What is an Air Quality Index (AQI)?

by Kieran Berryman December 26, 2024

An Air Quality Index (AQI) is a scale that conveys the health impact of air quality. Air pollutants are typically expressed using different units of measurement. For example, you’ll see particulate matter in units of micrograms per cubic meter or µg/m3. This is a value that describes the mass concentration of PM. But if someone told you that PM outside was at 4.5 µg/m3, would that mean anything to you? It wouldn't to me.

For the vast majority of people, it’s like you’re being told something without context. A number doesn’t mean anything if you don’t know what it is supposed to mean. This is what an AQI is designed to do. It takes the µg/m3 value (or whatever measurement the pollutant is in) and turns it into something meaningful. If you look at the US EPA AQI, for example (which we’ll talk more about below), you’ll see that it displays values from 0 - 301 and higher. Logical breakpoints in the scale (increments of 50) correspond with health recommendations. If the US EPA AQI is at 37, for instance, air quality is “good.” But if it’s at 137, the air is “unhealthy.”

AQIs allow you to process air quality data in a way that makes sense and assists you in making relevant decisions about your health.

The US EPA Air Quality Index (AQI)

While there are a number of AQIs available on the PurpleAir map, the default selection is the US EPA AQI. This is an air quality index created by the United States Environmental Protection Agency, and its goal, similar to that of other AQIs, is to convey health-based air quality information to the public.

1 - 50

Air quality is satisfactory, and air pollution poses little or no risk.

51 - 100

Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.

101 - 150

Members of sensitive groups may experience health effects. The general public is less likely to be affected.

151 - 200

Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.

201 - 300

Health alert: The risk of health effects is increased for everyone.

301 and higher

Health warning of emergency conditions: everyone is more likely to be affected.

The US EPA AQI uses a tiered and colored system to reflect the health effects of current air pollution. The first tier is green for good air quality, and the last tier is maroon for hazardous air quality. This association with colors allows for a quick and easy understanding of air quality.

The US EPA AQI and PurpleAir

As mentioned above, the EPA AQI is the default selection on the PurpleAir map. This means that as you navigate to the map and look at the sensors in your area, you'll be looking at EPA AQI data. The numbers correspond with the 0 - 300 scale, and the colors do as well. However, it is important to note that the PurpleAir map, as well as sensors with colored LEDs, use a dynamic color system that gradually changes the color of a sensor as its AQI value increases or decreases. This means that if your sensor is at 49 on the AQI, it is going to be much more yellow than green.

Additionally, while AQIs often take multiple pollutants into account, PurpleAir sensors only measure particulate matter. So, when you look at AQI data on the PurpleAir map, those numbers are being produced using only PM values.

Check out the PurpleAir map to view EPA AQI data from sensors near you and expand your knowledge of air quality!