The impact of respiratory droplet volume and their evaporation kinetics on environmental stability of respiratory viruses

By Neha MathurAug 2 2022Reviewed by Danielle Ellis, B.Sc.

In a recent study posted to the bioRxiv* preprint server, researchers measured the impact of initial respiratory droplet volume and relative humidity (RH) on the environmental stability of respiratory viruses, including influenza A and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Additionally, they examined a bacteriophage, Phi6, a  common surrogate for enveloped viruses. Other determinants of the environmental stability of these viruses are virion structure, droplet composition, fomite surface material, and temperature.

Background

During the coronavirus disease 2019 (COVID-19) pandemic, studies overestimated the transmission risk of SARS-CoV-2 on contaminated surfaces. They showed the significance of fomite transmission based on SARS-CoV-2 stability estimations in up to 50 µL droplets. However, they hardly explained virus decay in smaller, more physiologically relevant droplet volumes.

Droplet size typically determines the distance traveled by respiratory discharges and the host infection site. Smaller droplets or aerosols travel farther, and those smaller than 10 μm in diameter are more likely to deposit deep inside the respiratory tract. Past studies measuring virus stability in the environment created droplet volumes ranging from five to 50 μL, while discharged droplets from the respiratory tract are less than 0.5 µL. Therefore, these studies could not appropriately mimic a physiological volume of a droplet created by a respiratory expulsion.

About the study

In the present study, researchers measured the environmental stability of the H1N1 strain of influenza A virus and Phi6 in 50, five, and one µL droplets at 40%, 65%, and 85% RH in a humidity-controlled chamber. Additionally, the researchers explored droplet evaporation rates for which they used a micro-balance to measure droplet mass every 10 minutes for up to 24 hours and performed all the evaporation experiments in duplicate.

For experiments estimating SARS-CoV-2 stability, the team used an airtight desiccator at room temperature and 55% RH. For Phi6 and H1N1 virus, they first pipetted droplets onto six-well polystyrene tissue culture-coated plates. Next, they resuspended droplets at seven-time points – zero minutes, 20 minutes, 40 minutes, one hour, four hours, eight hours, and 24 hours. Lastly, the team investigated the effect of droplet morphology and drying pattern at 24 hours on varying droplet volumes.

Study findings

The authors observed that the droplet drying pattern at 24 hours depended on RH but not initial droplet volume, so any differences in viral decay by initial droplet size were not due to final physicochemical differences. At all RHs (40%, 65%, 85%), the droplets lost mass linearly over time before plateauing, referred to as a quasi-equilibrium stage. The researchers defined the period before and after the quasi-equilibrium stage as the wet and the dry phase, respectively. The decay of enveloped viruses was likely dependent upon complex interactions of media components with the viral glycoprotein and its changes during and after drying.

Evaporation was faster for smaller droplets and at lower RH. The time to reach quasi-equilibrium at 40% and 85% RH ranged from 0.5 to 11 hours for one µL and 50 µL droplets, respectively. The study data indicated that initial droplet volume changed drying kinetics, which affected virus stability. SARS- CoV-2 and H1N1 virus decayed similarly at 65% RH (intermediate RH), and the differences were only evident in larger droplets.

Further, for all droplet sizes tested, while a droplet was wet and evaporation was still occurring, the viruses were subject to a faster decay rate than after they reached the quasi-equilibrium. A previous preprint showed that biphasic virus decay likely occurs in aerosols too. Therefore, the first phase of viral decay was significant for transmission at close range, while both phases seemed important for viral transmission at a farther range. The first viral decay phase occurred within seconds, and further decay occurred at the quasi-equilibrium stage.

Conclusions

The study highlighted the importance of using physiologically relevant media and careful use of surrogates for a precise assessment of the transmission risk of future emerging pathogens. The study results showed that RH had a greater impact on viral decay in 50 µL droplets than in one µL droplets. Further, viral decay rates during the wet phase were greater than or similar to dry phase decay rates, irrespective of droplet size and RH. The differences in virus decay were more common in 50 µL droplets than in one µL droplets and at low RH.

The study findings questioned prior studies estimating viral stability employing large droplet volumes. According to the authors, the results of those studies would have differed had they used smaller droplet volumes, especially over shorter periods. For 24 hours, viral decay was similar across all three droplet volumes. Physical and chemical properties of the droplets, initial volume, and ambient humidity were likely causing their evaporation at different rates and resulted in these differences.

The study findings also cautioned against extrapolating survival times from surrogates to other viruses and strain selection. In the current study, Phi6 decayed quicker than H1N1 virus and SARS-CoV-2 under experimental conditions; thus, relying on only Phi6 data could lead to potentially wrong conclusions about pathogenic viruses. In fact, H1N1 decayed more like SARS-CoV-2 and could serve as its surrogate while extrapolating its persistence in more physiologically relevant conditions.

Future studies should focus on creating real-world conditions for respiratory droplet volume (ranging from sub-micron to hundreds of microns in diameter) and chemical composition of respiratory fluid to improve public policy on optimal SARS-CoV-2 transmission mitigation strategies.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources