What is ground-level ozone? The latest air quality warning, explained

The southern two-thirds of Minnesota is facing another round of hazardous air quality this week, the Minnesota Pollution Control Agency announced. While last week’s warnings were driven by smoke from Canadian wildfires, this week’s pollution alert is driven by a different culprit: ozone.

Ozone is a colorless gas formed when certain compounds in the air mix under intense sunlight. It naturally occurs in the upper reaches of the atmosphere, where it helps reflect solar radiation back into space. But at the ground level it’s a lung irritant linked with numerous health and environmental problems.

Ground-level ozone tends to be a bigger issue in hotter parts of the country where the sun is more intense, but the right conditions can transpire to create hazardous quantities of ozone in places like the Upper Midwest. The precursor chemicals necessary for ozone formation include molecules found in car exhaust and industrial emissions.

This week’s alert is being driven in part by pollutants blowing into southern Minnesota from elsewhere in the Midwest.

“Southeast [air] flow will bring pollution from the Chicago area into Minnesota,” the MPCA explained, “and there will also be some ozone precursors in the light wildfire smoke persisting over the area.” 

Tracking data from the Environmental Protection Agency show that ozone readings in the Twin Cities metro area have been much higher than normal for most of the past few months. Lingering smoke from Canadian wildfires coupled with hot, dry weather have set the stage for ground-level ozone formation.

One quirk of this particular event is that ozone levels are projected to be higher in the Twin Cities suburbs than they are within the urban core itself. “The precursors to ozone mainly form in urban areas — but those precursors react with solar radiation and then fallout in the suburbs,” according to the MPCA.

Ozone generally brings health impacts similar to wildfire smoke: coughing, breathing difficulties, and aggravation of respiratory conditions like asthma. Long-term exposure is linked with asthma development and higher rates of premature death. Children and older adults are generally at higher risk of adverse effects, as are those who work or do strenuous activity outdoors.

Unlike wildfire smoke, however, ozone can’t be filtered from the air using a standard air purifier or a mask because the molecules are too small and easily pass through the filter mesh. Some types of air “purifiers,” particularly those that claim to use “ionization” or “energized oxygen” to clean the air, actually add ozone to their environment. Some do so intentionally, to the frustration and bafflement of air quality researchers.

People can, however, stay inside to limit ozone exposure as levels are generally lower within buildings.

Individuals can also take steps to limit their contributions to ozone pollution on warning days. Those steps include using less electricity, traveling less, and not letting their car engines idle for long periods of time, according to the EPA.