Buzz
If you've only recently started paying attention to the air quality index, you're likely catching up fast. Both Philadelphia and Washington, D.C. are currently experiencing an air quality level referred to as 'code purple,' based on its color in the AQI system. Here's what it means.
What is the Air Quality Index (AQI)?
The AQI, or air quality index, is a numerical value that gauges multiple factors affecting air quality:
Particle pollution, which includes both coarse particles (10 microns or smaller) and fine particles (2.5 microns or smaller). Both types are harmful to inhale, but PM2.5 particles are especially concerning as they can penetrate deeper into our lungs.
Ground-level ozone, created when pollutants from vehicles, factories, and other sources chemically react in sunlight. While ozone in the ozone layer protects us from UV radiation, ground-level ozone can have significant health risks.
Carbon monoxide (CO), a pollutant produced during the burning of various materials, such as fossil fuels in cars.
Sulfur dioxide (SO2), mainly emitted by power plants and industrial activities.
Nitrogen dioxide (NO2), another harmful pollutant from sources like vehicles and power plants.
These are just a few examples of common air pollutants, but they are the most frequently regulated by the Clean Air Act.
Air quality is partly determined by the amount of pollution being generated—such as from raging wildfires in places like Canada or California. However, it’s also influenced by weather patterns. For example, high and low pressure systems affect wind direction, and heat and sunlight can cause chemical reactions that lead to ozone formation. Additionally, air quality tends to be worse in the afternoon and evening each day.
How can I check the current AQI in my area?
The AQI is like other weather metrics such as temperature, precipitation, and the UV index: it changes over the course of the day and from one day to the next, with measurements taken at various stations nationwide.
Many weather apps now feature the AQI in their reports, and often include a map showing AQI levels across different locations. The EPA offers the AirNow app, which provides current readings, maps, forecasts, and detailed information on the various air quality components.
If your preferred app doesn’t display air quality data, a fast alternative is to visit airnow.gov. Simply click “monitors near me” under your air quality result to view a local map with AQI readings. A fire and smoke map is also available, especially useful during wildfire events.
What do the colors represent?
An AQI of 50 or below is considered “good,” but the scale extends into the hundreds, with anything over 300 considered “hazardous.” The EPA categorizes AQI levels with color codes, as follows:
Good (green, 0-50) indicates air quality with minimal or no risk.
Moderate (yellow, 51-100) means air quality is “acceptable” for most people, though those who are highly sensitive to air pollution may want to limit exposure.
Unhealthy for sensitive groups (orange, 101-150) signals that individuals at greater risk—such as children, the elderly, pregnant women, or those with asthma, heart issues, or other respiratory conditions—should take precautions. If you have asthma, keep your inhaler close by.
Unhealthy (red, 151-200) suggests that even people not in the “sensitive groups” may experience health effects like coughing or irritation of the eyes, nose, and throat. It’s advisable to reduce or avoid strenuous outdoor activities, especially those that involve heavy breathing, like intense exercise or outdoor labor.
Very unhealthy (purple, 201-300) means an increased health risk for everyone. Sensitive individuals should avoid outdoor exercise, and all others should minimize exposure to outdoor air. Staying indoors with air conditioning is recommended.
Hazardous (maroon, 300 or higher) indicates that outdoor exercise should be avoided by everyone, and sensitive groups should stay indoors, making every effort to maintain clean indoor air.
A “code red” day refers to when air quality reaches “unhealthy” levels, while “code purple” indicates that the air quality is classified as “very unhealthy.” During such high-risk days, cities may issue alerts recommending the use of masks or other protective measures.
