How Much Do Ventilation Systems Help Reduce COVID Transmission?

air filtration system on ceiling of corporate office

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Key Takeaways

  • Proper air flow can help reduce COVID-19 transmission indoors, even as mask mandates fall away.
  • Air filters can protect again all three types of COVID transmission: inhalation, deposition, and touching.
  • Research shows adding portable air purifiers to spaces that already have HVAC systems can significantly enhance air circulation.

Mask mandates are gradually disappearing. That leaves many people wondering about whether improved ventilation strategies in indoor public spaces help reduce transmission of COVID-19.

Experts say yes. Improved ventilation helps and is crucial going forward, especially as fewer people are wearing masks.

“It’s not an airtight guarantee that you’re not going to get COVID,” Andrew Noymer, PhD, MSc, epidemiologist and associate professor of population health and disease prevention at the University of California Irvine, told Verywell. “It’s just a much-reduced probability because the air isn’t as stale.”

Why Good Ventilation Is Crucial

Last month, a federal judge struck down the Centers for Disease Control and Prevention (CDC) mask mandate for planes, trains, and busses. Some carriers still have their own mandates and some cities still have mask rules that are also in effect for local public transit. But a large swath of the nation is now mandate free.

Experts say research shows enhanced indoor ventilation via filters and portable air purifiers can help reduce virus transmission in indoor public spaces. Plus, people can take steps to improve and monitor ventilation in private spaces as well, especially when planning to host guests.

The virus that causes COVID-19 is spread in three ways: inhalation, deposition, and touching, according to the CDC. The virus is released in tiny droplets or aerosols when an infected person exhales, speaks, coughs, or sneezes. Transmission primarily occurs when someone else inhales the infected aerosols.

Infected aerosols can also be deposited from the air directly onto mucous membranes, such as the eyes, nose, or mouth. And aerosols can also contaminate surfaces that we touch. While surface transmission is unlikely, it is possible that we could become infected if we transfer the virus to a mucous membrane.

“When you’re in an indoor, climate-controlled building, those little droplets can just waft around on air currents for minutes on end,” Noymer said.

If air is circulated through a ventilation system’s filter many times per hour, however, that can help reduce transmission. That’s because a good filter will trap aerosols, preventing them from recirculating. But all ventilation strategies, systems, and filters aren’t created equal.

The Environmental Protection Agency (EPA) recommends that building operators upgrade their filters to the highest efficiency that’s compatible with the building’s HVAC system and use portable air cleaners to enhance overall circulation to reduce viral transmission.

Portable Air Purifiers Enhance Ventilation

In a recent study, researchers from the Well Living Lab, a collaboration between the Mayo Clinic and Delos, simulated a classroom environment. They strategically placed three recirculating portable air filtration units around the room, which also had HVAC.

The researchers then used dye-tagged particles and a breathing simulator to test how the aerosols dispersed and deposited. They tested conditions with just the HVAC unit running and with the HVAC running in tandem with the portable units.

“What we found is that the use of those portable air purification units really allowed for a reduction of up to five times lower particle concentration,” Zachary Pope, PhD, MS, a research scientist and study author, told Verywell. “So obviously having that reduction in the amount of particles in the air is good because you theoretically do not have as many potentially infectious particles in the air.”

In their simulation, the researchers also placed items commonly used in a classroom, like desks, a whiteboard, digital tablets, and more—all surfaces that can potentially become contaminated.

“Our study was able to help show that portable air purification—because it reduces the amount of particles in the air—also reduces the amount of particles that deposit on the surfaces,” Pope said. “And those two factors really help us limit both direct inhalation transmission as well as transmission via different surfaces in a room.”

Although the study tested a mock classroom, Pope says portable air purifiers are useful for other settings, like offices, cubicles, homes, and more. “They’re very cost effective and they’re very impactful,” he said.

The use of portable air purifiers is especially important in buildings where ventilation systems cannot be upgraded. And even when HVAC systems can be improved, portable units further enhance air circulation and help clean the air.

Buying a Portable Air Purifier

If you’re buying a portable air purifier, Pope recommends first knowing the dimensions of your space. Then, check the specifications for the air purifier’s cubic feet per minute (CFM) rating.

“Ideally what you want to have is for that CFM value to be at least two-thirds of the square footage of your room,” he said. For example, a unit with a 100 CFM rating can adequately ventilate 150 square feet. If you have a larger room, you may need two or more units.

If you need an even more budget-friendly option, Noymer says there’s what he calls a “hack” if you make a Corsi-Rosenthal Box. The method involves using a box fan and HVAC filters.

“It’s basically a DIY,” he said.

A study from researchers at the University of California Davis shows that the Corsi-Rosenthal Box reduces particle concentration. The study is available as a preprint and has not yet been peer-reviewed.

The university offers instructions for making a Corsi-Rosenthal Box on its website.

What About Opening Windows?

Opening windows can also greatly improve ventilation.

“In the summertime or anywhere where that’s possible and practical [opening windows] is something I highly recommend,” Noymer said.

Sometimes that’s not always feasible, adds Pope, as in the case of skyscrapers. And window-opening may not be ideal or advisable when air quality or weather is poor, like during wildfires, high pollen counts, or storms.

Monitoring Air Circulation

One way to monitor whether air is stale in a space is to use a carbon dioxide meter, which can be purchased online, Noymer said.

“CO2 is a proxy measure of how well the air is circulating in the room,” he explains. “If the CO2 keeps going up and up, then it is not being circulated very well. And if the CO2 is remaining at relatively low levels, then it is being circulated very well.”

A CO2 meter might be a useful tool if you’re hosting an indoor private event this summer, for example. A rising level might prompt you to open an extra window.

Ultimately, improving ventilation will remain a vital strategy for reducing viral transmission going forward as we forge on in the third year of the COVID-19 pandemic and beyond.

“It needs to be part of health and public safety codes,” Noymer said. “In the long run, we need a rethinking of air in public spaces.”

What This Means For You

While masks offer significant protection against COVID-19 transmission, high efficiency HVAC systems can further reduce your risk in places ranging from offices to airplanes. Unsure what the HVAC situation is in your office? Bringing in a portable air purifier is helpful, too.

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.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Kong M, Li L, Eilts SM, Li L, Hogan CJ, Pope ZC. Localized and whole-room effects of portable air filtration units on aerosol particle deposition and concentration in a classroom environmentACS ES T Eng. 2022;2(4):653-669. doi:10.1021/acsestengg.1c00321

  2. Centers for Disease Control and Prevention. Scientific brief: SARS-CoV-2 transmission.

  3. Environmental Protection Agency. Air cleaners, HVAC filters, and coronavirus (COVID-19).

  4. Dal Porto R, Kunz MN, Pistochini T, Corsi RL, Cappa CD. Characterizing the performance of a DIY air filter. medRxiv. Preprint posted online January 11, 2022. doi:10.1101/2022.01.09.22268972

By Jennifer Chesak
Jennifer Chesak is a medical journalist, editor, and fact-checker with bylines in several national publications. She earned her Master of Science in journalism from Northwestern University's Medill School. Her coverage focuses on COVID-19, chronic health issues, women’s medical rights, and the scientific evidence around health and wellness trends.