The virus that causes COVID-19 spreads most easily when the indoor relative humidity falls outside a “sweet spot” between 40 and 60%, say researchers at the Massachusetts Institute of Technology (MIT) in the US. This finding, which is based on a comparison of meteorological data and population-level COVID-19 statistics, suggests that indoor humidity plays a significant role in the spread of COVID-19, and should be considered alongside ventilation and other measures to reduce disease transmission.
Like other respiratory viruses, the SARS-CoV-2 virus responsible for the COVID-19 pandemic is transmitted via virus-laden droplets and aerosols produced when infected people breathe, talk, sneeze, cough or sing. Some of these microscopic particles remain suspended in the air for minutes or even hours, during which time other people may inhale them and become infected themselves. However, the exact length of time these viral particles linger – and how easily they infect new hosts – depends on a complex mix of environmental and biological factors.
Early in the pandemic, many infectious disease specialists predicted that COVID-19 might eventually become a seasonal virus, similar to flu and a clutch of older coronaviruses that usually cause mild, cold-like illness. However, many regions of the world have experienced major outbreaks even in summer months, and efforts to tease out links between COVID-19 rates and weather variables such as temperature, humidity and solar radiation have produced inconclusive and sometimes contradictory results.
Humans are indoor creatures
In the latest study, Connor Verheyen and Lydia Bourouiba of the Harvard-MIT Division of Health Sciences and Technology took a different approach. Rather than focusing on what the weather was like outside, they concentrated on conditions indoors, where most people spend much of their time, and where most respiratory disease transmission takes place.
More specifically, the pair looked for a connection between COVID-19 rates and indoor relative humidity (RH). “Compared to other variables in the problem of respiratory cloud emissions that are laden with pathogen-bearing droplets, it’s the relative humidity that really governs the physics,” explains Bourouiba, who leads an MIT laboratory dedicated to studying the fluid dynamics of respiratory disease transmission.
To explore the possible relationship between virus transmission and indoor RH, Verheyen and Bourouiba compared meteorological data from 121 countries (67 in the northern hemisphere, 50 at tropical latitudes and four in the southern hemisphere) with records of COVID deaths from the same countries between the start of the pandemic and August 2020. By concentrating on deaths rather than cases, and on the period before vaccines were manufactured, they reduced the chances of clouding the picture with unrelated factors such as limited testing capacity and local vaccine availability.
The researchers found that outdoor RH was fairly consistent across all three regions during the study period. However, in tropical countries, the indoor RH (which they calculated based on outdoor temperature, dewpoint and an assumption that indoor temperatures would mostly fall within a comfortable 19–25 °C) rose from intermediate to high (>60%) between March and August 2020. Northern and southern hemisphere countries, meanwhile, experienced sharp dips in indoor RH during their respective winters – a phenomenon that will not surprise anyone who spends the colder months applying lotion to their dry, cracked skin.
A sweet spot for reduced viral impact
When Verheyen and Bourouiba superimposed these indoor RH trends on graphs of new COVID deaths in each region, they observed a clear association between worse COVID outcomes and indoor RH that was either very low (<40%) or very high (>60%). According to Bourouiba, this association “continued to emerge despite us trying to poke holes in every hypothesis we made and every part of the analyses” by controlling for other factors, including weather parameters such as temperature, outdoor RH or absolute humidity and public health interventions aimed at limiting coronavirus spread. The association also endured after they collected new data aimed at confirming the limits of validity of their central assumptions.
As for why COVID spreads more easily at high and low indoor RH, and less easily in between, Bourouiba says that U-shaped trends like this typically appear when physical mechanisms compete. “Essentially, you have alignment of mechanisms in one regime and misalignment in another,” she explains.
Commenting on the research on Twitter, Linsey Marr, an environmental engineer at Virginia Tech, US, who also studies airborne virus transmission but was not involved in this work, said that while aerosol particles stay aloft more at relative humidities below 40%, the virus may also maintain its infectivity longer in such conditions. “40-60% is considered the sweet spot for more rapid virus decay, but there’s still a lot we don’t understand about this relationship,” Marr observed.
Ventilation remains vital
Asked about the research’s implications for public health, Bourouiba notes that this “sweet spot” of 40–60% humidity is “quite aligned with building management codes” and humidity sensors are relatively cheap. However, she cautions that controlling humidity is not a silver bullet: “If the humidity is controlled, but the air is not cleared at high enough frequency (that is, there are few air changes per hour), then of course the effect of humidity can become secondary.”
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Under those circumstances, Bourouiba says that viral load in the air may remain high despite the mitigating effect of humidity, meaning that a person’s cumulative exposure would still lead to an increased chance of transmission. A better solution, she says, would be to combine humidity controls with improvements to ventilation. “Hopefully, the pandemic has taught us that retrofitting current designs with comparatively low-cost means and incorporating these measures into new designs and upgrades is a worthwhile investment,” she says. “The cost is minimal compared to the cost of entire societies coming to full lockdowns due, in part, to a lack of preventive measures planned or implemented in time.”
The research is described in J Roy Soc Interface.
