Wildfire Smoke and AQI: What the Numbers Actually Mean for Your Lungs

Quick Answer: What Is the AQI?

The Air Quality Index (AQI) converts pollutant concentrations into a single number with a color and a health message attached. EPA defines breakpoints from 0 to 500, and during severe wildfire smoke you will sometimes see values above that: AirNow calls those "beyond the AQI," and they are computed by extending the same slope. During wildfire season, the pollutant driving the number is almost always PM2.5, fine particles small enough to reach deep into your lungs.

Category	AQI	24-hr PM2.5 (µg/m³)	What It Means
Good	0–50	0.0–9.0	Little or no risk
Moderate	51–100	9.1–35.4	Unusually sensitive people may notice effects
Unhealthy for Sensitive Groups	101–150	35.5–55.4	Sensitive groups affected; most others aren't
Unhealthy	151–200	55.5–125.4	Everyone may start to feel it
Very Unhealthy	201–300	125.5–225.4	Health alert — risk is increased for everyone
Hazardous	301+	225.5+	Emergency conditions

Those PM2.5 numbers are the current EPA breakpoints, tightened in May 2024 — which means an AQI of 75 today represents cleaner air than an AQI of 75 did a few years ago. If you're comparing against a memory of a past smoke event, the goalposts moved.

The Scale Is Not a Ruler

Here's the part almost nobody is told: equal steps on the AQI are not equal steps in pollution. The index is stretched wide at the clean end and squeezed tight at the dirty end.

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Going from AQI 50 to AQI 100 means the air picked up roughly 26 µg/m³ of extra fine particles. Going from 150 to 200 — the same 50 points — means it picked up about 70 µg/m³, more than two and a half times as much. The higher you go, the more each point costs you.

The practical takeaway: don't reason about smoke in AQI points. "It only went up 40 today" means something completely different at 60 than it does at 180.

Why PM2.5 Is the Number That Matters

PM2.5 means particulate matter 2.5 micrometers across or smaller — roughly one-thirtieth the width of a human hair. Size is the entire problem.

Your nose and throat catch larger particles like dust and pollen. PM2.5 sails straight past those defenses, down into the alveoli where oxygen crosses into your blood, and the smallest fraction crosses with it. That's why smoke exposure shows up not just as coughing but as heart attacks, strokes, and emergency-room visits for people who don't have any lung condition at all.

Wildfire smoke may also be worse than its mass suggests. A study of Southern California hospital admissions found that PM2.5 specifically from wildfire smoke was associated with respiratory admissions rising far more steeply — by some estimates up to ten times more — than the same mass of ordinary urban PM2.5. Smoke isn't just dust; it's a chemical soup of partially combusted organic material, and increasingly of burned structures, vehicles, and plastics when fires reach towns.

And it travels. Smoke from Canadian fires has pushed AQI values past 400 in New York City, more than a thousand miles downwind, under skies that were otherwise perfectly pleasant. Research published in 2023 found smoke has erased a meaningful share of the air-quality gains that decades of the Clean Air Act delivered across much of the country. A clear forecast is not an all-clear.

The AQI Is a Maximum, Not an Average

The reported AQI isn't a blend of pollutants. It's the single highest sub-index among ozone, PM2.5, PM10, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Whichever is worst becomes "the" AQI, and the rest are invisible.

This means the same number can describe two very different days:

AQI 130 driven by…	What's happening	When it's worst
PM2.5	Smoke or haze — particles you inhale deep	Often overnight and early morning
Ozone	A hot, sunny, stagnant day cooking pollution into smog	Mid-to-late afternoon

That timing difference is genuinely useful. Ozone needs sunlight, so it builds through the day and peaks in the afternoon — an early-morning run is the safe move. Smoke behaves in reverse: as the sun goes down the atmosphere's mixing layer collapses, and particles that were spread through a deep column get concentrated into the shallow air you're actually breathing. In valleys, cool drainage winds pool it further. On a smoke day, the dawn run is often the worst choice of the day.

So check which pollutant is driving the number, not just the number. A good air quality app shows the individual pollutants, not only the headline AQI.

Why the Number Sometimes Disagrees With Your Eyes

Two separate things make the AQI lag reality, and they're worth understanding before you decide the sensor must be broken.

The AQI is deliberately smoothed. Real-time AQI from monitoring networks is computed with an algorithm called NowCast, which averages up to the previous 12 hours and weights recent hours more heavily when conditions are changing fast. It exists to stop the number from flickering wildly, but the cost is lag. When a plume arrives in an hour, the AQI walks up behind it. If the air outside looks and smells much worse than the number says, believe the air.

Smoke aloft isn't smoke you breathe. NOAA's HRRR-Smoke model forecasts two different things: near-surface smoke, the concentration in the air at roughly head height, and vertically integrated smoke, the total amount in the entire column above you. High column smoke with low surface smoke gives you the eerie milky sky, the orange sun, the flat gray light — and perfectly breathable air. Both high, and the air is genuinely dangerous. This is the single most common reason people misjudge a smoke day in either direction. (Smoke plumes are easy to spot from orbit, too — our guide to reading satellite imagery covers what they look like.)

