What Does the “purple Code” Mean and Other Air Quality Terms

If you haven’t paid attention to the air quality index until now, you probably learned a lot about it pretty quickly. Philadelphia and Washington, D.C. are under an air quality level that people refer to as “code purple” because of its color on the AQI scale. That’s what it all means.

What is AKI?

AQI, or Air Quality Index, is a number that measures several aspects of air quality:

• Particle contamination , both large particles (10 microns or less) and small particles (2.5 microns or less). Both are harmful to inhale, but PM2.5 particles can penetrate deeper into our lungs and are of particular concern.
• Ground level ozone is formed when pollutants from cars, factories and other sources react chemically in the presence of sunlight. The ozone in the ozone layer of the sky protects us from ultraviolet radiation, but the ozone below, where we can breathe it, has serious health effects .
• Carbon monoxide (CO) produced when things are burned, including fossil fuels in vehicles.
• Sulfur dioxide (SO2) is mainly produced in power plants and industrial facilities.
• Nitrogen dioxide (NO2) , another pollutant from vehicles and sources such as power plants.

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

Air quality depends in part on the amount of pollution produced, such as from raging wildfires in Canada or California. But your air quality also depends on the weather. For example, high and low pressure systems affect where the wind blows, and heat and sunlight can cause chemical reactions that produce ozone. There is also a time effect: air quality tends to be worst during the day and evening every day.

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

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

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

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

What do the colors mean?

“Good” air quality is a score of 50 or lower, but the numbers continue to run into the hundreds, with an AQI over 300 being considered “dangerous”. According to the EPA , there are different levels of color coding:

• Good (green, 0–50) indicates that the air quality is considered to be of little or no risk.
• Moderate (yellow, 51–100) means the 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 you should take precautions if you are at greater risk of health effects from air pollution. This group includes children (under 18), the elderly (over 65), and anyone who is pregnant or has asthma, heart disease, or other lung disease. If you have asthma, keep an inhaler handy.
• Unhealthy (red, 151-200) means that some people outside of these “susceptible 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 ” (for example, exercise or work that makes you breathe heavily).
• Very unhealthy (purple, 201-300) means an increased health risk for everyone. People from “sensitive groups” should definitely avoid outdoor sports, and everyone should try to limit their exposure to outdoor air. If you can stay inside with air conditioning, that’s better.
• Dangerous (maroon, 300 or more) means everyone should avoid outdoor activities, and if you are in a sensitive group, stay home and do your best to keep indoor air clean.

Thus, “code red” is a day when air quality reaches “unhealthy” levels, while “purple code” means that air quality is in the “very unhealthy” zone. At these higher levels, cities sometimes issue warnings recommending the use of masks or other precautions.