With growing concerns over wildfire smoke and widespread access to air quality monitoring via smartphones, more people are becoming aware of the Air Quality Index, or AQI. 

This measure of pollution risk may be especially useful in Detroit, which has been affected by smoke from Canadian wildfires and received and ‘F’ for air quality in the American Lung Association’s 2024 State of the Air report.

But what exactly do the number and color-coded air quality scale mean? And how should Detroiters dealing with year-round air quality issues use them?

Planet Detroit spoke with air quality experts to get a better sense of what the AQI’s 0 to 500 scale tells residents about air pollution and what else they may need to know to protect themselves. 

This information may be especially important for the large portion of the population that the AQI refers to as “sensitive groups,” which includes those with asthma or COPD, young children, pregnant people and adults over 65.

“It’s kind of like a traffic light,” Stuart Batterman, an environmental health researcher at the University of Michigan, said of the AQI. He added that users shouldn’t take it as a definitive measure of air quality, but as a signal to adjust their activity and stay indoors if necessary.

What’s in the AQI?

Although the air quality index is supposed to show all six of the Environmental Protection Agency’s “criteria pollutants,” Batterman said in practice it really just shows fine particulate matter or PM 2.5 and ozone.

“The way the AQI is formulated, it takes the pollutant score which is the highest and uses that only,” he said. Unlike PM 2.5 and ozone, Batterman said other criteria pollutants like nitrogen oxides and carbon monoxide rarely approach air quality standards and that there aren’t many monitors for these pollutants.

PM 2.5 is 30 times smaller than the width of a human hair and can lodge itself in lung cells and move into the bloodstream. It comes from sources like roads, smokestacks and wildfires and it’s linked to cardiopulmonary illness, premature mortality and mental health issues.

Ozone pollution, which is formed by the chemical reaction of various pollutants from vehicles, factories and other sources, is associated with respiratory problems and can increase the frequency of asthma attacks.

The scale itself is broken into six categories with recommendations.

• Green (Good: 0-50):  Air quality is satisfactory and poses little or no risk.
• Yellow (Moderate: 51-100): Air quality is acceptable, except for those most sensitive to air pollution.
• Orange (Unhealthy for Sensitive Groups: 101-150): Members of sensitive groups should limit their outdoor activity and those with asthma or heart disease should watch for symptoms. The general public may be fine.
• Red (Unhealthy: 151-200): The air is unhealthy for everyone. Sensitive groups should avoid being outdoors, while others should limit their time outside.
• Purple (Very Unhealthy: 201-300): Health Alert: the risk of health effects is increased for everyone. Everyone should consider moving their activities indoors.
• Maroon (Hazardous: 301 and higher): This is a health warning for emergency conditions. Everyone should avoid outdoor physical activity. If you're sensitive to pollution, you should remain indoors.

What the AQI doesn't show

The downside of the AQI is that it doesn't let the user know if levels of multiple pollutants are high or account for the cumulative effects of pollution, Batterman said. He added that while the cumulative impact of various pollutants is still being researched and lacks clear EPA standards, people ought to be aware that they can be affected by multiple pollutants at the same time.

It's also possible for both PM 2.5 and ozone to be high at the same time. The Michigan Department of Environment, Great Lakes and Energy accounts for this when issuing air quality alerts and advisories, which evaluate regional air quality based on 8-hour averages for ozone and 24-hour averages for PM 2.5.

The 8-hour and 24-hour time periods reflect the EPA standards for the pollutants. However, they pose a problem because spikes in pollution can occur that may not be reflected in the averages, according to Jim Haywood, a senior meteorologist for EGLE's Air Quality division.

Haywood said EGLE would try to include these expected spikes in pollution in their air quality forecast but won't issue an alert or advisory unless the levels are expected to climb above the 8-hour and 24-hour standards for ozone and PM 2.5, respectively.

EGLE issues an “Air Quality Advisory” on days when ozone, PM 2.5 or both are expected to be in the “unhealthy for sensitive groups” range. The agency will issue an “Air Quality Alert” when one or both pollutants are in the “unhealthy” range.

The breakpoints for different categories on the AQI present another issue, potentially obscuring significant jumps in pollution levels, Haywood said. 

For example, a reading on the high end of the "unhealthy for sensitive groups" range borders on "unhealthy" for the general public, potentially posing a risk for those who don't consider themselves especially sensitive to air pollution.

The AQI also depends on relatively few monitors to offer a picture of air quality that may fail to account for disparities in PM 2.5 levels experienced by many low-income areas and communities of color who are disproportionately near sources like factory smokestacks and roadways, Batterman said. However, he pointed out that the AQI does a better job representing ozone pollution, which is well distributed across geographical areas.

How Metro Detroiters can cope with poor air quality?

By observing air quality and learning what pollutants trigger breathing problems and other symptoms, Metro Detroiters can learn when they need to protect themselves and what times of day are most likely to present an issue.

"Everybody's got to know their own body and when they're susceptible to," Haywood said.

PM 2.5 can often accumulate overnight with 'atmospheric inversions', with warmer air rising and creating a cap that holds colder air in place. This is a common winter issue in Metro Detroit, where inversions can trap particulate matter.

Ozone, on the other hand, is much more likely during hot and sunny weather and tends to occur in the afternoon.

On high PM 2.5 days, Detroiters can wear and N95 or KN95 to protect themselves when going outdoors and use air conditioning and an air filter when indoors. However, since ozone is a gas, masking won't protect against the pollutant while outside. 

On high ozone days, it's better to remain indoors with windows and doors closed. Ozone poses less of a problem indoors because it reacts with building materials, resulting in lower ozone levels.

Residents should avoid using gas-powered lawn equipment, refueling vehicles and driving on high ozone days because these activities emit pollutants that contribute to ozone formation.

Metro Detroiters can check on their air quality at the government site, airnow.gov, or via neighborhood monitors offered through JustAir’s app or PurpleAir’s online map.

Although the government site is considered the most accurate, the private monitoring networks give a more localized picture of air quality.