What 'Code Purple' and Other Air Quality Terms Mean

If you haven’t paid attention to the air quality index until now, you’re probably learning a lot about it rather quickly. Philadelphia and Washington, D.C. are under an air quality level that people are calling “code purple” after its color on the AQI scale. Here’s what that all means.

What is the AQI?

The AQI, or air quality index, is a number that measures several aspects of air quality:

• Particle pollution, both coarse particles (10 microns or less) and fine particles (2.5 microns or less). Both are harmful to breathe in, but the PM2.5 particles can get deeper into our lungs and are especially concerning.
• Ground-level ozone, produced when pollutants from cars, factories, and other sources chemically react in the presence of sunlight. Ozone in the ozone layer of the sky protects us from ultraviolet radiation, but ozone down here, where we can breathe it in, has serious health effects.
• Carbon monoxide (CO), produced when things are burned, including fossil fuels in vehicles.
• Sulfur dioxide (SO2), mainly produced from power plants and industrial facilities.
• Nitrogen dioxide (NO2), another pollutant from vehicles and sources like power plants.

These aren’t the only types of air pollution, but they are the most common of the pollutants that are regulated by the Clean Air Act.

Air quality depends in part on how much pollution is being produced—for example, if there are raging wildfires in Canada or California. But your air quality also depends on what the weather is doing. For example, high and low pressure systems affect where the wind blows, and heat and sunlight can trigger chemical reactions that create ozone. There’s also a timing effect: Air quality tends to be at its worst in the afternoon and evening each day.

How do I find out what the AQI is near me right now?

AQI is like temperature, precipitation, UV index, or other weather metrics: It changes from day to day and throughout the day, and the data for it is measured at different stations throughout the country.

Weather apps will often include the AQI in the report, and you can see a map of AQI in different places. The EPA has an AirNow app that includes current readings, maps, forecasts, and a breakdown of the levels of the different air quality components.

If your favorite app doesn’t show air quality, a quick way to get the air quality near you is airnow.gov. Click “monitors near me” under your air quality result to see a map of local air quality readings. There is also a fire and smoke map that can be especially helpful during wildfires.

What do the colors mean?

A “good” air quality is a score of 50 or below, but the numbers continue into the hundreds, with an AQI over 300 considered “hazardous.” There are different color-coded levels, according to the EPA:

• Good (green, 0-50) means the air quality is considered to carry little or no risk.
• Moderate (yellow, 51-100) means that air quality is “acceptable” for most people, but people who are very sensitive to air pollution may want to reduce their exposure.
• Unhealthy for sensitive groups (orange, 101-150) means that you should take precautions if you are at greater risk of health effects from air pollution. This group includes children (under 18), older people (over 65), and anyone who is pregnant or has asthma, heart conditions, or other lung conditions. If you have asthma, keep your inhaler handy.
• Unhealthy (red, 151-200) means that some people who aren’t in those “sensitive groups” may notice health effects such as coughing or irritation of the eyes, nose, and throat. Outdoor activities should be reduced or avoided if they are “long and intense” (such as exercise or work that gets you breathing heavily).
• Very unhealthy (purple, 201-300) means there is an increased health risk for everyone. People in “sensitive groups” should definitely avoid exercising outside, and everyone should try to limit their exposure to outside air. If you can stay inside with air conditioning, that’s best.
• Hazardous (maroon, 300 or more) means that everyone should avoid outdoor exercise, and if you’re in a sensitive group, stay inside and do your best to keep your indoor air clean.

So a “code red” day would be one in which the air quality hits “unhealthy” levels, and “code purple” means that air quality is in the “very unhealthy” zone. At these higher levels, cities sometimes issue warnings advising the use of masks or other precautions.

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