What is a safe AQI level?

Green, 0 to 50, poses little or no risk. Yellow, 51 to 100, is acceptable for most people. At orange, 101 to 150, sensitive groups including children, older adults, and people with asthma should limit outdoor exertion.

Who counts as a sensitive group for air quality?

People with asthma or other lung conditions, people with heart disease, older adults, children, and anyone pregnant. That covers a large share of most households, so orange days are worth acting on.

Do masks help with wildfire smoke?

A well-fitted N95 filters the fine particles in smoke. Cloth and surgical masks do not. Indoors, a HEPA air cleaner in the bedroom and a MERV 13 furnace filter both reduce particle levels.

How to check the air quality near you, and what the numbers mean

Orange is the number that matters for most households. Here is what the AQI colors actually mean.

Smoke haze and hot, still summer days both put the same question in front of people: is it safe to go outside right now? The answer has a number, and the number is free to look up.

The Air Quality Index is a scale from 0 to 500 maintained by the Environmental Protection Agency. Look it up by ZIP code at AirNow.gov, or in most weather apps, which pull the same government data.

The colors

AirNow publishes six categories, and the color is usually all you need.

Green, 0 to 50, Good. "Air quality is satisfactory, and air pollution poses little or no risk."
Yellow, 51 to 100, Moderate. "Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution."
Orange, 101 to 150, Unhealthy for Sensitive Groups. "Members of sensitive groups may experience health effects. The general public is less likely to be affected."
Red, 151 to 200, Unhealthy. "Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects."
Purple, 201 to 300, Very Unhealthy. "Health alert: The risk of health effects is increased for everyone."
Maroon, 301 and above, Hazardous. "Health warning of emergency conditions: everyone is more likely to be affected."

Sensitive groups means people with asthma or other lung conditions, people with heart disease, older adults, children, and anyone pregnant. That's a larger share of any household than people assume, which is why orange is the number most families should watch for rather than red.

What the number is made of

The AQI combines several pollutants and reports whichever is worst at that moment. On a smoky day it's fine particle pollution, written as PM2.5, which is small enough to get deep into the lungs. On a hot, sunny, stagnant day it's more often ground-level ozone, which peaks in the afternoon and eases after sunset.

That difference changes the advice. Ozone days are better early in the morning. Smoke days aren't better at any hour.

On a bad air day

Move hard exercise indoors or to a cleaner day, since heavy breathing pulls more of everything into your lungs. Keep windows closed and run air conditioning on recirculate. A portable air cleaner with a HEPA filter in the room where people sleep does real work, and a furnace filter rated MERV 13 helps if your system can handle it.

If you go out in heavy smoke, a well-fitted N95 filters fine particles. A cloth or surgical mask does not.

Anyone with asthma should keep rescue medication close and follow the action plan their clinician set. Smoke is a common trigger for symptoms that show up hours later, not right away.

Wildfire smoke can travel thousands of miles, so an orange day in Chicago or New York often has nothing to do with anything burning nearby. Checking the number beats checking the sky, because moderate smoke frequently looks like ordinary haze.