What is AQI?
The Air Quality Index (AQI) is a standardized number used by government agencies to communicate how polluted the air currently is or how polluted it is forecast to become.
How AQI is Calculated
The AQI is calculated based on the concentrations of five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide.
For each pollutant, the EPA has established national air quality standards to protect public health. The AQI converts pollutant concentrations to a scale from 0 to 500, where higher values indicate greater health risks. The overall AQI for a location is the highest individual sub-index value.
Key fact
An AQI of 100 generally corresponds to the national ambient air quality standard for each pollutant. Above 100, the air quality is considered unhealthy — first for sensitive groups, then for the general population.
AQI Scale
Key Pollutants Measured
- PM2.5
- Fine inhalable particles with diameters 2.5 micrometers and smaller. These particles can penetrate deep into the lungs and enter the bloodstream.
- PM10
- Inhalable particles with diameters 10 micrometers and smaller, including dust, pollen, and mold.
- Ozone (O₃)
- Ground-level ozone is a main ingredient in smog and is formed by reactions between sunlight and pollutants from vehicles and industry.
- Nitrogen Dioxide (NO₂)
- A reddish-brown gas produced by burning fossil fuels, primarily from vehicles and power plants.
- Sulfur Dioxide (SO₂)
- A colorless gas with a sharp odor, produced mainly by burning coal and oil.
- Carbon Monoxide (CO)
- An odorless, colorless gas produced by incomplete combustion of fossil fuels.
Frequently Asked Questions
What AQI is safe to be outside?
AQI 0–100 is generally safe for most people. At 101–150, sensitive groups (asthma, heart disease, older adults, children) should limit prolonged outdoor exertion. Above 150, everyone should reduce time outside.
What is the difference between AQI and PM2.5?
PM2.5 is one specific pollutant. AQI is a composite index that includes PM2.5 plus ozone, PM10, NO₂, SO₂, and CO. In most cities, PM2.5 is the primary driver of the AQI.
Does a higher AQI always mean the air is more dangerous?
Yes — a higher AQI corresponds to greater health risk. However, the specific risks vary by pollutant. High ozone mainly affects the respiratory system; high PM2.5 affects both lungs and the cardiovascular system.