Indoor Air and Illness Risk

If a friend with a cold came over, you’d probably avoid drinking from the same glass, right? That said, you can’t exactly avoid breathing the same air. 

A growing body of research suggests respiratory infections—including the flu, a cold, and COVID-19—spread significantly by inhaling virus-containing aerosols, particularly indoors.

This means that the quality of your indoor air is integral to avoiding sickness. Here are research-backed ways to reduce exposure to airborne viruses when indoors.

What the research tells us

“Clean-looking air is not necessarily clean air.” That’s according to Mojtaba Zabihi, PhD.

He’s a postdoctoral research fellow at the University of British Columbia researching aerosols—invisible floating particles all around us—and ways to improve air quality. Zabihi also helps revise Canada’s national standards for healthier indoor air.

Indoor air naturally moves less than outdoor air, where wind continuously disperses particles.   In enclosed spaces, airborne contaminants can accumulate.

As Zabihi puts it, airflow “strongly affects our exposure to airborne contaminants and pathogens.” Because of that, ventilation and filtration play major roles in avoiding airborne viruses. 

Clean up your air

One of the easiest ways to reduce airborne contamination is by properly keeping your space clean. 

Start with what’s floating all around you. Dust should be removed regularly: recent research from UC Davis and the Icahn School of Medicine at Mount Sinai found that influenza viruses can travel through the air on dust and other microscopic particles. This indicates that if your space is accumulating dust, you may be accumulating viral pathogens, too.

The more viral contaminants that build up in your air, the greater your risk of infection can be. That’s partly why colds are more common in the fall and winter when people spend more time inside.

Improve your airflow

Ventilation is a key lever for improving your air quality, but it works best when the incoming air is cleaner than what’s already indoors.

Hospital researchers in Kansas City and at the Centers for Disease Control and Prevention (CDC) recently assessed indoor air quality across local elementary schools and found 83 percent of classrooms had elevated carbon dioxide (CO₂) levels. 

High CO₂ can indicate exhaled air is accumulating indoors. So, if someone is sick, their viral aerosols are recirculated rather than displaced outside. The research found that classrooms with excessive CO₂ also had higher rates of airborne viruses—suggesting an increased chance of getting sick.

A basic CO₂ monitor is a relatively affordable way to measure your air’s freshness. The CDC suggests keeping CO₂ levels below 800 parts per million (ppm). Health Canada recommends avoiding exposure to CO₂ levels above 1,000 ppm for 24 hours or longer, as this can increase the likelihood of symptoms such as congestion, runny nose, sneezing, coughing, and postnasal drip.

A quick fix? Opening your doors and windows. But ventilation alone won’t necessarily be enough to outright prevent getting sick. That’s because it’s not just about how much air you’re moving, it’s also about where the air is moving.

“Airflow does not directly strengthen or weaken the immune system, but it strongly affects our exposure to airborne contaminants and pathogens,” Zabihi says.

Filter out impurities

If you’re replacing old air with new air that’s still filled with contaminants, then there’s not much use to circulating the air at all. That’s where air filters come in. They’re physical barriers that catch particles before you breathe them in. 

Zabihi recommends proper maintenance of HVACs, replacing air filters regularly, and avoiding overcrowded indoor spaces with poor ventilation. 

“Even relatively small improvements in ventilation and filtration can meaningfully reduce airborne exposure indoors,” Zabihi says.

High-efficiency particulate air (HEPA) filters are a research-backed method to reduce viral transmission. A true HEPA filter differs from more standard air filters because it can continuously capture airborne particles that may contain viruses, reducing the extent they accumulate indoors. Make sure you’re buying a filter that’s genuinely HEPA-rated—true HEPA filters are held to stricter filtration standards than those marketed as “HEPA-like.”

Where you place the HEPA purifier matters, too. Zabihi’s research found that placing air cleaning devices closer to people who are sick can help reduce airborne exposure in shared indoor spaces.

Taken together, these strategies show that small, intentional changes to your indoor environment can meaningfully reduce your exposure to airborne viruses.

This article was originally published in the September 2026 issue of alive magazine.