Air Quality Index-PM2.5

The 1977 amendments to the Clean Air Act called for daily analysis and reporting of air quality based on a uniform air quality index. The Air Quality Index (AQI) is EPA’s index for reporting air quality.

The higher the AQI value, the greater the level of air pollution, and the greater the health concern. For example, an AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality. The AQI for PM2.5 is an average value based on 24-hour concentrations.

To create the AQI, raw sensor measurements are converted into a separate AQI value for each of five major pollutants (ground-level ozone, particle pollution, carbon monoxide, nitrogen dioxide, and sulfur dioxide). The highest of these AQI values are reported as the AQI value for that day, the primary pollutant. Each component is normalized to the same scale with a health threshold of 100. The AQI is not a composite metric.

For each pollutant, an AQI value of 100 generally corresponds to an ambient air concentration that equals the level of the short-term national ambient air quality standard for the protection of public health.

The chart below shows the relationship between the calculated PM2.5 AQI value and the particulate matter concentration measured in ug/m3:

Values above 250ug/m3 or an AQI value of 300 is considered hazardous.

Particulate Matter (PM): PM2.5 describes fine inhalable particles, with diameters that are generally 2.5 micrometers and smaller.