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Editor's note: This article was originally published July 12, 2021, and updated June 30, 2026.��

Coloradans awoke to hazy skies this week, as smoke from multiple large fires burning across the West deemed air quality “poor” and “unhealthy for sensitive groups” in some regions,��.

Not long ago, researchers had little understanding of how such smoke impacts human health—if at all, says Colleen Reid, an assistant professor of geography who, in 2008, became one of the first researchers in the country to start studying it.��

“At the time, there was a sense that it was ‘natural’ air pollution, and that it was only short-term so it was not as much of a concern as pollution from things like factories, refineries and vehicles,” she said. “What we are realizing now, as these episodes of exposure become more frequent and last longer, is that wildfire smoke can have measurable and potentially lasting health impacts.“

Reid spoke with��CU ������ý���� Today about what those impacts are and how we can minimize them.

It seems like smoke is getting worse each summer. Is it?

It is. When I started this work, people collecting data from monitors in Northern California said "disregard anything above 400 micrograms per cubic meter (ug/m3)" because they assumed it was impossible. They thought the monitors were malfunctioning. Now, in some areas of the country, levels have reached the thousands during extreme wildfire smoke events near the fire.��

There is now�� that have been achieved by regulating industrial and vehicular emissions under the Clean Air Act.����

What is wildfire smoke made of?

It’s a complex mix of gases and solid and liquid particles. From a health perspective, the particles—that dark cloud that you see when you look at the smoke plume—are most concerning. They are so tiny that, unlike larger particles, they don’t get stopped in the trachea or nose when you breathe them in.

When assessing air quality, we tend to focus on PM2.5, particulate matter that is 2.5 microns or smaller in diameter. The smaller the particle, the deeper they get into the lungs, and the more likely they are to get into the bloodstream.��

Most particles created during wildfires are 1 micron or smaller, not only affecting respiratory health but potentially having systemic effects—including inflammatory effects. They have found particles in the brains of cadavers, so we also know they can get into the brain.

Is there anything else in there of concern?

Some researchers have also become concerned about hazardous air pollutants such as formaldehyde, benzene and hydrogen cyanide present in wildfire smoke.��We also have found these contaminants in ash after wildfires that burn structures, such as the Marshall Fire and the L.A. fires. Ground-level ozone is also a concern, as it is a respiratory irritant and can be formed through chemical reactions of other air pollutants in wildfire smoke.

Is wildfire smoke worse for you than other forms of air pollution?

The case is not yet settled on this.��

suggest that PM2.5 found in wildfire smoke is more harmful than other sources of PM2.5, but these findings do not take into account differences in what burned (forest, grassland or human-made materials) or the fact that wildfire smoke is very bad for short periods, whereas other sources of PM2.5 contribute lower levels but regularly throughout the year.��

At least�� in mice found that particles in wildfire smoke are more toxic than particles from other sources.��

How does wildfire smoke impact short-term health?

Probably everyone exposed to wildfire smoke is impacted in some way, even if they don’t recognize it. They might be a little short of breath or have some heart rate variability or lung function decline. We know that it can cause a sore throat, cough, itchy/watery eyes, congestion and shortness of breath. If you can’t breathe as deeply and get as much oxygen, especially at this altitude, you might feel tired.

For people with pre-existing respiratory disease (such as asthma or emphysema), wildfire smoke can lead to increased need to use medication and, in some cases, needing emergency care. Generally, there are more people who die during wildfire events than would be expected if the wildfire was not there—these are probably individuals who were very sick and the air pollution from the wildfire smoke was the proverbial straw that broke the camel’s back.

Are there longer-term effects?

found that babies whose gestation is during a wildfire event are more likely to be born with a lower birth weight and more likely to be born early. There is also some evidence that wildfire smoke can trigger heart attacks and impact the immune system.��

���Ա��� looking at eight years of data in Montana found that areas with more intense wildfire smoke exposure in the summer had worse flu seasons the following winter. It could be that wildfire smoke irritates the lungs and makes people more susceptible to worse infection upon exposure.�� suggests that areas with worse wildfire smoke may experience more COVID-19 cases or more severe cases of the disease.

Researchers are also studying whether wildfire smoke is affecting metabolic health and cognition.

What should people do on smoky days?

Make sure that your medication is up to date and that you have enough of it.

Use indoor air cleaners and make sure to check the filter on portable air cleaners and on your HVAC system. After just one bad wildfire smoke episode, the filter can be filled with particulates and needs to be changed. Get the highest MERV (minimum efficiency reporting value) rating on your HVAC filter that your system can handle.��

On very smoky days, people with pre-existing conditions or those who are more sensitive should wear an N95, KN95 or KF94 mask (good for smaller sizes or children) that fits them well.

What if the fire is really far away? Should I worry?

A study from researchers at Colorado State University found there were more asthma hospitalizations along the Front Range when there was wildfire smoke in the area from the Pacific Northwest than when there was smoke from a local fire.��

The difference is likely due to people being more likely to protect themselves when fires are close than when there is only smoke. That means people should take precautions when there is smoke in the area, regardless of how close the fire is.��

Should I exercise outdoors when it’s smoky?

That is a challenging question. We know from studies of cyclists that when the air pollution levels are not too high, the benefits of exercise outweigh the risk of air pollution exposure. But when the air quality gets really bad, it is challenging.

I recommend following the guidelines of the air quality index (AQI):��

• For the average person—curb outdoor exercise when levels exceed an AQI level of 150 or PM2.5 concentrations of 55 micrograms per meter cubed.
• For those with pre-existing respiratory or cardiovascular disease—the recommended levels are lower, 100 on the AQI or 35 micrograms per meter cubed PM2.5 concentration.

If you can’t see to the end of your street, that might be a time to curb your outdoor exercise.

Can the smoke get inside?

From research I have done looking at indoor and outdoor PM2.5 in normal times and during wildfire smoke episodes, we see that PM2.5 can infiltrate indoor environments—but this happens less so in newer constructions (which are less leaky or tighter) and those with central HVAC systems. Indoor spaces do protect people from outdoor smoke.��

But even with a really tight home, if the air pollution is bad for many days at a time, the smoke will get in. HEPA filters can help a lot. When it is really smoky, try to keep your windows and doors closed. But you also don’t want the indoor temperature to get so high that it is a health threat.

With heat and wildfires happening at the same time, that gets really challenging, especially for people who don’t have air conditioning. Some cities are looking into creating public clean-air shelters with air conditioning and HEPA filters.