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Banner image: Ground level ozone, shown here in Denver, is a growing problem in the western United States. (Credit: National Renewable Energy Laboratory)听

It鈥檚 no secret: The air quality is bad in Colorado this summer. 听

Thus far, the state has issued 59 Ozone Action Day alerts since May 31鈥攖he most in a single year since record-keeping began a decade ago. At least one day in August, Denver boasted the worst air quality of any major city in the world.

But what鈥檚 in that air?

鈥淚n the Front Range, we are mostly concerned about two types of air pollutants, especially in the summer: particulate matter and ozone. And then when you're indoors, there's now this extra concern about the COVID viruses spreading through the air,鈥� said Marina Vance, an assistant professor in the Paul M. Rady Department of Mechanical Engineering who was awarded an NSF CAREER Award to study how pollutants transform as they move between indoors and out.

So how do we keep ourselves healthy?听

鈥淚t can be overwhelming for people to figure out what the least risky situation is,鈥� said Vance, who is also faculty in the environmental engineering program. 鈥淲e're hoping that with some simple measures, people can alleviate some anxiety that's driven by this co-occurrence of the wildfires and COVID persisting.鈥澨�

On high ozone days, go inside and shut the windows

First, ozone鈥攊t鈥檚 been a big part of this summer鈥檚 air quality warnings.听

Ozone is a 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, .听

Notably, it鈥檚 not emitted directly by any air pollution source: , and can be amplified by hot temperatures. Colorado鈥檚 abundance of sunlight and heat, combined with VOCs emitted from numerous chemical plants and gasoline pumps (among other sources), and NOx emitted from power plants and motor vehicles鈥攐f which Colorado has plenty鈥攎akes it a particular hot spot for ozone.听

This ground-level ozone doesn鈥檛 just gray our blue skies and obscure our beloved mountains. It doesn鈥檛 feel good to breathe. And studies suggest that chronic exposure to this pollutant can lead to lower birth weights, asthma and higher rates of premature death.听

Vance鈥檚 advice for high ozone days: Stay inside and close the windows and doors.听

鈥淭he ozone is so reactive it will react with the building exterior before it gets inside,鈥� said Vance.听

While VOCs can make their way indoors (no building is completely airtight), ozone cannot form indoors because windows filter out the UV light needed to catalyze the chemical reaction.听

Can鈥檛 smell it? Don鈥檛 ignore it.听

While the air quality is worse this year compared to last year, there鈥檚 one major difference: We鈥檙e not smelling the smoke.听

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鈥檙e not paying enough attention, according to Vance.听

鈥淩elying on smell to detect air pollution can be really deceiving,鈥� said Vance.听

Smoke from wildfires as far away as California doesn鈥檛 carry its smell when it arrives in Colorado, but it can still carry particulate matter, according to , associate director for science at the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU Boulder.听

And we know wildfire smoke can be quite harmful to our health, whether from near or far away, something studied by Colleen Reid, assistant professor of geography.

To check if smoke is at unhealthy levels, Vance recommends using . The overall 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.听

This regularly updated index ranges from 鈥淕ood鈥� to 鈥淢oderate鈥� at the low end, all the way up to 鈥淗azardous.鈥� If PM2.5 or the overall rating is at , it鈥檚 wise to take appropriate precautions inside and out.

To catch and filter wildfire smoke 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 one). And if you have air conditioning, recirculate the indoor air within the house to further reduce particulate matter.听

If living in an apartment or condo, she recommends purchasing a portable air cleaner, which often use high quality filters, a type of pleated mechanical air filter. Keep the portable filter in the room you鈥檙e in.听

If you don鈥檛 have central air conditioning or a window unit, and can鈥檛 keep doors and windows closed while indoors, Vance recommends a DIY approach. Place a high-quality furnace filter, such as a MERV13, on the intake side of a box fan blowing air into the room; then seal the window opening surrounding the fan. This will let outdoor air in to cool the room but filter out much of the particulate matter from wildfire smoke. Adding a portable air cleaner (with a HEPA filter) can help further reduce particulate matter and viruses within the room. However, this approach does not filter out ground-level ozone.

Pay attention to more than one pollutant听

What about days with high ozone and high smoke levels?听

Be careful with what online tool you use, because it might be only telling you half the story, Vance notes. She recommends , which is based on good data from , and measures both ozone and particulate matter.听

While Purple Air is a popular online site many reference, it does not measure ozone levels.听

鈥淭he overall air quality index is only driven by whatever is the worst thing. But if both ozone and particulate matter are rated orange, that's extra bad,鈥� said Vance.听

But we can take advantage of how these pollutants vary throughout the day. For example, ozone levels are higher in the afternoons and evenings. So if you can鈥檛 resist exercising outdoors, do it early.

Get clean air, no matter where

COVID-19 further complicates the equation, as spending time indoors with other people is something we鈥檝e now been advised to avoid.听

But fortunately, the same MERV13 or HEPA systems which filter out particulate matter from fires can also help catch and keep viruses out of the air. And when changing your filter, you don't need to worry that it鈥檚 COVID-laden, because the virus is simply not going to survive on it, said Vance. 听

Either way, avoid ionizing bipolar, ionizers and UV light air purifiers at home鈥攖here鈥檚 no need for them, and some types of UV light can be harmful, said Vance.听