What's Actually in Wildfire Smoke (And What You Can Do About It)

Photo by Johel Espana on Unsplash

When wildfire smoke rolls in, most people make a simple decision: is it bad enough to stay inside? The honest answer depends on what is actually in the air, and one number tells you most of what you need to know.

PurpleAir sensors measure that number. It is called PM2.5, and during a wildfire it is the single best real-time indicator of how risky the air is to breathe.

What PM2.5 Is

PM2.5 is shorthand for fine particulate matter with a diameter of 2.5 micrometers or less, roughly 30 times smaller than the width of a human hair. The particles are small enough to bypass your nose and throat, slip deep into the lungs, and in some cases pass directly into the bloodstream.

The U.S. Environmental Protection Agency links short-term PM2.5 exposure to:

• Asthma attacks and worsened breathing
• Irregular heartbeat and heart attacks
• Reduced lung function in children and adults
• Premature death in people with heart or lung disease

The World Health Organization attributes roughly 4.2 million premature deaths globally each year to ambient air pollution, with fine particulate matter the main contributor.

During wildfires, PM2.5 concentrations can spike from the low single digits to several hundred micrograms per cubic meter in a matter of hours. That is the part of smoke your body feels most directly, and the part you can act on right away.

Smoke Is More Than PM2.5, But PM2.5 Tells You Most of the Story

Wildfire smoke is a complex chemical mixture. According to the U.S. CDC and EPA's wildfire smoke guide, smoke can contain:

• Fine and ultrafine particles (PM2.5 and smaller)
• Carbon monoxide and carbon dioxide
• Nitrogen oxides
• Volatile organic compounds (VOCs)
• Aldehydes and other irritant gases
• Trace amounts of heavy metals when structures and vehicles burn

These other components matter, and researchers continue to study how they affect long-term health. But for the day-to-day question of "is it safe for my kid to play outside right now?" PM2.5 remains the strongest, best-validated, most-actionable indicator. Health agencies in the U.S., E.U., Canada, Australia, and the WHO have all built their wildfire smoke guidance around PM2.5 concentrations.

It is also the pollutant that low-cost community sensors can measure reliably and in real time. That combination, broad scientific support and dense local coverage, is why PM2.5 dominates the wildfire smoke story.

What the Numbers Mean

Most U.S. tools, including the PurpleAir map, express PM2.5 using the EPA's Air Quality Index (AQI). The categories are color-coded:

• Green (0-50): Good. Air quality is satisfactory.
• Yellow (51-100): Moderate. Unusually sensitive people may want to limit prolonged outdoor exertion.
• Orange (101-150): Unhealthy for sensitive groups. People with asthma, heart disease, or other lung conditions, plus children and older adults, should reduce outdoor activity.
• Red (151-200): Unhealthy. Everyone may begin to feel effects. Sensitive groups should avoid outdoor activity.
• Purple (201-300): Very Unhealthy. Health warnings for everyone.
• Maroon (301+): Hazardous. Emergency conditions. Everyone should stay indoors.

These categories were designed for general exposure, not specifically for wildfire smoke. The EPA notes that smoke from wildfires can drive PM2.5 levels into the Purple and Maroon ranges during severe events, and that even brief exposure at those levels can have measurable health effects.

If you want a walk-through of each category with a little more personality, our Jedi's Guide to the AQI breaks down what each color means and who it affects most.

How to Use PurpleAir During a Smoke Event

A few practical patterns make the data more useful when it matters most:

• Check your immediate area, not just the regional forecast. Smoke distribution is rarely uniform. Two neighborhoods five miles apart can be in different AQI categories at the same time. Zoom in on the PurpleAir map and look at sensors within a mile or two of your location.
• Watch the trend, not just the snapshot. A reading of 80 climbing fast tells you something different than a reading of 80 that has been steady for hours. Tap a sensor to see its recent history.
• Use the EPA conversion when available. PurpleAir offers an "EPA" data layer that applies the agency's correction equation for wildfire smoke, which more closely matches what regulatory monitors would report.
• Cross-check with official forecasts. AirNow and your state or local air agency issue smoke and AQI forecasts for the next 24-48 hours. Pair their forecast with your community sensors' real-time readings.
• Re-check before opening windows. Surface-level smoke often spikes overnight when air cools and stabilizes. A clear afternoon does not always mean a clear evening.

What to Do When PM2.5 Climbs

Recommendations from the EPA wildfire smoke guide scale with the AQI category:

• Orange / Red: Limit prolonged outdoor exertion. Close windows. Run HVAC on recirculate with a high-grade filter (MERV 13 or higher when possible). Sensitive groups should stay indoors as much as practical.
• Purple / Maroon: Stay indoors. Use a portable HEPA purifier in the room where you spend the most time. A box fan paired with a furnace-grade filter is a low-cost backup that can dramatically lower indoor PM2.5.
• Outdoors when you have to be: A well-fitted N95 or KN95 filters fine particles. Cloth and surgical masks do not.
• All categories: Watch infants, older adults, pregnant people, and anyone with asthma, COPD, or heart disease. They feel the effects first and recover the slowest.

The Bottom Line

Wildfire smoke is more complicated than any single number. But during an active smoke event, you do not need to track every chemical in the plume to make smart decisions. You need a number that updates fast, reflects what your body is actually breathing, and lines up with the guidance health agencies give for protecting yourself.

That number is PM2.5, and the PurpleAir map puts thousands of community-level readings at your fingertips for free.

When the air looks hazy or the smell of smoke arrives, check your block. Then decide.

PurpleAir Map · Air Quality Monitors · EPA AirNow · EPA Wildfire Smoke Guide