Air Quality Index (AQI) Basics

The AQI was developed by the Environmental Protection Agency (EPA) to help people understand outdoor air quality and how it affects health. It uses a scale of 0 – 500 to show how clean or polluted the outdoor air is, and a six color-scale to indicate when health impacts could be a concern. The health impacts are based on what someone might experience within a few hours after breathing polluted air.

The EPA calculates the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter or PM), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The EPA has set national ambient air quality standards for each of these pollutants to protect public health. Ground-level ozone and PM are the two pollutants that pose the greatest threat to human health and the ones most often shared on DEQs air quality monitoring site.

How Does the AQI Work?

Think of the AQI like a thermometer for pollution. The higher the AQI, the more pollution in the air and the greater the health risk. For example, an AQI value of 50 is the highest level considered “good” air quality, meaning it’s unlikely to cause health problems for anyone. An AQI value over 300 is in the “hazardous” category, where pollution poses serious health risks for everyone.

An AQI value of 101 or higher is significant because it is the transition point from the moderate air quality category, which is considered acceptable, to “unhealthy for sensitive groups” or USG category. USG usually corresponds to the level of the short-term national ambient air quality standards designed to protect public health. In Oregon, when the 24-hour average for PM or the 8-hour average for ozone is forecasted to reach USG, DEQ issues an air quality advisory. Local officials use this advisory to provide information to help people reduce their exposure to air pollution.

The table below shows each AQI category along with its color, level of concern, index range, and details about air quality and who may be impacted.

Understanding the AQI

The AQI helps explain how local air quality may affect public health. To make it easier to read, the AQI is divided into six color-coded categories. Each category represents a different level of health concern.

Good: Color is green. AQI range is 0 to 50. Air quality is satisfactory, and air pollution poses little to no health risk.
Moderate: Color is yellow. AQI range is 51 to 100. Air quality is acceptable. Some people may have moderate health concerns. For example, people who are unusually sensitive to ozone might have breathing issues.
Unhealthy for Sensitive Groups (USG): Color is orange. AQI range is 101 to 150. Most people are not affected, but those with lung disease, older adults, and children are at a higher risk from exposure to ozone. People with heart and lung disease, older adults, and children are more affected by PM in the air at this level.
Unhealthy: Color is red. AQI range is 151 to 200. Everyone may start to feel some adverse health impacts. Sensitive groups may experience more serious problems.
Very Unhealthy: Color is purple. AQI range is 201 to 300. Everyone may experience more serious health impacts.
Hazardous: Color is maroon. AQI range is 301 and higher. Everyone is very likely to be affected.

Health Information

Information on reducing health effects of wildfire smoke can be found through the Oregon Health Authority.

Oregon Health Authority has also published a guidance for youth titled, Air Quality and Outdoor Activity Guidance for Infants, Children and Youth.

The Oregon School Activities Association maintains a guide with Recommendations Regarding Air Quality.

The Environmental Protection Agency developed a picture book for children to help explain how wildfires can affect air quality called Why Is Coco Red?. This book is for all children, especially those with asthma, and their caretakers. Children’s lungs are still growing and developing, so they are especially vulnerable to wildfire smoke.

Visit AirNow to read Why is Coco Red? in Spanish, Simplified Chinese, Traditional Chinese, Korean, Navajo, Tagalog, and Vietnamese. The Why is Coco Red? video is available on YouTube in English or Spanish.