Using a Humidifier May Ward Off COVID

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As people crank up the thermostat to combat the cold this winter, indoor spaces can become uncomfortably dry. According to a new study, keeping your relative humidity between 40% and 60% may help more than your skin—it may also lower COVID-19 transmission rates.

The study, published last month in the Journal of the Royal Society Interface, suggests that indoor humidity levels are a bigger drive of respiratory viral outbreaks than outdoor conditions, regardless of where you live. Researchers from the Massachusetts Institute of Technology analyzed meteorological data from 121 countries from January to August 2020 and information about COVID-19 outbreaks and mitigation tactics.

They found that maintaining a relative humidity sweet spot of 40-60% coincided with fewer COVID-19 cases and deaths. The virus proliferated more quickly in areas with higher or lower relative humidity, despite other ventilation or air purification efforts.

But what is relative humidity, and how does it affect us? Roger Seheult, MD, medical advisor for On/Go and associate clinical professor of internal medicine at the University of California, Riverside, School of Medicine, explains that relative humidity is more than just how moist the air feels.

“Relative humidity is a ratio expressed as a percent of the amount of water dissolved in the air divided by the maximum amount of water that can be dissolved in the air at that temperature,” Seheult, who was not involved in the study, told Verywell.

Air temperature plays a significant role in relative humidity. Since warmer air can hold more water, warmer places tend to be more humid, both inside and outside. Nitrogen and oxygen particles in the air move quicker when they are warmer, and have the potential to hold more moisture before the liquid condenses.

When left to its own devices, the air inside and outside a building will equalize to reach the same level of humidity based on temperature. Once heat or air conditioning is applied, the temperature change either reduces or increases the amount of moisture the air will hold, making the humidity inside the building relative to the air outside the building.

“Because we try to keep indoor temperature stable regardless of the temperature outside, our indoor relative humidity will be lower in the winter and normal or higher in the summer,” Seheult said.

While it’s easy to understand how relative humidity can affect comfort levels, it also impacts healthy breathing. Seheult said that our breathing processes rely on a specific temperature and humidity ratio to work correctly.

The lungs are lined with hair-like appendages called cilia that act as a clearing crew for any inhaled particles or pathogens. Mucus is produced by cells embedded among the cilia, creating another barrier to entrap and filter the millions of pathogens and particles we inhale with every breath. The mucociliary system is present in the nose, sinus passages, and trachea.

When air is inhaled, it is humidified in the trachea before traveling to the lungs.

“If the relative humidity of the air is too low, the humidification process will be inadequate,” Seheult said. “The resulting drier air will disrupt the lungs’ protective system and cause symptoms of dry eyes and itchy skin.”

Too much humidity can also present problems by allowing the growth of mold, bacteria, and dust mites, which are dangerous to inhale.

Seheult doesn’t think that COVID-19 virus particles are more virulent or infectious at lower or higher relative humidities. Rather, our bodily systems operate more efficiently and effectively within the middle range. The intermediate humidity range is associated with more significant decay in virus particles, Seheult said.

While the evidence is compelling, the study authors are clear that they only show correlations so far. Further study is needed. This is not the first study to investigate a link between humidity and virus transmission; a 2018 study found that influenza A was also less transmissible within a humidified environment.

If you live in a colder climate and use a heater, there are actions you can take to maintain the ideal relative humidity, Valerie Bradt, marketing manager for DriSteem, a source for industrial humidifiers, told Verywell.

The first step is to measure the humidity in your home using an affordable tool called a hygrometer. Bradt said hygrometers are easy to find on Amazon or other big box retailers for around $10. Other indicators may be increased static electricity and dry eyes or skin.

Once you have an accurate reading of your relative humidity, humidifiers can help boost relative humidity by sending water vapor into the air. While most people are familiar with smaller units meant to humidify a single room, Bradt says that whole-house systems are also available. They work with the home’s current HVAC system.

For those using single-room humidifiers, Bradt said that each unit will cover a different range, with some that include the ability to shut off when the desired relative humidity is reached using an embedded hygrometer.

Relative humidity is easier to control in areas where temperatures don’t vary as much. Still, for those that need seasonal heating and cooling, humidity may play a big part in your health. Investing in a good humidifier may keep you healthy and comfortable this winter.

Maintaining relative humidity of 40-60% may be able to lower the transmission rates of COVID-19, but a more extensive randomized study is needed to prove it. In the meantime, the humidity sweet spot is ideal for healthy breathing, especially for those susceptible to allergies, asthma, and seasonal respiratory illnesses. A humidifier can help maintain consistent relative humidity in your home.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

Verheyen CA, Bourouiba L. Associations between indoor relative humidity and global COVID-19 outcomes. J R Soc Interface. 2022;19(196):20210865. doi:10.1098/rsif.2021.0865

Kuek LE, Lee RJ. First contact: the role of respiratory cilia in host-pathogen interactions in the airways. Am J Physiol Lung Cell Mol Physiol. 2020;319(4):L603-L619. doi:10.1152/ajplung.00283.2020

Reiman JM, Das B, Sindberg GM, et al. Humidity as a non-pharmaceutical intervention for influenza A. PLOS One. 2018;13(9):e0204337. doi:10.1371/journal.pone.0204337

By Rachel MurphyRachel Murphy is a Kansas City, MO, journalist with more than 10 years of experience.