH. We measured droplet pH using gold nanoprobes in combination with surface-enhanced Raman spectroscopy and found that pH increased in low CO2 and decreased in high CO2 at 80% RH by ∼1 pH unit in both cases. Results were consistent with chemical equilibrium modeling, which indicated that both carbonate and phosphate buffering were important. Changes in pH were smaller or insignificant at 30 and 55% RH. At these low and medium RHs, rapid evaporation of water from the droplets and the resulting increase in viscosity may limit changes in pH. Measured changes in pH did not appear to be sufficient to drive virus inactivation under any tested condition. This finding suggests that pH likely does not impact influenza transmission by fomites.
Longest et al. (Thu,) studied this question.