Air Quality in St. Louis, Missouri — Live

Live readings from PurpleAir sensors across St. Louis — with free alerts the moment the air turns.

Live St. Louis AQI from nearby PurpleAir sensors loads when this page opens in a browser, along with a live sensor map covering St. Louis and Clayton, Kirkwood, Florissant, Belleville.

St. Louis is the one metro in this part of the Midwest that the EPA formally says is failing an air quality standard. The bi-state St. Louis area is listed in the EPA's Green Book as a Serious nonattainment area for the 2015 eight-hour ozone standard, covering the City of St. Louis, St. Louis County, St. Charles County, Jefferson County and part of Franklin County on the Missouri side, plus Madison, Monroe and St. Clair counties in Illinois. That classification has been getting worse rather than better: the area was designated in 2018, reclassified from Marginal to Moderate on 7 October 2022, and bumped again after a finding that it had failed to attain. The American Lung Association's 2025 State of the Air put the metro 21st most ozone-polluted of 228 US metros, up from 30th the year before.

Canairy watches the sensors around your home, your kids' school, and everywhere else you care about, and tells you in plain English when the air changes enough to matter.

What the ozone nonattainment listing actually means for a normal week

Ozone is not smog you can see and it is not something a particle sensor detects. It forms when nitrogen oxides and volatile organic compounds react in sunlight, so it builds on hot, still, sunny afternoons and fades after dark. Missouri's Department of Natural Resources runs the ozone forecast season from 1 March through 31 October, and in the St. Louis region the alerts come from the St. Louis Clean Air Partnership, relayed by the National Weather Service office in St. Louis, covering the same eight bi-state counties as the nonattainment area. The first orange alert of 2025 went out on 17 June for the following day, warning that conditions would be unhealthy for sensitive groups.

The named sources are ordinary ones: power plants, industrial plants, gas-powered cars, and, less obviously, lawn mowers. Susannah Fuchs of the American Lung Association has framed the practical response in terms of timing rather than avoidance, advising people to limit hard outdoor exertion between roughly 10 a.m. and 7 p.m. on an orange day, and putting it plainly: not going out running at 4 o'clock on an orange air quality day would be a good idea. All three of the core Missouri counties, the city, St. Louis County and St. Charles County, carried F grades for ozone in the association's most recent report.

The air here is far better than it was, and the ozone problem is still real

Both things are true and it is worth holding them together. St. Louis had one of the worst coal-smoke problems in the country in the early twentieth century; in April 1940 the city banned the burning of soft coal mined in nearby states and hired officials to enforce a switch to anthracite, and by 1946 air pollution in the city had been cut by roughly 75 percent. The ozone trend has moved the same direction more recently. In the 1996 to 1998 period the region averaged around 50 high ozone days a year. In the 2021 to 2023 period used for the 2025 State of the Air report it was 10.5.

What has not improved is the metro's position relative to everyone else. The 2025 report moved St. Louis from 30th to 21st worst for ozone and from 30th to 17th worst for short-term particle pollution, one of the two largest rank changes in the country that year. That is partly the metro standing still while other cities improve, and partly the smoke and heat years that fed into the averaging period.

Smoke arrives from elsewhere, and it looks nothing like an ozone day

In 2023 dense smoke from Canadian wildfires pushed south into the St. Louis area more than once, first in early June and again later in the summer, dropping visibility to between two and three miles and putting the region under orange air quality alerts. Chicago took the worse of it that week, at one point registering the worst air quality of any city in the world, but the smoke reached far enough down the Mississippi valley to matter here.

Telling the two kinds of bad-air day apart is genuinely useful in St. Louis, because the right response differs. A smoke day is a particle day: you can smell it, visibility drops, PM2.5 sensors like the ones feeding this page rise sharply, and closing the windows and running a filter helps. An ozone day is invisible and odourless, PM2.5 sensors will not show it, and the thing that helps is moving exertion to the morning or evening. The alert colour can be the same orange on both.

St. Louis air quality FAQ

Why is there smoke in St. Louis today?

If you can genuinely smell it and visibility looks short, the usual answer is wildfire smoke that has travelled here. In 2023 smoke from Canadian fires dropped south into the St. Louis area repeatedly, cutting visibility to two or three miles and putting the region under orange alerts. If instead the air just looks flat and hazy on a hot, still, sunny afternoon between March and October, and there is no smell, the more likely explanation is ozone. Ozone is a gas rather than a particle, so it will not show up on the sensor readings on this page even on a day the region is under an alert for it.

What is the air quality in St. Louis right now?

This page shows the live AQI for St. Louis, calculated from PurpleAir sensors around the metro and updated continuously. The regulatory nonattainment area stretches across eight counties in two states, which is far too wide for any one number to describe, so it is worth saving the specific addresses you care about. What a sensor reads in Soulard on a smoky afternoon can be quite different from one in Chesterfield or across the river in Belleville.

What does it mean that St. Louis is in ozone nonattainment?

It means the region's monitors have not met the federal eight-hour ozone standard of 70 parts per billion, and that Missouri and Illinois are required to submit plans to fix it. The area was designated nonattainment in 2018, covering the City of St. Louis, St. Louis County, St. Charles County and Boles Township in Franklin County, with Jefferson County added in July 2021 and Madison, Monroe and St. Clair counties on the Illinois side. It was reclassified from Marginal to Moderate in October 2022 and is now listed as Serious. In practical terms it means more orange days each summer than a comparable metro, not that the air is dangerous year-round.

Does the old Herculaneum lead smelter still affect St. Louis air?

No, and it is worth being precise about why. The Doe Run smelter at Herculaneum, about 25 to 30 miles south of St. Louis in Jefferson County, ran from around 1895 until it closed in December 2013, and it was the last primary lead smelter operating in the United States. Its air emissions did contaminate the surrounding area, and the cleanup work there continues. But that is a soil and site issue near the plant, not an ongoing airborne one, and it is far enough south that it is not what a PM2.5 sensor in the city is reading. Nothing on this page tracks lead; these sensors measure fine particulate matter only.

What AQI level is unhealthy?

Under the US EPA scale, AQI 0–50 is Good and 51–100 is Moderate. From 101 the air is Unhealthy for Sensitive Groups — kids, older adults, and people with asthma or heart conditions — and above 150 it's Unhealthy for everyone. Canairy translates these numbers into a plain-English verdict, like whether to close the windows or move a workout indoors.

Is Canairy free?

Yes. The Canairy app is free to download and includes live air quality for all your saved places, plain-English verdicts, and free push alerts when the air crosses your threshold. A paid plan adds extras like SMS text alerts.

Get Canairy free on Google Play.