Was Chicago's air quality actually bad Wednesday? No. Here's what happened

Users across Chicago were met Wednesday with warnings from their smartphones about the city's air quality that led to much confusion, but was the air quality actually that bad?

Across social media, residents reported an unusually high air quality warning, with some weather apps indicating "very unhealthy" levels.

The issue appeared for both Google and iPhone users.

Still, commonly used air quality tracking pages like AirNow.gov showed air quality levels were good Wednesday. On Thursday, they rose to "moderate" levels.

So why didn't it match up?

According to Abigail Jaffe with Google, Wednesday's issue stemmed from "a third party monitoring station inaccurately reporting high measurements." The issue was resolved, Jaffe said.

A spokesperson for Google said its air quality information is compiled using government monitoring stations, commercial sensor networks, fire smoke and dust models, weather patterns and more.

Apple did not immediately respond to NBC Chicago's requests for comment.

The Chicago Department of Public Health said no unhealthy levels were confirmed Wednesday, though it did note "there was a technical glitch on Google and Apple that has been corrected."

"When emergencies, severe storms, air quality issues or extreme heat arise, the Office of Emergency Management and Communications (OEMC) will coordinate the City’s response by working with departments citywide to assist residents and vulnerable populations," the department told NBC Chicago. "When an Air Quality Alert is issued by the IEPA, OEMC and the Chicago Department of Public Health (CDPH) will communicate to residents that air quality levels are expected to be at or above the 'Unhealthy for Sensitive Groups' category of the air quality index and will share safety tips."

The reported issues came just as Chicago’s air quality was ranked among the worst in the U.S.

According to the 2025 State of the Air Report, released by the American Lung Association on Wednesday, the Chicago-Naperville metropolitan area is one of the most polluted in the U.S. in terms of both ozone and year-round particle pollution.

The metro area rose from 17^th in the country in ozone pollution to 15^th, and is now 13^th in the country in terms of year-round particle pollution, the ALA said in their annual report.

How to check air quality in your area

To check the air quality level in your area right now, click here.

What is an unhealthy air quality level?

AirNow said its air quality index determines the level of air pollution and the correlating health concerns.

"When AQI values are above 100, air quality is unhealthy: at first for certain sensitive groups of people, then for everyone as AQI values get higher," the website states.

Once levels reach above 300, they enter the highest level of concern known as "hazardous."

In total, there are six categories: green, or good; yellow, or moderate; orange, or unhealthy for sensitive groups; red, or unhealthy; purple, or very unhealthy; and maroon, or hazardous.

Here's a breakdown of the AQI levels:

But what exactly do those numbers mean, and where do they come from?

While the numbers don't directly represent the number of pollutants in the air (for example, an AQI of 300 doesn't mean there are 300 pollutants in the air), the number is more of a yardstick to show pollution levels.

The higher the AQI value, the greater the level of air pollution and the greater the health concern, according to AirNow.

"For each pollutant an AQI value of 100 generally corresponds to an ambient air concentration that equals the level of the short-term national ambient air quality standard for protection of public health," according to AirNow. "AQI values at or below 100 are generally thought of as satisfactory. When AQI values are above 100, air quality is unhealthy: at first for certain sensitive groups of people, then for everyone as AQI values get higher."

The scale goes from 0 to 500, with 500 being the worst.

The AQI measures five major air pollutants, according to AirNow. Those are:

• ground-level ozone
• particle pollution (also known as particulate matter, including PM2.5 and PM10 — this is what was causing Wednesday's historically high levels)
• carbon monoxide
• sulfur dioxide
• nitrogen dioxide