Air Quality Index (AQI) Explained

The Air Quality Index, or AQI, turns measurements of several different air pollutants into a single number from 0 to 500 with a color attached. It was designed so that you don't need to know what a microgram per cubic meter of fine particulate matter is to decide whether to go for a run. This guide explains what the number means, which pollutants drive it, and how weather makes the difference between a clear day and an air quality alert.

The AQI scale

The U.S. Environmental Protection Agency sets the AQI categories. A value of 100 corresponds to the national health standard for a pollutant, so anything above 100 means the air is worse than the standard for at least one pollutant.

The reported AQI is the highest of the individual pollutant values, not an average. If ozone is at 45 and fine particles are at 130, the AQI is 130 and the "main pollutant" is listed as particulate matter.

The pollutants behind the number

The AQI covers five pollutants regulated under the Clean Air Act, but two drive almost all of the bad air days in the United States:

• Ground-level ozone. Ozone high in the stratosphere protects us from ultraviolet light, but ozone at ground level is a lung irritant. It isn't emitted directly. It forms when nitrogen oxides (largely from vehicles and power plants) and volatile organic compounds react in strong sunlight. That's why ozone is a summer, afternoon problem: it builds through the day, peaks in mid-to-late afternoon, and drops overnight. Hot, sunny, stagnant days in and downwind of cities are the classic setup.
• Fine particulate matter (PM2.5). Particles 2.5 micrometers or smaller, about 1/30th the width of a human hair, are small enough to travel deep into the lungs and even into the bloodstream. Sources include wildfire smoke, vehicle exhaust, industry, and wood burning. Unlike ozone, PM2.5 can be high at any hour and in any season. Winter inversions trap wood smoke in valleys, and wildfire smoke can cover half the country for days.

The other three, coarser particles (PM10), carbon monoxide, and sulfur dioxide, rarely drive the AQI except near specific sources such as dusty roads, heavy industry, or volcanic areas.

How weather makes bad air days

Emissions don't change much from one day to the next, but air quality can swing from Good to Unhealthy overnight. The difference is almost always the weather.

• Stagnant high pressure. Under a strong high-pressure system, winds are light and air sinks, acting like a lid that keeps pollution close to the ground. Several stagnant days in a row are when ozone and particles accumulate.
• Temperature inversions. Normally air cools with height and rising warm air mixes pollution upward. In an inversion, a layer of warmer air sits above cooler air at the surface and blocks that mixing. Inversions are strongest on clear, calm nights and in valleys.
• Heat and sunshine speed up the chemistry that makes ozone, which is why ozone alerts so often coincide with heat waves.
• Fronts and rain clean the air. A cold front's stronger winds sweep pollution away, and rain washes particles out of the air. The day after a front passes is often the cleanest air of the week. See Weather Fronts Explained.
• Upper-level winds carry smoke. Wildfire smoke can travel thousands of miles. Smoke high in the atmosphere turns sunsets red without affecting what you breathe; it only matters for health when it mixes down to the surface. That's why a hazy sky and the AQI don't always agree.

Wildfire smoke

Wildfire smoke has become the most common cause of the highest AQI readings in the United States. It's mostly PM2.5, and it can push the AQI into Very Unhealthy or Hazardous territory far from any fire. On smoky days:

• Keep windows and doors closed, and run central air on recirculate if you have it.
• A portable HEPA air purifier in a bedroom can make one room much cleaner. Avoid frying, candles, and vacuuming, which add particles indoors.
• A well-fitted N95 or KN95 respirator filters smoke particles; cloth and surgical masks do not.
• Reschedule strenuous outdoor exercise. Heavy breathing pulls far more air, and far more particles, into your lungs.
• Check on neighbors with heart or lung conditions, who are most at risk.

Reading the air quality layer on RadarBug

RadarBug's air quality layer, part of pollen mode, draws outlines from the EPA's AirNow program using the latest hourly readings from monitors across the country.

• Only areas worse than Good are outlined. No outline means the air is in the Good range, or that no monitor data is available there.
• Orange outlines mark Moderate air quality. Note that this is a different color from the EPA's yellow, chosen so the outline stands out against the base map.
• Red outlines mark Unhealthy for Sensitive Groups or worse. If you're inside a red outline, tap inside it to see which category applies, since red covers everything from Unhealthy for Sensitive Groups upward.
• Use the air quality opacity setting to make the outlines stronger when you need them or fainter when they get in the way of other layers.

The outlines are interpolated from monitors, which are concentrated in cities. Air quality can vary within an outline, especially near highways, industrial sites, or a nearby fire. For the official forecast for tomorrow, check AirNow.gov or your state air agency, which issue "Air Quality Alert" or "Ozone Action Day" notices when bad air is expected.

Air quality and allergies

Air pollution and pollen often make each other worse. Ozone and particles inflame the airways, which can make allergic reactions to pollen stronger, and the same warm, dry, stagnant weather that raises ozone also tends to bring high pollen counts. That's why RadarBug shows the two together in pollen mode. If you're sensitive to both, check both before planning a long day outside. See Pollen Types, Seasons & Counts Explained.