Since the beginning of the Covid-19 pandemic, scientists have wondered whether the virus’ infectiousness and virulence may change with the seasons, as it happens in the case of influenza and other respiratory viruses. However, studies attempting to link the virus’ patterns to seasonal outdoor conditions have led to mixed results.
Now, starting with the hypothesis that most societies spend over 90 percent of their time indoors, where the majority of viral transmission occurs, a team of researchers from the Massachusetts Institute of Technology (MIT) has investigated whether Covid-19 is more influenced by indoor rather than outdoor conditions, and, specifically, by indoor relative humidity – defined as the amount of moisture in the air compared to the total moisture the air can hold at a given temperature before saturating or forming condensation.
For their analysis, the researchers focused on the early period of the pandemic (January to August 2020) – reasoning that vaccinated populations would obscure the influence on other factors such as indoor relative humidity – and collected data from 121 countries where Covid outbreaks with at least 50 deaths occurred. For each day, they used meteorological data to compute the country’s outdoor relative humidity and estimate its average indoor relative humidity, based on guidelines on temperature ranges for human comfort.
The analysis revealed that maintaining an indoor relative humidity between 40 and 60 percent was associated with relatively low rates of Covid-19 infections and deaths, while indoor conditions outside this range – thus, either too dry or too humid – were associated with worse Covid outcomes.
“Indoor ventilation is still critical,” said study co-author Lydia Bourouiba, the director of the MIT Fluid Dynamics of Disease Transmission Laboratory “However, we find that maintaining an indoor relative humidity in that sweet spot — of 40 to 60 percent — is associated with reduced Covid-19 cases and deaths.”
“We were very skeptical initially, especially as the Covid-19 data can be noisy and inconsistent. We thus were very thorough trying to poke holes in our own analysis, using a range of approaches to test the limits and robustness of the findings, including taking into account factors such as government intervention. Despite all our best efforts, we found that even when considering countries with very strong versus very weak Covid-19 mitigation policies, or wildly different outdoor conditions, indoor — rather than outdoor — relative humidity maintains an underlying strong and robust link with Covid-19 outcomes.”
While it is still unclear how indoor relative humidity affects the transmissibility and virulence of the new coronavirus, some of the team’s follow-up studies suggest that pathogens may survive longer in respiratory droplets or aerosols in both very dry and very humid conditions.
“Our ongoing work shows that there are emerging hints of mechanistic links between these factors. For now however, we can say that indoor relative humidity emerges in a robust manner as another mitigation lever that organizations and individuals can monitor, adjust, and maintain in the optimal 40 to 60 percent range, in addition to proper ventilation,” Bourouiba concluded.
The study is published in the Journal of the Royal Society Interface.
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