# Influence of Carbon Dioxide and pH on Influenza Virus in Sessile Saliva Droplets

## TL;DR

This study shows how CO2 and pH levels in saliva droplets affect the survival of the influenza virus, especially at high humidity.

## Contribution

The study reveals that pH changes due to CO2 levels have minimal impact on influenza virus inactivation in droplets.

## Key findings

- At 80% RH, low CO2 maintained greater virus infectivity compared to high CO2.
- pH changes were significant at 80% RH but not at lower RHs due to rapid evaporation.
- pH changes alone were insufficient to drive virus inactivation in any tested condition.

## Abstract

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 (<0.005% CO2 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.