Keeping indoor humidity in the “sweet spot” could reduce the spread of COVID-19


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We know that proper indoor ventilation is key to reducing the spread of COVID-19. Now a study by MIT researchers shows that the relative humidity indoors could also affect the transmission of the virus.

Relative humidity is the amount of moisture in the air compared to the total moisture that the air can hold at a given temperature before it saturates and forms condensation.

In a study published today in Journal of the Royal Society InterfaceThe MIT team reports that maintaining an indoor relative humidity between 40 and 60% is associated with a relatively lower rate of COVID-19 infections and deaths, while indoor conditions outside this range are associated with worse COVID-19 outcomes. To put this into perspective, most people are comfortable between 30 and 50% humidity, and an airplane cabin is around 20% humidity.

The findings are based on the team’s analysis of COVID-19 data along with weather observations from 121 countries, from January 2020 to August 2020. Their study suggests a strong link between regional outbreaks and indoor humidity.

In general, the researchers found that whenever an area experienced an increase in pre-vaccination COVID-19 cases and deaths, the estimated indoor humidity in that area was on average either lower than 40% or higher than 60% regardless of season. Almost all areas of the study experienced fewer COVID-19 cases and deaths during periods when the estimated indoor humidity was within the “sweet spot” between 40 and 60%.

“There is a potential protective effect of this intermediate humidity indoors,” suggests lead author Connor Verheyen, Ph.D. student in medical engineering and medical physics in the Harvard-MIT program in health sciences and technology.

“Indoor ventilation is still important,” says co-author Lydia Bourouiba, director of the MIT Fluid Dynamics of Disease Transmission Laboratory and associate professor in the Departments of Civil and Environmental Engineering and Mechanical Engineering, and at the Institute for Medical Engineering and Science at MY “However, we found that maintaining indoor humidity in that sweet spot – 40 to 60% – is associated with lower COVID-19 cases and deaths.”

Seasonal fluctuations?

Since the beginning of the COVID-19 pandemic, scientists have considered the possibility that the virulence of the virus fluctuates with the seasons. Infections and associated deaths appear to increase in winter and decrease in summer. But studies seeking to link the virus’s patterns​​​​​​with outdoor seasonal conditions have had mixed results.

Verheyen and Bourouiba looked at whether COVID-19 was instead affected by indoor rather than outdoor conditions, and in particular, relative humidity. After all, they note that most communities spend more than 90% of their time indoors, where the majority of viral transmission has been shown to occur. What’s more, indoor conditions can be quite different from outdoors due to climate control systems, such as radiators, which significantly dehumidify indoor air.

Could indoor relative humidity affect the worldwide spread and severity of COVID-19? And could it help explain regional differences in health outcomes?

Track humidity

To get answers, the team focused on the early periods of the pandemic when vaccines were not yet available, arguing that a vaccinated population would mask the effects of any other factor such as indoor humidity. They collected global COVID-19 data, including the number of cases and reported deaths, from January 2020 to August 2020, and identified countries with at least 50 deaths, indicating that at least one outbreak occurred in those countries .

In total, they targeted 121 countries where there was an outbreak of COVID-19. For each country, they also tracked local regulations related to COVID-19, such as isolation, quarantine, and testing measures, and their statistical association with COVID-19 outcomes.

For each day for which COVID-19 data was available, they used meteorological data to calculate outdoor relative humidity. They then estimated average indoor humidity based on outdoor relative humidity and human comfort temperature guidelines. For example, guidelines say that humans feel comfortable between 66 and 77 degrees Fahrenheit indoors. They also assumed that, on average, most residents had the means to heat indoor spaces to a comfortable temperature. Finally, they also collected experimental data that they used to validate their estimation method.

For each case when the outdoor temperature was below the typical human comfort range, they assumed that indoor spaces were heated to reach that comfort range. Based on the increased heating, they calculated an associated decrease in indoor relative humidity.

During the warmer periods, the relative humidity both outside and inside each country was about the same, but during the colder periods it quickly returned. While outdoor humidity remained around 50% throughout the year, indoor relative humidity for Northern and Southern Hemisphere countries dropped below 40% during colder periods, when COVID-19 cases and deaths also increased in these regions.

For countries in the tropics, relative humidity was about the same indoors and outdoors throughout the year, with a gradual increase indoors during the region’s summer season, when high outdoor humidity probably raised indoor relative humidity above 60%. They felt this increase reflected a gradual increase in COVID-19 deaths in the tropics.

“We saw more reported deaths from COVID-19 at the low and high ends of indoor humidity and less at that sweet spot of 40 to 60 percent,” says Verheyen. “This intermediate relative humidity window is associated with a better outcome, meaning fewer deaths and slowing the pandemic.”

“We were very skeptical at the beginning, especially since the COVID-19 data can be noisy and inconsistent,” says Bourouiba. “So we were very thorough in trying to poke holes in our own analysis and used a variety of methods to test the limits and robustness of the findings, including taking into account factors such as government intervention. Despite our best efforts, we found that even When considering countries with very strong and very weak mitigation policies against COVID-19, or very different outdoor conditions, indoor relative humidity—rather than outdoor—holds the underlying strong and robust association with COVID-19 outcomes.

It remains unclear how indoor relative humidity affects the outcome of COVID-19. The team’s follow-up studies indicate that pathogens can survive longer in respiratory droplets in both very dry and very humid conditions.

“Our ongoing work shows that there is emerging evidence for a mechanistic relationship between these factors,” says Bourouiba. “For now, however, we can say that indoor relative humidity is powerfully emerging as another countermeasure that organizations and individuals can monitor, adjust, and maintain in the optimal 40 to 60% range, in addition to proper ventilation.”

More information:
The relationship between indoor relative humidity and the global consequences of COVID-19, Journal of the Royal Society Interface (2022). DOI: 10.1098/rsif.2021.0865. … .1098/rsif.2021.0865

Provided by the Massachusetts Institute of Technology

This story is republished with permission from MIT News (, a popular site that covers news about MIT research, innovation, and teaching.

Quotation: Keeping Indoor Humidity in ‘Sweet Spot’ Could Reduce Spread of COVID-19 (2022, November 15) Retrieved November 15, 2022 from covid-.html

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