How to Read the US Air Quality Index

The US Air Quality Index (AQI) is the US Environmental Protection Agency’s (EPA) scale for reporting how polluted the air is, from 0 to 500. An AQI of 100 corresponds to the national standard for a pollutant; above 100, air quality is unhealthy, first for sensitive groups and then for everyone. The higher the number, the greater the health concern.

This article is general information, not medical advice.

The Categories

The EPA’s AirNow describes six categories, each with a colour:

AQI	Category	Colour	What it means
0 to 50	Good	Green	Little or no risk
51 to 100	Moderate	Yellow	Acceptable; a risk for some unusually sensitive people
101 to 150	Unhealthy for Sensitive Groups	Orange	Sensitive groups may have health effects
151 to 200	Unhealthy	Red	Some of the public may have effects; sensitive groups more serious ones
201 to 300	Very Unhealthy	Purple	Health alert: higher risk for everyone
301 and above	Hazardous	Maroon	Emergency conditions; everyone is likely to be affected

Sensitive groups include people with heart or lung disease, such as asthma, older adults, children and teenagers, and pregnant people.

One Number for Several Pollutants

The AQI is calculated separately for each major pollutant: ozone, fine particles (PM2.5), coarse particles (PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each pollutant’s concentration is converted to its own index, and the reported AQI is the highest of them, so it reflects the worst pollutant at the time. The responsible pollutant is usually named alongside.

NowCast

Health standards are set as averages over 8 or 24 hours, but conditions can change within an hour during wildfire smoke or summer smog. AirNow reports current air quality with the NowCast, the EPA’s method for relating hourly readings to the daily AQI: it uses longer averages when air quality is stable and shorter ones when it’s changing fast, such as during a wildfire.

What to Do

• Moderate: unusually sensitive people may want to cut back on long or heavy outdoor exertion.
• Unhealthy for sensitive groups and above: sensitive groups should reduce outdoor exertion; at “Unhealthy” and above, everyone should.
• Indoors: keep windows closed during smoke events and use an air cleaner, but watch indoor CO₂; see What Counts as a Healthy CO₂ Level at Home?

Other countries use their own indices with different scales.

Following It in Kaizora

US Air Quality Index is on Air Quality. The page’s view of the air around you now shows outdoor air from the nearest public stations, through the EPA’s AirNow (in the US, Canada, Mexico, and at US embassies), with the AQI as the highest of the station’s pollutants’ AQIs, NowCast for ozone and particles, named by its EPA category in the EPA’s colours. It’s preliminary data, labelled so, crediting the agency. Station data is never your own exposure: a station kilometres away describes the air outdoors there. See What Is PM2.5?

The Short Version

• The AQI runs from 0 to 500; 100 matches the national standard.
• Good is 0 to 50, Moderate 51 to 100, and above 100 is unhealthy, first for sensitive groups.
• The reported AQI is the worst pollutant’s; NowCast keeps it current.

Sources

• AirNow, US Environmental Protection Agency, AQI Basics.
• AirNow, US Environmental Protection Agency, Using Air Quality Index.
• US Environmental Protection Agency, Technical Assistance Document for the Reporting of Daily Air Quality – the Air Quality Index (AQI).