We know that good indoor ventilation is essential to reduce the spread of Covid-19. Now, a study by MIT researchers reveals that indoor relative humidity can also influence virus transmission.
Relative humidity is the amount of moisture in the air compared to the total humidity the air can hold at a given temperature before it saturates and forms condensation.
In a study published today in the Royal Society Interface Journal, the MIT team reports that maintaining indoor relative humidity between 40 and 60% is associated with relatively lower rates of Covid-19 infections and deaths, while indoor conditions outside this range are associated with worse Covid-19 outcomes. To put that into perspective, most people are comfortable between 30-50% relative humidity, and an airplane cabin is around 20% relative humidity.
The findings are based on the team’s analysis of Covid-19 data combined with weather measurements from 121 countries, from January 2020 to August 2020. Their study suggests a strong link between regional outbreaks and indoor relative humidity. .
In general, the researchers found that whenever an area experienced an increase in Covid-19 cases and deaths before vaccination, the estimated indoor relative humidity in that area averaged below 40% or above 60. % whatever the season. Almost all regions in the study experienced fewer Covid-19 cases and deaths during periods when the estimated indoor relative humidity was in a “sweet spot” between 40 and 60%.
“There is potentially a protective effect of this intermediate indoor relative humidity,” suggests lead author Connor Verheyen, a doctoral student in medical engineering and medical physics in the Harvard-MIT program in health science and technology.
“Indoor ventilation is always critical,” says co-author Lydia Bourouiba, director of MIT’s Fluid Dynamics of Disease Transmission Laboratory and associate professor in the Departments of Civil and Environmental Engineering and Mechanical Engineering, and the Institute for Medical Engineering and Science at MIT. “However, we find that maintaining indoor relative humidity in this sweet spot – 40-60% – is associated with reduced Covid-19 cases and deaths.”
Since the start 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 virus patterns to seasonal outdoor conditions have had mixed results.
Verheyen and Bourouiba examined whether Covid-19 was instead influenced by indoor – rather than outdoor – conditions and, in particular, relative humidity. After all, they note that most societies spend more than 90% of their time indoors, where the majority of viral transmission has been shown to occur. Also, indoor conditions can be very different from outdoor conditions due to air conditioning systems, such as radiators, which significantly dry out indoor air.
Could indoor relative humidity have affected the spread and severity of Covid-19 around the world? And could it help explain differences in health outcomes across regions?
To get answers, the team focused on the early period of the pandemic when vaccines were not yet available, believing that vaccinated populations would mask the influence of any other factors such as indoor humidity. They collected global data on Covid-19, 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 is occurring. was produced in these countries.
In total, they focused on 121 countries where Covid-19 outbreaks have occurred. For each country, they also tracked local policies related to Covid-19, such as isolation, quarantine and testing measures, and their statistical association with Covid-19 outcomes.
For each day that Covid-19 data was available, they used weather data to calculate a country’s outdoor relative humidity. They then estimated the average indoor relative humidity, based on the outdoor relative humidity and guidelines on temperature ranges for human comfort. For example, the guidelines state that humans are comfortable between 66 and 77 degrees Fahrenheit indoors. They also assumed that, on average, most populations can afford to heat indoor spaces to comfortable temperatures. Finally, they also collected experimental data, which they used to validate their estimation approach.
For each instance where outdoor temperatures were below the typical human comfort range, they assumed that indoor spaces were heated to reach that comfort range. Based on the added heating, they calculated the associated drop in indoor relative humidity.
During warmer periods, the outdoor and indoor relative humidity for each country was about the same, but they quickly diverged during colder periods. While outdoor humidity remained around 50% throughout the year, indoor relative humidity in northern and southern hemisphere countries fell below 40% during their respective colder periods, when Covid-19 cases and deaths have also increased in these regions.
For countries in the tropics, the relative humidity was about the same indoors and outdoors throughout the year, with a gradual increase indoors during the region’s summer season, when the high outdoor humidity probably increased the indoor relative humidity by more than 60%. They found that this increase mirrored the gradual increase in Covid-19 deaths in the tropics.
“We’ve seen more reported Covid-19 deaths in the lower and upper limits of indoor relative humidity, and fewer in that sweet spot of 40-60%,” Verheyen says. “This intermediate relative humidity window is associated with a better outcome, which means fewer deaths and a deceleration of the pandemic.”
“We were very skeptical at first, especially since Covid-19 data can be noisy and inconsistent,” says Bourouiba. “We have therefore been very thorough in trying to poke holes in our own analysis, using a range of approaches to test the limitations and robustness of the results, including taking into account factors such as government intervention. Despite our best efforts, we have found that even when considering countries with very strong or very weak Covid-19 mitigation policies, or very different outdoor conditions, the indoor – rather than outdoor – relative humidity maintains a strong and robust underlying link to Covid-19 results.
It is still unclear how indoor relative humidity affects Covid-19 results. The team’s follow-up studies suggest that pathogens can survive longer in respiratory droplets under both very dry and very humid conditions.
“Our ongoing work shows that there are emerging clues to mechanistic links between these factors,” says Bourouiba. “For now though, we can say that indoor relative humidity is robustly emerging as another mitigating lever that organizations and individuals can monitor, adjust, and maintain within the optimal 40-60% range, by more than adequate ventilation.”
This research was made possible, in part, by an MIT Alumni Class Fund, the Richard and Susan Smith Family Foundation, the National Institutes of Health, and the National Science Foundation.
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