The air quality index explained

Air quality reports have been happening in the US since 1976

A regular assessment of air quality in the United States started in 1976, about six years after the Environmental Protection Agency was created.

According to, the law requires any metropolitan area with a population of more than 350,000 to report air quality on a daily basis.

The Air Quality Index, better known as AQI, as we know it today was released in 1999, according to It has gone through several updates over the years, but the goal has always been the same — to offer people an easy-to-understand daily report about the air they’re breathing.

The report provides an air-quality number for five major air pollutants that are regulated by the Clean Air Act:

  • Ground-level ozone.
  • Particle pollution (also known as particulate matter).
  • Carbon monoxide.
  • Sulfur dioxide.
  • Nitrogen dioxide.

A sixth air pollutant that is monitored by the EPA is lead, but that is not included in the daily air quality reports. According to an EPA spokesperson, that’s because the effects of lead pollution are cumulative.

"The AQI focuses on health effects that may be experienced within a few hours or days," the spokesperson said in a written statement. "Health effects from airborne lead more usually accumulate over longer time periods, so daily reports about air quality and potential health effects from lead exposure are not possible."

The AQI scale

The AQI ranges from 0 to 500 and has six color-coded categories to correspond to a different level of health concern, according to Basically, lower numbers equate to better air quality and higher numbers equate to poorer air quality.

The reporting period for each of the five pollutants is different, according to For example, the AQI for ozone is valid for eight hours, while the AQI for particle pollution is valid for 24 hours.

Categories explained

Each of the categories on the AQI corresponds to a different set of health concerns that people should consider when being outside for extended periods of time.

Here’s a closer look at each category and how you should react to each of them, according to


This means that the air quality is good, and pollution poses little to no health risks. Feel free to enjoy the great outdoors.


This color means that air quality is moderate. While the quality of the air is satisfactory, it may pose a risk to people who are unusually sensitive to pollution. Those people should consider reducing their time outside.


This means the air quality is unhealthy for sensitive groups. The general public is less likely to experience any adverse effects, but people who are sensitive to air pollution may experience problems. Sensitive groups should reduce their time outdoors. People with asthma should keep quick-relief medicine handy. People suffering from heart disease should immediately contact their health care provider if they experience palpitations, fatigue or shortness of breath.


When the AQI is red, it means the air is unhealthy. The general public may experience health problems, and people who are sensitive to air pollution may experience more serious effects. Sensitive groups should avoid prolonged time outside, while everyone else should reduce their time spent outdoors. People with asthma should keep quick-relief medicine handy.


If purple shows up on the AQI report, it means the air is very unhealthy. Officials will usually issue health alerts at this level because the risk of health problems from air pollution is increased for everyone. Sensitive groups should avoid all outdoor activities, while everyone else should avoid prolonged time outside.


This is the worst level on the AQI. This color means the air is considered hazardous. Officials will usually describe this as a health emergency because just about everyone will experience problems from air pollution. Everyone should avoid all physical activity outside. Sensitive groups should remain indoors and keep their activity levels low.

How to check your air quality provides air-quality information based on your location. It also includes a map of surrounding regions and a forecast for the following day’s AQI.

The FOX Weather app also provides you with air-quality data based on your location.