Show 1454: Stopping Airborne Viruses: Simple Steps to Cleaner Indoor Air

Do you worry about things you can’t see, smell or taste? Most of us don’t. Yet particles we can’t detect with our five senses are often present in the air we breathe. They have the power to make us sick. How can we achieve cleaner indoor air so that we have less chance of coming down with a serious infection?

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The Importance of Cleaner Indoor Air:

When we talk about air pollution, the image that may arise is factories belching dark plumes of smoke. While the particles generated by industrial processes can indeed be dangerous for our health, sometimes the greatest danger is from particles we can’t even see.

The COVID-19 pandemic brought this into sharp focus, as we realized that people who had not yet begun to experience symptoms could be spreading infectious viruses. But the need for cleaner indoor air is not limited to COVID, or even to an epidemic like measles or the flu. Many infections spread primarily on viral particles wafting through the air. We are reminded of this every winter, as cases of influenza start to rise. But respiratory syncytial virus, human metapneumovirus and dozens of rhinoviruses and coronaviruses that cause colds also travel on the air. So do measles viruses.

Our guest, Dr. Linsey Marr, is one of the country’s leading environmental engineers. She got interested in airborne transmission of infection even before SARS-CoV-2 appeared. Then, with COVID, it became clear that the advice to the public about maintaining 6 feet of distance was inadequate to protect people from coming down with the infection. It was developed based on an outdated understanding of how infectious particles travel.

Can You Tell If Indoor Air Is Contaminated?

Given the extremely small size of viral particles, we might have to use our imagination to understand how they could be present. We can’t smell viruses. But if you imagine someone smoking a cigar in the room, you know that the smell will linger for quite a while after the smoker has left. Viral particles can float around like the smell of cigar smoke, which is why they can still be present even after an infected person has left the space.

This viral behavior means that the riskiest places are those where many people congregate, especially during a season when infections are spreading. Think of grocery stores, hospitals, or athletic event venues. Wearing a tightly fitted N95 or KN95 mask could provide some protection (especially if others also wore masks). It is not a magic bullet, though. Japanese people accept mask protocol during flu season, and they have still experienced the spread of influenza. In the US, it is very unlikely that most people will accept wearing masks, even if it could help reduce their risk of infection.

While we can’t measure viral particles in the air without complicated equipment, we can use a simple relatively inexpensive piece of equipment to check the ventilation in a space with multiple people. It is called a carbon dioxide (CO2) monitor. Because people exhale CO2, high levels of this harmless gas indicate lots of people breathing in the space without much ventilation. Fresh outdoor air runs about 400 ppm CO2; once indoor air reaches 1,000 ppm or higher, you may want to take action.

Moving Toward Cleaner Indoor Air:

Ventilation:

Improving ventilation would be very advantageous. Most public places should strive to achieve at least 4 to 6 air exchanges per hour. More sensitive spaces such as health care facilities might benefit from a higher level of ventilation.

Filtration:

The other way to deal with airborne viruses is through filtration. Home air handling systems could be equipped with a high-efficiency particulate arresting (HEPA) filter. This is ideal, but it may not be practical in every space. Ordinary air filters carry a MERV number such as 8, 11 or 13. Higher numbers indicated better filtration capacity. In general, you’d want to use the highest MERV number your HVAC system will tolerate. Too high a number can create too much pressure and cause problems.

What if you don’t have access to the filters for your air? That is the case for many apartment dwellers who have to share their air with everyone else in the building. One affordable option is to build and use a Corsi-Rosenthal box. It can be assembled at home for $50 to $70 and it works quite well to provide cleaner indoor air in the space where it is operating. Dr. Marr describes how to build one. Here is a link to our interview with Dr. Corsi, including instructions on building a Corsi-Rosenthal box.

Elimination:

Another step toward cleaner indoor air might be to utilize ultraviolet (UV) light as a disinfectant. A unit that uses germicidal UV at a wavelength of 250 nanometers needs to be tucked into air ducts. That wavelength can damage eyes and skin. New technology is being developed using a slightly different wavelength of 222 nanometers. While still germicidal, it is supposed to be safe for human eyes.

This Week’s Guest:

Linsey Marr, PhD, is a professor of civil and environmental engineering at Virginia Tech, where she leads the Applied Interdisciplinary Research in Air (AIR2) laboratory. Her research group focuses on the dynamics of biological aerosols like viruses, bacteria, and fungi in indoor and outdoor air. Marr teaches courses in environmental engineering and air quality, including topics in the context of global climate change, as well as health and ecosystem effects. She has been thinking and writing about how to avoid airborne viral transmission since the pandemic began, as in this article published in Environment International (Sep. 2020). Photo by Peter Means, courtesy of Virginia Tech.

Dr. Linsey Marr of Virginia Tech. Photo by Peter Means, courtesy of Virginia Tech

Dr. Marr mentioned her publication, with many colleagues, advocating for cleaner indoor air in public buildings. Here is a link.

Joe Graedon conducted this interview, as Terry was unavailable.

Listen to the Podcast:

The podcast of this program will be available Monday, Dec. 8, 2025, after broadcast on Dec. 6. You can stream the show from this site and download the podcast for free. This week’s episode contains some additional discussion of outside air, including the dangers of smoke from wildfires, along with particulates from car tires or microplastics.

Citations

• Morawska L et al, "Mandating indoor air quality for public buildings." Science, March 29, 2024. DOI: 10.1126/science.adl0677