Wildfire smoke

What wildfire smoke is made of

Wildfire smoke is a moving mix of gases and fine particles. It contains carbon monoxide, nitrogen oxides, volatile organic compounds, and the pollutant that concerns health officials most, fine particulate matter known as PM2.5.

PM2.5 particles measure 2.5 micrometers or smaller, roughly 30 times thinner than a human hair, the EPA reports. That size is the whole problem. The particles are small enough to slip past the nose and throat, travel deep into the lungs, and pass into the bloodstream. The EPA identifies PM2.5 as the main health threat from wildfire smoke, because it reaches places in the body that larger particles never touch.

Smoke does not respect distance. When a fire burns hot, it lifts particles high into the atmosphere, where wind carries them across states and even oceans. A fire in California can dim the sun in Colorado, and a fire in Canada can foul the air in Georgia.

That reach is why wildfire smoke is a shared exposure rather than a local one. The National Oceanic and Atmospheric Administration tracks smoke plumes by satellite as they drift across the continent, and researchers have found that a growing share of PM2.5 pollution in the eastern United States now traces back to fires burning thousands of miles away in the West and in Canada.

How smoke gets inside your home

Closing the door does not keep smoke out. Fine particles drift indoors through open windows, doorways, gaps around vents, and the small cracks in every building, so on a heavy smoke day the air in your living room follows the air on your street.

Once inside, those particles stay. Indoor air moves slowly, so smoke that gets in has nowhere to go and can linger for hours after the outdoor air starts to clear. The EPA notes that indoor particle levels rise during smoke events, and homes with older windows or weak seals see the sharpest jump. That is why a smoky day outside becomes a smoky day inside unless you take steps to filter the air.

What wildfire smoke does to the body

The effects start fast. Breathing wildfire smoke can cause coughing, a scratchy throat, burning eyes, headaches, and shortness of breath within hours, the CDC reports. For people with asthma or heart disease, even a short exposure can trigger an attack or a cardiac event.

The deeper danger is what smoke does over time. Long-term exposure to PM2.5 is linked to reduced lung function, chronic heart and lung disease, and earlier death, according to the World Health Organization. Children breathe faster and take in more air for their size, older adults have less reserve, and people with existing heart or lung conditions have the least room to absorb the strain. A 2021 study in The Lancet Planetary Health estimated that wildfire smoke contributes to more than 30,000 deaths worldwide each year, a number that climbs as fire seasons grow longer.

Why it matters more each year

Fire seasons are getting longer and hotter. The U.S. Forest Service reports that the average wildfire season now runs about 78 days longer than it did in the 1970s, and the acreage burned each year has more than doubled since the 1980s. More fire means more smoke, and more smoke reaching more people.

The trend changes who is exposed. Smoke that once affected a handful of western communities now blankets major cities across the country several times a year. For millions of households, a few smoky days each summer has become part of the calendar, which makes knowing how to protect the air at home a year-round concern rather than an emergency skill.

What a household can do

Check the air before you plan your day. The Air Quality Index at AirNow.gov reports local particle levels in real time and tells you when smoke has reached a level worth staying inside for. When the number climbs into the orange range or higher, keep outdoor activity short and skip hard exercise outside.

Seal the house and clean the air indoors. Close windows and doors, set your air conditioner to recirculate so it stops pulling smoke in, and run a portable air cleaner with a HEPA filter sized for the room, since HEPA filtration captures the fine particles smoke carries. Upgrade your furnace filter to a MERV 13 rating if the system can handle it. If you need to be outside in heavy smoke, wear an N95 respirator, which the CDC says filters fine particles when it fits snugly, unlike a cloth or surgical mask. On the worst days, pick one room, close it off, and run a filter there to create a clean air space where the whole household can breathe easier.