Air Quality

Air quality is based on the amount of pollution that is in the air. Air pollution can be made up many different pollutants and can come from many sources, such as combustion engines, factories, power plants, construction, agriculture and naturally occuring sources, such as volcanic eruptions.

The EPA actively monitors six principal pollutants:

  • particulate matter smaller than 2.5 microns (PM2.5)
  • particulate matter smaller than 10 microns (PM10)
  • sulfur dioxide (SO2)
  • nitrogen dioxide (NO2)
  • ozone (O3)
  • carbon monoxide (CO)

The EPA evaluates the averages of these pollutants to come up with an area’s Air Quality Index (AQI) score. Each principal pollutant is given its own AQI score, and the single highest AQI score among the the six pollutants determines the AQI for a given area.

EPA Air Quality Index

AQI Value Suggested Actions
Good (0 - 50) None.
Moderate (51 - 100) Unusually sensitive people should consider reducing prolonged or heavy exertion.
Unhealthy for Sensitive Groups (101 - 150) People with heart or lung disease, children, and older adults should reduce prolonged or heavy outdoor exertion
Unhealthy (151 - 200) People with heart or lung disease, children, and older adults should reduce prolonged or heavy outdoor exertion
Very Unhealthy (201 - 300) People with heart or lung disease, children, and older adults should avoid all physical activity outdoors. Everyone else should avoid prolonged or heavy exertion.

Particulate Matter (PM)

SensorGrid specifically measures particulate matter (PM), which affects more people than any other pollutant, according to the World Health Organization.

PM are small, inhalable particles that are suspended in the air. They can be a mixture of solid and liquid particles of organic or inorganic substances. Components of PM can include of ammonium, sodium chloride, sulfates, nitrates, carbon, mineral dust, water and hundreds of other different chemicals.


PM can enter the environment through natural causes, such as wildfires, but the majority of PM is caused by human activity. Particulate matter is released by sources such as construction sites, agriculture, combustion engines, unpaved roads, factories and the burning of coal. Even everyday products such as hairspray and incense can produce potentially harmful particulate matter.


PM is a particularly dangerous pollutant because of its extremely small size. When inhaled, it is small enough that it can be absorbed into the bloodstream. Chronic exposure to particulate matter has been attributed to increased risk of death due to heart disease, lung cancer and stroke.

In addition to being harmful to human health, PM can also negatively impact the environment. Depending on its chemical composition, PM can deplete nutrients in soil, increase the acidity of bodies of water, damage crops, affect ecosystems and contribute to acid rain.

Measuring PM

PM Size Illustration
Relative size of particulate matter.

PM is measured in units of micrograms per cubic meter (µg/m³) and is usually measured in two size categories: 1) PM10, or particles 10 micrometers in diameter or smaller, and 2) PM2.5, or particles 2.5 micrometers in diameter or smaller.

According to the EPA, the AQI scores for particulate matter correspond as follows:

PM 10
An AQI of 100 corresponds to 150 micrograms per cubic meter (averaged over 24 hours).
PM 2.5
An AQI of 100 corresponds to 35 micrograms per cubic meter (averaged over 24 hours).