Relative humidity
An association between RH and virus transmission has been reported. For example, the incidence of influenza A in Hong Kong increased with higher RH, and the number of positive test results for influenza A was negatively correlated with RH in Singapore [68, 69]. Results from these epidemiological studies suggest that RH affects virus transmission. Furthermore, studies have demonstrated the effect of RH on the survival of human viruses (e.g., SARS-CoV-2 [70, 71]), underlining the importance of understanding the effect of RH on virus transmission.
RH has large impacts on the viability of viruses in droplets, larger than the effect of chemical composition in some cases. We observed U-shaped patterns in the viability of MS2 against RH, and monotonically decreasing relationships between the viability of Φ6 and RH, respectively, in droplets of different compositions. We reported previously that the viability of MS2 and Φ6 in droplets composed of culture medium follows U-shaped patterns, in which the lowest viability occurs at 55% and 85% RH, respectively [13]. Many other studies have reported a similar pattern with greater decay at intermediate RHs than at lower or higher RHs [17, 18, 25, 27, 72]. The viability patterns observed in this study for Φ6, decreasing with RH rather than U-shaped, seem to conflict with results in the literature. However, we examined the viability of Φ6 between 20% and 80% RH in the current study. Over this range, the viability of viruses also decreased monotonically in previous studies; we have shown that the minimum viability of Φ6 in droplets occurs around 85% RH, beyond the range examined in the present study [13].
As we concluded in our prior study [13], RH affects the viruses’ viability mainly by controlling droplet evaporation kinetics, inducing changes in solute concentrations and the cumulative dose of harmful compounds to which viruses are exposed. At intermediate RH, the cumulative dose is higher because the solute concentrations increase relatively quickly and are then maintained at a high level throughout the experiment. While our previous work focused on viruses in their prescribed culture medium, results of the present study indicate that their viability follows the same pattern in droplets consisting of culture medium diluted 100x in ultrapure water and lacking salt, protein, and surfactant. Components in LB medium that are potentially harmful for viruses, though diluted, can accumulate as droplets evaporate and eventually cause virus inactivation over time.
RH is the major factor that determines droplet evaporation kinetics, as shown in Fig 6. The initial evaporation rate was much higher at 20% RH than at 50% and 80%. At 20% and 50% RH, droplets fully desiccated in 1 h. At both conditions, the evaporation rates were relatively steady at the beginning (the first ~10 min and ~15 min for 20% and 50% RH, respectively), but later gradually decreased. However, at 80% RH the evaporation rate was more consistent throughout the experiment, and droplets did not fully evaporate within 1 h.
Besides ambient RH, droplet composition can affect evaporation rates as well. At certain RH conditions, the evaporation kinetics varied with chemical composition and initial solute concentration. Droplets containing BSA generally evaporated faster than droplets containing other components at 20% and 50% RH. Droplets containing 1 g/L NaCl evaporated faster than those containing 35 g/L NaCl. A previous study demonstrated that the evaporation rates of droplets containing less than 5.8 g/L NaCl was almost two times higher than for droplets containing 58 g/L NaCl at RH < 60% [73]. The authors concluded that Marangoni flows induced by surface tension gradients, which originated from local peripheral salt enrichment, caused the difference in evaporation rate. We observed that droplets with higher initial SDS concentration evaporated slower than those with lower initial SDS concentration at RHs of 20% and 50%. However, the result was completely the opposite at 80% RH, at which droplets containing 10 μg/mL SDS evaporated significantly faster than those containing 1 μg/mL SDS. Droplets containing different initial BSA concentrations and at different initial pH values had similar evaporation rates across all RH levels. Since the evaporation kinetics determine the change in solute concentrations and cumulative dose, it is necessary to understand the influence of droplet chemical composition and concentrations on the evaporation rates of virus-containing droplets.