What is an Air Quality Index (AQI)? Measuring Air Pollution

Most of us are not air quality scientists. We aren’t familiar with air quality data, nor have we spent the time learning how to interpret it. Think about it, what does air quality data actually mean? Take particulate matter, for example, the primary air pollutant measured by PurpleAir sensors. Particulate matter (PM) is typically measured in micrograms per cubic meter (µg/m³). But on their own, these values don’t provide much context. If someone told you the PM level outside was 4.5 µg/m³, would that mean anything to you? It probably wouldn’t to most people.

An AQI, or Air Quality Index, is designed to give meaning to these values, so that you can properly interperet air pollution levels and make informed decisions.

What is an Air Quality Index (AQI)?

An AQI is a scale that translates air quality data into something more understandable and actionable for the average person. For example, the US EPA AQI displays values ranging from 0 to 301 and higher. Logical breakpoints in the scale, typically in increments of 50, correspond to specific health recommendations.

If the US EPA AQI is 37, for example, air quality is considered “Good.” If it rises to 137, however, the air is considered "Unhealthy." In this way, AQIs help people quickly understand both current air quality and potential health impacts.

No single organization is responsible for the creation of AQIs. As their primary purpose is to provide health-based guidance at different levels of air quality, most have been developed by governmental agencies. A majority of AQIs available on the PurpleAir map fall into this category. For example, the US EPA AQI was created by the United States Environmental Protection Agency, and the Canadian AQHI was developed by the Canadian government.

The US EPA AQI

The US EPA AQI is defined below. Information was taken from the EPA's AQI basics page.

What Should You Do at Each AQI Level?

While the information above explains how air quality is categorized, it doesn’t describe what actions you should take at each level. Below are general recommendations associated with each AQI category.

Information taken from the EPA's Guide for Particle Pollution.

The US EPA AQI and PurpleAir

The US EPA AQI is the default AQI selection on the PurpleAir map. This means that when you view sensors in your area, the values you see are displayed using the EPA’s AQI scale. Both the numbers and colors correspond to the standard 0–300+ AQI range.

It’s important to note that the PurpleAir map, and PurpleAir sensors with colored LEDs, use a dynamic color scale. Instead of switching abruptly between colors at fixed breakpoints, sensor colors gradually shift as AQI values increase or decrease. For example, a sensor reading 49 AQI will appear more yellow than green, reflecting its proximity to the next category.

Additionally, while many AQIs are calculated using multiple pollutants, PurpleAir sensors measure only particulate matter. As a result, the AQI values shown on the PurpleAir map are calculated exclusively from PM measurements.

Air quality data can be difficult to understand without context. While pollutants like particulate matter (PM) are measured in scientific units such as micrograms per cubic meter, those numbers alone don’t clearly explain how clean or unhealthy the air is. Air Quality Indexes solve this problem by translating air quality measurements into easy-to-understand scales providing health guidance.