Most people have seen an AQI number at some point — on a weather app, a news alert, or a widget on their phone. But knowing the number and knowing what to do with it are two different things.
This is a practical guide. By the end, you'll know when to go outside, when to stay in, what to wear, and what to do inside when the air is bad.
The number tells you about right now
The AQI is a snapshot of current conditions, not a long-term average. A 42 this morning means the air is clean right now. It doesn't mean it'll stay that way — a wildfire 200 miles away, an afternoon traffic peak, or a change in wind can push it above 100 by lunchtime.
Checking once at 8am and assuming it holds all day is one of the most common mistakes. Check it before you head out, especially if you're planning outdoor exercise, yard work, or leaving kids outside.
What the numbers actually mean
0–50: Go ahead.
The air is fine. Exercise outside, open your windows, let the kids play. Nothing to think about.
51–100: Mostly fine, with one exception.
For most people, no change needed. But if you have asthma, a heart condition, or serious allergies, pay attention. You might want to cut a long run short or avoid the part of the day when AQI peaks (usually mid-afternoon for ozone, morning rush hour for PM2.5).
101–150: Sensitive groups, take action.
If you have asthma, COPD, a heart condition, or you're pregnant — move strenuous activity inside. This level won't cause problems for healthy adults on a quick walk, but it will for someone whose lungs or heart are already working harder than average. Kids and older adults fall in the sensitive group too.
151–200: Everyone should notice this.
At this level, healthy adults will start to feel it — a scratchy throat, dry eyes, or that slightly off feeling in your chest after being outside. Limit outdoor exercise. Keep windows closed. If you have a HEPA air purifier, run it.
201–300: Stay inside.
This is the level where you'd smell smoke or see haze even on a sunny day. Don't exercise outdoors. If you have to go outside, wear an N95 or KN95 — cloth masks and surgical masks don't filter the fine particles that cause harm at this level. Recirculate air in your car rather than pulling from outside.
301+: Treat it like an emergency.
Hazardous conditions. Stay indoors with windows and doors sealed. Run your air purifier on its highest setting. If you don't have one, makeshift options — like sealing window gaps with tape or running central AC on recirculate — can help. Check on elderly neighbors and anyone you know with a respiratory condition.
What to do inside when the air is bad
Closing your windows helps, but indoor air can still degrade during prolonged bad air events. A few things that make a real difference:
- Run a True HEPA air purifier. Size it for the room — check the CADR rating against the room's square footage.
- Avoid adding to indoor pollution. Skip candles, incense, frying in open pans, and vacuuming without a HEPA-filter vacuum. All of these push PM2.5 up indoors.
- Use recirculate mode on your HVAC. Most systems pull air from outside by default. Switch to recirculate so you're not pumping bad outdoor air in.
Smoke is a different situation
Wildfire smoke can push AQI from 30 to 250 in a few hours as a plume arrives. The warning signs before monitors update: visible haze, the smell of campfire, or eyes that start burning when you go outside.
During smoke events, the guidance is more aggressive than regular pollution. Even at 100–150, sensitive individuals should stay inside. Cloth masks don't help with smoke. Your N95 works, but only if it fits your face well — leaks around the edges defeat the purpose.
When to check
The most useful habit is a quick check before anything outdoor and prolonged — exercise, yardwork, kids at practice, morning commute with windows down. It takes five seconds and it's the only way to use the information.
You can check current AQI for any US city on LocalAirData. The number updates hourly from EPA monitoring stations across the country.