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Outdoor Air Quality

To protect human health and the environment, the Clean Air Act amendments of 1990 established standards for six air pollutants. Two of those air pollutants, ozone and particulate matter, can reach unhealthy concentrations in the Washington Metropolitan Region.


Current Regional Air Quality





What is an Air Quality Action Day?

Air Quality Action Days occur when air pollution in the region is expected to reach unhealthy levels. They occur primarily during the summer months when hot, humid, and stagnant weather conditions contribute to the formation of air pollution.



Health Concerns on Air Quality Action Days

Ground-level ozone is an air pollutant that damages human health, vegetation, and is a key ingredient of urban smog. Repeated exposure may cause permanent damage to the lungs and trigger a variety of health problems including chest pains, coughing, nausea, throat irritation, and congestion. Exposure can also worsen bronchitis, heart disease, emphysema, and asthma, and reduce lung capacity.

Particulate matter pollution includes dust, dirt, soot, smoke, and liquid droplets. Particulate matter can either be directly emitted into the air or formed in the atmosphere by reactions of fuel combustion gases. Inhaled particles can evade the respiratory system's natural defenses and penetrate deep into sensitive regions of the lungs. Many scientific studies have linked particulate matter to a series of significant health problems, such as aggravated asthma, chronic bronchitis, decreased lung function, and heart disease.


What is the Air Quality Index?

The Air Quality Index (AQI) is EPA's tool for communicating air quality to the public. The AQI uses both a color-coded and numerical scale to report how clean or polluted the air is and what associated health effects might be of concern. The higher the AQI value, the greater the level of air pollution and the greater the health concern.

Air Quality monitors measure the concentrations of criteria air pollutants in our region, and EPA calculates the AQI for each criteria pollutant, each day.The highest of the AQI values for the individual pollutants becomes the AQI for that day. The index is graphically represented on a color scale, which ranges from lower values (green = good air quality levels) to higher values (red = unhealthy or purple = very unhealthy levels).The Air Quality for a particular day may be referred to as a Code (Green, Yellow, Orange, Red or Purple) Day, which corresponds to the color category on the AQI.

The AQI can advise persons most susceptible to unhealthy air, primarily the elderly, children, and persons with existing respiratory problems, of air quality conditions that may affect them. An AQI value of 100 for any pollutant corresponds to the primary (health based) standard for that air pollutant, and is a "Code Orange" forecast. An Air Quality Action Day is a Code Orange, Code Red or Code Purple Day, where air pollution is forecasted to be greater than the health based criteria air pollution standard set by the Clean Air Act.


Air Quality Index

Air Quality Index Levels of Health Concern




Good 0-50 Green Air quality is considered good, and air pollution poses little or no risk.
Moderate 51-100 Yellow Air quality may pose a moderate health risk, especially for those who are unusually sensitive to air pollution.
Unhealthy for
Sensitive Groups
101-150 Orange Members of sensitive groups, children and adults with respiratory and heart ailments, may experience health effects and should limit time spent outside. The general public is not likely to be affected.
Unhealthy 151-200 Red Everyone may experience health effects and should limit their outdoor activity; members of sensitive groups may experience more serious health effects.
Very Unhealthy 201-300 Purple Everyone may experience more serious health effects and should avoid outdoor activities, especially individuals with heart and breathing ailments, children, and older adults.


What is an Air Quality Action Days Participant?

Air Quality Action Days participants are public and private individuals and organizations that have agreed to develop voluntary Air Quality Action Day plans. These plans range from notifying employees and customers of an Air Quality Action Day to rescheduling emission producing operational activities that contribute to the Region's poor air quality, such as lawn mowing and pesticide application. Air Quality Action Days empower government agencies, businesses, and residents to do their share for cleaner air. During Air Quality Action Days, all sectors of the community will be called on to take voluntary actions to reduce air pollution.


How are Air Quality Action Days Announced?

A team of meteorologists and scientists produce air quality forecasts for the Washington Metropolitan area for the following day, usually by 3:00 pm. The Metropolitan Washington Area Council of Governments (MWCOG) receives the forecast and notifies Air Quality Action Day participants, other organizations, the news media, and the public in the event of a Code Orange/Red forecast. Current air quality forecasts are announced by Clean Air Partners.

For more information, read the Clean Air Partners Air Quality Action Guide (PDF, 653KB)