What to Actually Do at Each Level

The table below follows the general shape of AirNow's category guidance. Read it as orientation, not prescription: the right response depends on the pollutant, your own health and symptoms, how hard you're working, and what your local health authority is advising. If you have asthma, COPD, or heart disease, are pregnant, or are caring for young children or older adults, your clinician's advice beats any table.

AQI	Category	Healthy adults	Sensitive groups
0–50	Good	No restrictions	No restrictions
51–100	Moderate	Normal activity	Unusually sensitive people: watch for symptoms, consider easing long hard efforts
101–150	Unhealthy for sensitive groups	Consider shortening or easing very long outdoor workouts	Reduce prolonged or heavy outdoor exertion; keep rescue inhalers close
151–200	Unhealthy	Reduce outdoor exertion; keep windows closed	Avoid outdoor exertion; indoors with filtered air where you can
201–300	Very unhealthy	Avoid outdoor exertion; a well-fitted N95 helps if you must be out	Indoors with filtration; treat symptoms seriously
301+	Hazardous	Stay indoors where possible; N95 for any outdoor time	Follow local public-health guidance, which may include relocation advice

Symptoms outrank the number. Coughing, chest tightness, wheezing, or shortness of breath mean reduce your exposure and seek care, whatever the reading says.

Exertion is the multiplier nobody accounts for. Heavy exercise moves several times more air through your lungs than sitting does, so a hard hour at AQI 120 can deliver a bigger dose than a quiet afternoon at 180. Our AQI exposure planner works out the tradeoff for your specific AQI, sensitivity, duration, and effort level.

Masks: What Works and What Doesn't

• N95 / KN95 / P100 — yes. These filter at least 95% of fine particles when they seal. The seal is the whole product; a gap along the nose or cheeks lets air take the easy path around the filter. Facial hair along the seal line defeats it entirely.
• Surgical masks — barely. The material is reasonable, the fit isn't. Air flows around the open sides.
• Cloth masks, bandanas, wet towels — no. They do essentially nothing at the particle sizes that matter. A wet bandana is a folk remedy, not protection.

If you're outdoors in heavy smoke — walking the dog, clearing a yard, working a shift — an N95 that seals is worth far more than a better mask that doesn't.

Cleaning the Air Indoors

You'll spend most of a smoke event inside, so indoor air is where most of your dose is decided:

• Close it up. Windows and doors shut; set AC and car ventilation to recirculate so you're not pulling smoke in.
• Filter it. A portable HEPA purifier sized for the room, or a MERV-13 filter in central HVAC with the fan running continuously.
• Build one cheaply. A box fan taped to four MERV-13 filters — a Corsi-Rosenthal box — costs a fraction of a purifier and moves far more air than most of them. It's ugly and it works.
• Make one clean room. If you can't filter the whole house, filter the bedroom and sleep in it. Eight clean hours a night matters.
• Stop adding particles. No candles, incense, smoking, or gas stoves. Skip the pan-frying. Don't vacuum without a HEPA filter — you'll just relaunch settled dust.

Watch the heat, too. "Close every window" collides badly with a hot afternoon, and smoke events often ride along with the same stagnant high pressure that builds a heat dome. If you have to choose between smoke and dangerous indoor heat, heat is the faster threat — see our extreme heat guide for how quickly that one turns serious.

Who's Most at Risk

Smoke doesn't hit everyone equally. Take extra care with:

• Asthma, COPD, and other lung conditions — smoke is a direct trigger
• Heart disease — particles drive cardiovascular events, not just respiratory ones
• Children — they breathe more air per pound of body weight, and their lungs are still developing
• Older adults (65+) and people who are pregnant
• Outdoor workers — long hours plus exertion is the worst possible combination
• Anyone without filtration at home, who gets no break from the exposure

Always Verify With Official Sources

Smoke conditions shift hour to hour. Use authoritative sources when it matters: AirNow for official AQI, fire.airnow.gov for the combined smoke and fire map including temporary monitors near active fires, and your state or local health department for guidance specific to your area.

Track the Air With WeatherAI

WeatherAI puts air quality next to the rest of your forecast, on your iPhone:

• Real-time AQI plus the individual pollutants — PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide — so you can see what is driving the number
• Hourly updates and a 5-day air quality forecast, to plan the run, the ride, or the yard work around the smoke
• Active wildfire tracking with fire perimeters from CalFire and WFIGS, so you know what's burning upwind
• AI-powered answers — ask "Is it safe to run outside today?" and get a plain-English answer for your actual conditions
• An AQI exposure planner that factors in your sensitivity, how long you'll be out, and how hard you'll be breathing

Smoke season rewards planning over guessing. Know what you're breathing before you head out the door.

Download WeatherAI for iPhone →

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Keep learning about summer weather hazards with our guides to the heat index and extreme heat safety, what a heat dome is, and how to read satellite imagery.