The Role of Dry Winter Air in Spreading COVID-19

December 21, 2020

As Americans head indoors for the winter, they find themselves at increased risk of contracting and spreading COVID-19 due to lower levels of humidity in the air.

“Not only does breathing air that is below recommended relative humidity levels irritate respiratory passages,” says UH pulmonologist David Rosenberg, MD, MPH, “but doing so contributes to a heightened risk of contracting COVID-19 and influenza.”

But what exactly is relative humidity? And how does keeping your home at a proper level of relative humidity reduce the risk of contracting COVID-19?

What is Relative Humidity?

Relative humidity is a measurement of the amount of water vapor in the air compared to the total amount of water vapor the air can hold. Overall, colder air retains less water vapor than warmer air. When furnaces draw in cooler, less humid air, the heating process increases the air’s capacity to hold water, causing its relative humidity to plummet.

The ideal relative humidity for human-occupied buildings is between 40 and 60 percent. Unfortunately, indoor relative humidity can dip down to 20 percent or lower during winter months.

Relative Humidity’s Role in the Transmission of COVID-19

Relative humidity can play a role in the transmission of COVID-19 in three primary ways:

Drying out respiratory airways: Low relative humidity has a drying effect on the cells and mucous linings of our airways. These cells act as a defense against viral particles and other invading foreign substances. Drying out airway linings impairs their ability to function properly. When moisture levels are insufficient, viral particles have a greater “docking” potential at airway receptor sites, which can lead to an increased risk of infection.

Viruses die off faster in higher relative humidity: Research indicates that virus particles suspended in the air die more rapidly when the relative humidity is higher. In environments where viral particles decay faster, less viral material remains suspended in the air, leading to reduced risk of infection.

Lower relative humidity aids airborne transmission of water droplets and aerosols: When we cough, sneeze, talk or sing, water droplets and smaller water particles (aerosols) containing living virus disperse into the air. Droplets and aerosols tend to stay afloat longer and travel farther in air with lower relative humidity. Virus-containing aerosols, in particular, can penetrate more deeply into the lungs to increase the possibility of transmission.

In addition, research shows coronavirus has the potential to live longer on surfaces in colder environments.

“When you combine lower relative humidity with colder temperatures,” Dr. Rosenberg says, “one is potentially at even more increased risk for developing infection. This is supported by data collected with respect to COVID-19 outbreaks in both Seattle and China.”

What Can We Do About It?

Using home humidifiers during winter can be useful in keeping relative humidity levels in the recommended 40 to 60 percent range to potentially reduce the risk of contracting and spreading COVID-19 and other viral infections. However, controlled clinical trials investigating the extent of this benefit are still needed.

Dr. Rosenberg also cautions, “While home humidifiers may be useful in reducing risk of viral infection, improper use of humidifiers is associated with certain health issues. Specifically, when the relative humidity becomes too high (more 60 percent), it can promote mold growth in indoor environments.”

Excessive mold growth can lead to asthma and other respiratory problems. In addition, dangerous microbes such as aspergillus or legionella can grow inside the water reservoirs of humidifiers, leading to increased risk of other types of infections, particularly for immunocompromised people. As such, owners of humidifiers should strictly adhere to industry and manufacturer guidelines when using them and follow the U.S. Center for Disease Control guidelines for heating, ventilation and air conditioning systems.

If you have concerns about the safety of using a home humidifier, especially if you or a family member with whom you live has a chronic lung disease, consult a healthcare provider.

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