Is Indoor Air Really More Polluted Than Outdoor Air?

By Dan Dearden, CAFS, Certified Air Filtration Specialist

You’ve probably heard the claim that indoor air is more polluted than outdoor air — maybe even five times worse. That idea comes from widely circulated statements by the Environmental Protection Agency (EPA), such as:

“Americans, on average, spend approximately 90 percent of their time indoors, where the concentrations of some pollutants are often 2 to 5 times higher than typical outdoor concentrations.”

“In the last several years, a growing body of scientific evidence has indicated that the air within homes and other buildings can be more seriously polluted than the outdoor air in even the largest and most industrialized cities.”

These quotes are often highlighted by indoor air quality (IAQ) product manufacturers — and understandably so. But let’s take a closer look at the language being used.

Words Matter: "Some" vs. "All"

Notice that the EPA says “some pollutants” are “often” higher indoors — not all pollutants, and not always. Similarly, the air inside “can be” more polluted — not that it always is. These distinctions are important.

So, is indoor air more polluted than outdoor air? Sometimes. And it depends on a few key factors.

What’s in Your Indoor Air?

There are about 45 major indoor air pollutants, plus over 900 sub-pollutants, most of which fall into the category of volatile organic compounds (VOCs). Only pollutants that originate indoors — such as formaldehyde, nitrogen dioxide, ozone, mold, and other VOCs — have the potential to be more concentrated inside than out.

Together, these pollutants account for roughly 12% of the harm to humans caused by indoor air pollution. After years of in-home testing (excluding mold, which we refer to specialized mold inspectors), we’ve consistently found formaldehyde and VOCs to be the most common culprits. These are often found at higher levels indoors than is considered safe — but the good news is, you can fix it. Source control (eliminating what’s causing the pollution) combined with proper ventilation, such as an Energy Recovery Ventilator, can bring those levels back down to a healthy range.

What About Particulates?

Here’s something that surprises many homeowners: in all our testing, we’ve never found higher levels of particulate matter indoors than outdoors. However, that doesn’t mean you’re in the clear.

Particles from outdoors (like smoke, dust, and pollen) easily infiltrate the home. When outdoor air quality is poor, indoor levels follow — just slightly lower. And since particulate matter is responsible for a whopping 86% of the harm to humans caused by the indoor air environment, it’s absolutely something to take seriously.

In most homes we test, when the outdoor particle count is high, indoor levels also exceed the safe threshold of 5 µg/m³. But again, there’s good news: with near-HEPA-level filtration systems, like the NovusAer, combined with a depressurization duct, homeowners can achieve ultra-clean indoor air — even down to 0.1 µg/m³. That’s the level I maintain in my own home, year-round. And I live in Salt Lake City, which at times has the worst air quality in the country due to its unique valley climate.

So, What’s the Verdict?

Yes — indoor air can be more polluted than outdoor air, but only under certain conditions. The only way to truly know what you're breathing is to test your indoor air.

At Essential Air Products, we believe informed homeowners make healthier choices. If you're concerned about your air quality, we’re here to help with honest assessments, proven solutions, and expert advice.