St. Louis is currently experiencing elevated air pollution, with coarse particulate matter (PM10) reaching an AQI of 106, which falls in the “Unhealthy for Sensitive Groups” range. Fine particulate matter (PM2.5) sits at 58, in the moderate zone. The primary concern right now is particle pollution, not ozone, which points to a combination of local emissions and weather conditions trapping pollutants near ground level.
What’s Driving Today’s Pollution
When PM10 spikes like this, the culprit is usually some combination of dust, industrial emissions, and vehicle exhaust that isn’t dispersing the way it normally would. St. Louis sits in a river valley alongside the Mississippi, and while the terrain is relatively flat, atmospheric conditions can still trap pollution close to the ground. Temperature inversions, where a layer of warm air sits on top of cooler air near the surface, act like a lid that prevents pollutants from rising and scattering. When winds are calm and an inversion is in place, everything the metro area generates stays right where people breathe it.
Vehicle exhaust is one of the single greatest sources of air pollution in the metro area, contributing nitrogen dioxide and fine particles from the dense highway network and freight corridors that run through the region. Industrial sources add to the load: the St. Louis metro, which spans both Missouri and Illinois, is home to chemical manufacturing, refining, steel processing, and metal smelting operations. Studies tracking the chemical fingerprints of local particulate pollution have traced contributions to zinc smelting, copper production, steel mills, and paint pigment manufacturing on both sides of the river.
St. Louis Has a Chronic Air Quality Problem
Today’s numbers aren’t an isolated event. The American Lung Association’s 2025 “State of the Air” report ranked the St. Louis metro area 21st worst in the nation for ozone pollution out of 228 metro areas, with an average of 10.5 unhealthy ozone days per year and a failing grade. That ranking actually worsened from the previous year, when St. Louis came in 30th. For particle pollution, the metro earned a D grade and ranked 88th worst nationally, with 2.2 unhealthy days per year. Perhaps most striking, year-round average particle pollution levels ranked 17th worst in the entire country and exceeded the federal standard.
Madison County in Illinois consistently logs the worst ozone readings in the metro area, while St. Louis County in Missouri sees the highest particle pollution. If you live in either of those areas, poor air quality days are more common than the regional average suggests.
Why Ozone Builds Up in Summer
While today’s issue is particulate matter, St. Louis also struggles with ground-level ozone, especially from late spring through early fall. Ozone isn’t emitted directly. It forms when nitrogen oxides and volatile organic compounds react in the presence of sunlight. Cars, power plants, industrial boilers, refineries, and chemical plants all release those precursor chemicals. Hot, sunny, stagnant days are the perfect recipe for ozone to build up, which is why Missouri monitors ozone levels from March through October.
On high-ozone days, the pollution you’re breathing is essentially a secondary product of all the combustion and industrial activity in the region, cooked together by summer heat. St. Louis’s geography compounds this: the broad, flat river valley doesn’t channel winds effectively, so pollutants linger.
What the AQI Numbers Mean for You
An AQI between 101 and 150, the orange “Unhealthy for Sensitive Groups” category, means people with asthma, heart disease, lung conditions, older adults, children, and anyone who exercises heavily outdoors may notice symptoms. You might experience coughing, throat irritation, or shortness of breath during activity. The general public is less likely to feel effects at this level, but it’s not zero risk.
At moderate levels (AQI 51 to 100), most people are fine, though individuals who are unusually sensitive to air pollution may still react. Once readings climb above 150 into the red “Unhealthy” zone, even healthy adults can start experiencing irritation and breathing difficulty.
When the AQI crosses 100, the American Lung Association and the St. Louis Regional Clean Air Partnership issue formal Air Quality Alerts. On alert days, the practical steps are straightforward: reduce time spent outdoors, avoid strenuous outdoor exercise, and keep windows closed. If you have a respiratory condition, keep your inhaler accessible and pay attention to how you feel rather than pushing through symptoms.
How to Track Conditions
AirNow.gov provides real-time AQI readings for the St. Louis metro area broken down by pollutant. You can check both PM2.5 and PM10 levels separately, which helps you understand whether you’re dealing with fine combustion particles or coarser dust and industrial emissions. The site also posts forecasts, so you can plan outdoor activities for lower-pollution days. Local news stations typically announce air quality alerts during weather segments when the AQI is forecast to reach orange or red levels.