St. Louis, MO Air Quality & Smog

Dominant pollutants, when smog is worst, notable historical episodes, and local agency resources for St. Louis, MO — plus how to check the current AQI. For real-time readings, the free Smog Report iOS app pulls live air quality from EPA AirNow.

Dominant pollutants

ozone (summer), PM2.5 (industrial + transport).

Seasonal pattern: when smog is worst

St. Louis ozone elevates during hot stagnant Midwest summer weeks. PM2.5 baseline is elevated by I-70/I-44 corridor traffic and the Mississippi River industrial corridor. Episodic wildfire smoke transport from Canadian fires has become more common since 2021.

A recent or notable event

Summer 2023 — multiple weeks of Canadian smoke pushed metro St. Louis AQI past 150 repeatedly.

Local agencies and resources

MDNR runs the regulatory monitor feeding AirNow.

• Missouri Department of Natural Resources
• St. Louis County Department of Public Health

How to check air quality in St. Louis

The EPA's AirNow.gov publishes the canonical reading from the nearest regulatory monitor. For glanceable iPhone access — including widgets, Live Activities, Siri voice queries, and Apple Watch — install Smog Report. It's free, requires no account, and pulls from the same AirNow API.

For St. Louis-specific advisories and alerts that go beyond the federal AirNow feed, check the agency pages above directly.

Common questions about St. Louis air quality

Why does St. Louis have air-quality and smog problems?

St. Louis ozone elevates during hot stagnant Midwest summer weeks. PM2.5 baseline is elevated by I-70/I-44 corridor traffic and the Mississippi River industrial corridor. Episodic wildfire smoke transport from Canadian fires has become more common since 2021.

What are the main air pollutants in St. Louis?

St. Louis's dominant pollutants are ozone (summer), PM2.5 (industrial + transport). The AQI on any given day is usually driven by whichever of these is highest.

Has St. Louis had a major air-quality or smog event?

Summer 2023 — multiple weeks of Canadian smoke pushed metro St. Louis AQI past 150 repeatedly.

How can I check St. Louis's air quality right now?

The EPA's AirNow.gov publishes the official reading from the nearest regulatory monitor. For live AQI on your iPhone — with home-screen widgets, Lock Screen Live Activities, Siri, and Apple Watch — the free Smog Report app pulls from the same AirNow feed. No account is required.

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