Headed inside due to bad outdoor air? Here’s how to improve your indoor air quality.

To avoid breathing in unhealthy levels of ground-level ozone or wildfire smoke in Colorado, sometimes the best choice is to stay indoors. However, allergens, smoke, and airborne viruses—like those which cause influenza and COVID-19—can all find their way inside and build up indoors, degrading the quality of your indoor air. To make the most of your time inside as we head into the fall, be sure to freshen up your indoor air with some ventilation and filtration. 

Why stay indoors for ozone?  

Ozone is an invisible, highly reactive gas that forms naturally in the Earth’s upper atmosphere, where it protects us by reducing the amount of harmful UV radiation that reaches the Earth’s surface. But when it forms closer to where we breathe, known as ground-level ozone, it can be harmful.  

Notably, it’s not emitted directly by any air pollution source: It forms in a chemical reaction between volatile organic compounds (VOCs), nitrogen oxides (NOx) and sunlight, and can be amplified by hot temperatures. Colorado’s abundance of sunlight and heat, combined with VOCs and NOx emitted by chemical plants, power plants, gasoline pumps, motor vehicles, lawn and garden equipment, and oil and gas production, makes it a particular hot spot for ozone.

The easiest way to avoid breathing in higher levels of ozone on bad air days is to stay inside and close the windows and doors, as the molecule is likely to react with the building exterior before it gets inside. While VOCs can make their way indoors (no building is completely airtight), ozone has a much harder time forming indoors because windows filter out the UV light needed to catalyze the chemical reaction.  

To get alerts for ground-level ozone in the Front Range from the Regional Air Quality Council (RAQC), sign up here. When a high ozone day is anticipated the next day or happening now, the RAQC will send you a text or email to let you know. You can also take advantage of how ozone levels vary throughout the day. For example, ozone levels are higher in the afternoons and evenings. So if you can’t resist exercising outdoors, do it early.  

Can’t smell it? Don’t ignore it.  

Besides being colorless, ground-level ozone is also odorless. And sometimes, so is smoke from far away fires or that which has drifted for a long time in the atmosphere before settling down to ground level where we breathe. This can lead people to underestimate just how bad the air quality is. If you wait until your eyes feel itchy, you start coughing, you smell it or see ash falling, you’re not paying enough attention, according to Marina Vance, an assistant professor in the Paul M. Rady Department of Mechanical Engineering at the University of Colorado Boulder.  

“Relying on smell to detect air pollution can be really deceiving,” said Vance. 

To check if smoke is at unhealthy levels, she recommends using airnow.gov. The overall Air Quality Index incorporates five major pollutants that are regulated by the Clean Air Act into its rating, including PM2.5: The term used for tiny bits of particulate matter in smoke which can be harmful if breathed in. You can also check the Colorado Smoke Outlook from the Colorado Department to Public Health and Environment (CDPHE) and sign up to receive email updates about wildfire smoke in the state.  

Cleaning indoor air 

To catch and filter out wildfire smoke, allergens, and other particulate matter indoors, Vance recommends first sealing up your house: Close windows and doors. If you live in a house, replace your furnace filter with a high quality one (MERV13 is a popular and inexpensive filter). And if you have air conditioning, recirculate the indoor air within the house to further reduce particulate matter.  

If living in an apartment or condo, she recommends purchasing a portable air cleaner that uses a high-quality HEPA (high efficiency particulate air) filter, a type of pleated mechanical air filter. Keep the portable filter in the room you’re in. You can also add portable air cleaners to a house to further improve its indoor air quality.  

If you don’t have air conditioning and need to leave the windows open, you can still run an air cleaner using HEPA filters—it just won’t be as effective as with windows and doors closed, and you’ll need to run in on a higher setting, which uses more energy. Consider placing a box fan with a HEPA filter attached (on the intake side of the fan) in your window, and blowing outdoor air in, so that the air entering your home is filtered.  

Build your own DIY air filter 

Need a portable, affordable air filter that will do the trick in a pinch? Go the DIY route. You can create a Corsi-Rosenthal Box, which is an efficient and long-lasting DIY filter for removing airborne virus particles and particulate matter (like smoke) from large indoor spaces like homes, offices, and other shared indoor spaces—all for less than $100. Find more information and a how-to video here. A Spanish video tutorial is available here.  

A smaller Corsi-Rosenthal Box may be appropriate for smaller spaces, such as dorm rooms, bedrooms, and private offices. Find more information and a how-to video here.  

These DIY portable air purifiers do not filter out ozone.  

Avoid getting sick  

Due to the airborne transmission of the SARS-CoV-2 virus, which causes COVID-19, spending time indoors with other people is an activity that also comes with the risk of infection. Fortunately, the same MERV13 or HEPA air purification systems which filter out smoke and allergens can also help catch and keep viruses out of the air.  

When changing your filter, you don’t need to worry that it’s COVID-laden, because the virus is simply not going to survive on it, said Vance. But avoid ionizing bipolar, ionizers and UV light air purifiers at home—there’s no need for them, and some types of UV light can be harmful.  

This piece was adapted from an article originally published by Kelsey Simpkins for CU Boulder Today on Sept. 7, 2021: “Amid wildfires and a pandemic, here’s how to keep your indoor air clean.” Additional relevant reading: Indoor pollution can make you sick. Here’s how to keep your home’s air clean, from National Public Radio (NPR), published August 18, 2023.