When looking into air quality, you’ll commonly see two phrases: Raw PM and AQI. They both refer to air quality, but mean two different things.
Raw PM (Particulate Matter) values describe the mass of particulates in a given volume of air expressed as micrograms per cubic meter (µg/m³). These are raw numbers, meaning they are taken directly from the Plantower laser counters in PurpleAir devices. The laser counters estimate these mass concentration values using formulas called CF=1 and ATM, which you can learn about here: What is the Difference Between CF=1, ATM, and ALT?
On the PurpleAir Map, raw PM2.5 is displayed using a 0 → 250 scale as seen below. This is the same scale provided by the EPA in their AQI Technical Assistance Documentation (pp. 4-5).
An AQI, or Air Quality Index, is a measure of health created by a government body. An AQI value is calculated using the raw measurements mentioned above. The purpose of an AQI is to take raw PM measurements and apply them to a scale that directly indicates the impact of the air quality on human health. This turns the raw data into a measurement that can be useful and insightful for the average citizen.
Without much needed context, raw PM readings will appear as irrelevant numbers, with inconsequential meanings. An AQI, however, provides that context. Put differently, an AQI can tell you if the air is healthy or unhealthy, and to what degree.
There is no single best AQI, as different countries and regulatory agencies have different standards by which they judge air quality. By default, the PurpleAir Map displays the US EPA PM2.5 AQI, which was created by the United States Environmental Protection Agency. However, you can select from a number of different AQI scales, based on your preference or location. Information about other AQI scales can be found in this article: Air Quality Index Data Layers.
The PurpleAir Map displays the US EPA PM2.5 AQI by default. This AQI scale displays values from 0 → 300 as seen below. More information can be found on the US EPA website.