What is the Air Quality Index?

Why everyone in the Midwest is suddenly talking about the AQI as smoke from Canadian wildfires is choking the air in a large swath of the United States.

Dylan Stableford
·Senior Writer
Updated ·3 min read
6
A person rides a bicycle along the shore of Lake Michigan on Tuesday as the Chicago skyline is blanketed in haze from Canadian wildfires.
A person rides a bicycle along the shore of Lake Michigan on Tuesday as the Chicago skyline is blanketed in haze from Canadian wildfires. (Kiichiro Sato/AP)

With smoke from Canadian wildfires triggering air quality alerts across the Midwest, millions of people in cities like Chicago, Detroit and Milwaukee are becoming intimately familiar with the Air Quality Index, or AQI, a color-coded numerical scale that helps people understand the health risks and exposures to airborne pollutants at any given time.

The Environmental Protection Agency (EPA) has an online dashboard (Airnow.gov) where you can type in your location (or any location) to find the AQI.

“The U.S. Air Quality Index is getting a lot of media attention right now, but it is really fantastic,” Dr. Brady Scott, a fellow at the American Association for Respiratory Care, told Yahoo News earlier this month when wildfire smoke invaded the Northeast. “Because you can just plug in your ZIP code and kind of understand the air quality is where you're at.”

How it works

The AirNow system was created by the EPA in 1998 to serve as a "centralized, nationwide repository" for real-time data collected by local, state and federal agencies. The AQI uses that data to measure the amount of pollution in the air on a 0-500 scale. Higher numbers mean a higher concentration of the following pollutants regulated by the Clean Air Act:

• Ground-level ozone

• Particle pollution (also known as particulate matter)

• Carbon monoxide

• Sulfur dioxide

• Nitrogen dioxide

What it’s like right now

Haze from Canadian wildfires is seen over the Milwaukee Art Museum on Tuesday.
Haze from Canadian wildfires is seen over the Milwaukee Art Museum on Tuesday. (Morry Gash/AP)

The wildfires in Canada are releasing high concentrations of fine particulate matter (with diameters of less than 2.5 micrometers, virtually invisible to the naked eye) that drift south, causing the AQI in the Great Lakes region to spike.

On Wednesday morning, Chicago and Detroit were among the top three cities with the worst air quality in the world, each with AQI readings above 200.

However, those levels paled in comparison to the record 405 AQI that New York City experienced earlier this month.

What the numbers mean

An apocalyptic orange haze caused by smoke from Canadian wildfires is seen in New York City on June 7.
An apocalyptic orange haze caused by smoke from Canadian wildfires is seen in New York City on June 7. (Andy Bao/AP)

Any number over 100 is considered “unhealthy” for sensitive groups, such as children or people with heart or lung diseases. When the AQI goes above 200, everyone, even those without respiratory conditions, is at risk.

Or you can just go by the colors:

• Green: 0 to 50 (Good)

• Yellow: 51 to 100 (Moderate)

• Orange: 101 to 150 (Unhealthy for sensitive groups)

• Red: 151 to 200 (Unhealthy)

• Purple: 201 to 300 (Very unhealthy)

• Maroon: 301 to 500 (Hazardous)

“If it’s green or yellow, it’s OK for most individuals,” Brady explained. “When it’s orange, there’s concern that some people, especially those with respiratory conditions, could be impacted. When you’re in the red zone and certainly when we’re in the purple or maroon zones, everyone is at risk, even if you are a so-called healthy person.”