The air quality index (AQI) is an index for reporting air quality on a daily basis. It is a measure of how air pollution affects one's health within a short time period. The purpose of the AQI is to help people know how the local air quality impacts their health. The Environmental Protection Agency (EPA) calculates the AQI for five major air pollutants, for which national air quality standards have been established to safeguard public health.

1. Ground-level ozone
2. Particle pollution/particulate matter (PM2.5/pm 10)
3. Carbon Monoxide
4. Sulfur dioxide
5. Nitrogen dioxide
The higher the AQI value, the greater the level of air pollution and the greater the health concerns. The concept of AQI has been widely used in many developed countries for over the last three decades. AQI quickly disseminates air quality information in real-time.
How is AQI calculated?
Different countries use different point scales to report air quality. For instance, the United States uses a 500 point scale, wherein rating between 0 and 50 is considered good. Rating between 301 to 500 range is deemed hazardous. India too follows that the 500 point scale. Every day monitors record concentrations of the major pollutants. These raw measurements are converted into a separate AQI value for each pollutant (ground-level ozone, particle pollution, carbon monoxide, and sulfur dioxide) using standard formulae developed by EPA. The highest of these AQI values are reported as the AQI value for that day.

Air Quality Index Categories
Good (0–50) - Minimal Impact
Satisfactory (51–100) - May cause minor breathing difficulties in sensitive people.
Moderately polluted (101–200) - May cause breathing difficulties in people with lung disease like asthma, and discomfort to people with heart disease, children and older adults.
Poor (201–300) - May cause breathing difficulties in people on prolonged exposure, and discomfort to people with heart disease
Very Poor (301–400) - May cause respiratory illness in people on prolonged exposure. Effect may be more pronounced in people with lung and heart diseases.
Severe (401-500) - May cause respiratory issues in healthy people, and serious health issues in people with lung/heart disease. Difficulties may be experienced even during light physical activity.
Why is AQI important?
Awareness of daily levels of air pollution is important, especially for those suffering from illnesses caused by exposure to air pollution.
Objectives of Air Quality Index (AQI)
  • Comparing air quality conditions at different locations/cities.
  • It also helps in identifying faulty standards and inadequate monitoring programmes.
  • AQI helps in analysing the change in air quality (improvement or degradation).
  • AQI informs the public about environmental conditions. It is especially useful for people suffering from illnesses aggravated or caused by air pollution.
Who is most at risk from air pollution?
— People with lung diseases, such as asthma, chronic bronchitis, and emphysema
Children, including teenagers
Active people of all ages who exercise or work extensively outdoors
Some healthy people are more sensitive to ozone