Humidity – BioQuakes

The COVID-19 pandemic has taught us that indoor air quality matters. It has also taught us that indoor spaces need clean air in order to protect the health and well-being of the people inside. According to a study done by MIT, researchers found that indoor relative humidity may also influence the transmission of the virus.

In a recent study appearing in the Journal of the Royal Society Interface, the MIT team reports that maintaining an indoor relative humidity between 40 and 60 percent 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 this into perspective most people are comfortable between 30 and 50 percent relative humidity, and an airplane cabin is 20 percent relative humidity.

To conduct this research, the MIT team focused on the early period of the pandemic when vaccines were not yet available. This is important because with vaccinated populations it would be unclear the influence of any other factor, such as humidity.

In order for the MIT team to find this “sweet spot” between 40 and 60 percent relative humidity, the researchers found that whenever a region experienced a rise in COVID-19 cases and deaths, the estimated indoor relative humidity was below 40 percent or above 60 percent.

According to co-author Lydia Bourouiba, director of the MIT Fluid Dynamics of Disease Transmission Laboratory, “indoor ventilation is still critical.” However, “maintaining an indoor relative humidity in that sweet spot — of 40 to 60 percent — is associated with reduced Covid-19 cases and deaths.”

This study relates to AP Biology as COVID-19 affects the immune system. When the virus first enters the body, the body’s response starts by engaging two kinds of immune cells: B cells, which produce antibodies that fight off the virus, and T cells, which destroy infected cells. After this initial response, levels of antibodies in the bloodstream begin to fall. The humidity in the air also relates to biology because in low humidity environments below 40 percent our mucous membranes dry and this “mucociliary clearance” process is impaired. This leaves us more susceptible to airborne infections, like the flu or common cold. When the relative humidity is above 60 percent, as in the case of humid weather, the sweat your body produces cannot evaporate, leaving our bodies feeling hot and sticky. To cool off, our bodies must work even harder. This results in excessive sweating, increased rate and depth of blood circulation and increased respiration. This generates a negative feedback loop to help our body return to a normal state (homeostasis).

As “Covid Winter” approaches, especially in states with seasonal changes such as New York, it calls into question what this will mean for the virus in the coming months. When thinking about when the pandemic will end, temperature, humidity, and seasonal shifts are large factors which work against stopping the spread of the virus. Externally, as the air outside becomes colder, it is able to hold less water vapor, which decreases humidity. HVAC (heating, ventilation, and air conditioning) units inside office buildings work by taking in outside air and heating it to channel through the indoor space, which similarly dries the air out.

Why is humidity important in preventing the spread of the virus on a biological level? In an aerosol study conducted at Virginia Tech, the researches demonstrated that as humidity levels decrease, the particles of moisture released from actions such as talking, coughing, sneezing become smaller. This becomes a problem because the dry air causes the water in the molecules to evaporate faster, therefore becoming even smaller and staying in the surrounding air for a longer period of time. Any droplets can then travel around the closed, indoor space further. Their minuscule size allows them to be inhaled and move deeper into the lungs, where, as we learned in the video we watched in class, a spike on the virus will insert into a receptor molecule on a healthy cell membrane, allowing it to infect the healthy lung cell, leading to a susceptible person contracting COVID-19 and being able the virus further.

Other coronaviruses, like the common cold, influenza, and rhinoviruses, have exhibited similar spreading patterns dictated by the seasons, demonstrated by flu season occurring in the winter, calming down in summer, and coming back again in fall. Scientists believe COVID-19 could do the same, and are currently conducting research and gathering data to see the correlation between the virus and humidity levels. Stephanie Taylor, a physician and fellow at Har-

An example of how the virus remains in the air after released through talking, singing, etc

vard Medical School, is part of a joint study with the Massachusetts Institute of Technology that “found that the most powerful correlation between national numbers of daily new coronavirus cases and daily Covid-19 deaths was indoor relative humidity.” In reflecting upon their findings, she says that humidity “is so powerful, it’s crazy.”

The only way to know exactly how the coming winter months will affect the spread of the virus is through time and observation, but it is interesting to look at the biological processes and movement of particles in relation to humidity to understand how the virus may have an increased spread as it becomes colder. I also feel this background helps us be able to make intelligent, informed decisions about the risk of social gatherings as it becomes harder to stay outdoors and the weather changes. What do you think is lying ahead in “Covid Winter?” Do you think we will inevitably have to wait until the humidity changes in spring to declare an official end to the pandemic?

BioQuakes

AP Biology class blog for discussing current research in Biology

Tag: Humidity

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“Covid Winter” is Coming: The Power of Humidity in our Return to Normal

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