# St. Louis, MO Air Quality Index (AQI) Today > Source: https://breathecheck.com/air-quality/us/missouri/st-louis > Go, short outing, or stay in for St. Louis, plus its own 30-day AQI trend and why we can't yet confirm which side of the Mississippi its monitor sits on. That color and that number are today's read on St. Louis air quality. Decide by the color first, the number second. The actual question underneath it is smaller: the riverfront run, the Forest Park loop, whether tonight's windows stay open. This page tracks the air quality St. Louis has actually breathed over the past month, not just the single figure sitting above. Pick your own number and [get a message when the river valley crosses it](/alerts/). ## St. Louis doesn't spike the way its neighbors do Search interest for "air quality st. louis" averages 1,900 a month across the trailing year. The typical month runs 720. July 2026 peaked at 8,100 — a real spike, but only a 2.6x gap between average and typical month. Compare that to Madison's 13.8x or Milwaukee's 15.1x, two states north. August 2025 ran hot too, at 4,400. Demand here spreads across two elevated stretches instead of piling into one dramatic month. That fits the mechanism: St. Louis sits at the southern reach of the same Canadian smoke corridor, where the plume usually arrives thinner than it does in Wisconsin. This page rebuilds the air quality St. Louis actually breathed over the past 30 days, from the same hourly feed as the reading above. It also carries a running count of days above AQI 100 in the last 12 months — the shape IQAir, AccuWeather and Missouri's own monitor grid never show. ## Why is it hazy in St. Louis today? St. Louis sits where the Missouri and Mississippi Rivers meet, in a broad floodplain rather than a hemmed-in valley. The same basic trap still applies. A stagnant, high-pressure summer stretch can hold river-valley pollution close to the ground for days. Three more things can also make the sky look worse than the reading: 24-hour averaging, smoke sitting aloft, or a distant station. [Pittsburgh reports the same mechanism](/air-quality/us/pennsylvania/pittsburgh) further upriver, in its own hillier version. The metro itself straddles two states: Missouri to the west, Illinois to the east across the river, in what locals call Metro East. The AQI's 24-hour average dilutes a bad afternoon across the day's twenty calmer hours. Smoke from a distant fire can sit aloft above the monitor doing the measuring — increasingly a Canadian plume reaching this far south. Or the nearest working station sits miles off. This metro has an extra wrinkle most don't: it isn't yet confirmed which side of the river that station sits on. None of that makes a hazy sky fake, or a clear one trustworthy. One number averages one location, not your block. [River-valley air, drifting smoke, an averaged reading](/why-is-the-air-bad-today/) goes through them all. ![Bar chart of monthly US search volume for “air quality st louis” in St. Louis, twelve months from Aug 2025 to Jul 2026. Peak 8,100 in Jul 2026, floor 390, a 2.6x gap between the 1,900 average and the 720 typical month. Bars are neutral blue-grey with the peak month in accent blue — these are search volumes, not AQI readings.](/img/st-louis/inline-2.webp) *Two elevated stretches, not one dramatic month. Search volumes, not AQI readings — bars are neutral blue-grey with the peak month in accent blue, never EPA band colours, so demand data can never be read as a measurement of the air.* ## What is causing the bad air in St. Louis? Two mechanisms trade off through a St. Louis year. Ground-level ozone builds on hot, still summer afternoons — invisible and odorless, typical of a river-valley metro this far south. PM2.5, fine particulate matter, dominates the rest of the year: traffic, regional industry, and a Canadian wildfire plume. That plume arrives thinner here than in Wisconsin, at the southern edge of its usual reach. That's the wildcard behind the 2.6x average-to-typical gap — real, but smaller than what the same plume does two states north. The FAQ below carries today's live read on which pollutant is actually driving the number. ## Is the air safe to breathe in St. Louis today? **Go** needs nothing changed — most days clear that bar without a second thought. **Don't go** at the top of the range means exactly that, no negotiating. **Short and unloaded** covers everything in between: the quick version of whatever's already planned. Duration and intensity are two separate dials, not one. On the EPA's own breakpoints, 151–200 is Unhealthy — the band where the warning first covers everyone, not just sensitive groups. 201–300, Very Unhealthy, restates that same everyone-warning more urgently: "everyone, more seriously" in the agency's own wording. Past roughly AQI 200 the choice really does stop being optional. A 20-minute walk and a 90-minute run at that same number are two different decisions, not one. ## What AQI should you stay inside? No documented Missouri rule for daycares or schools exists in the material reviewed. Wisconsin's absence is confirmed — its licensing rules mandate outdoor time and stay silent on air quality. Missouri's is just missing from what's been reviewed, a different kind of gap, not filled with a guess. Minnesota sets 100, "because children are sensitive groups." Washington sets 80. If a provider in the St. Louis area has a written air-quality policy, it isn't one that's been confirmed against these three. [What's on record state by state, and what isn't](/daycare-air-quality-policy/) says so when a state isn't in it. Adults set their own lines, and they don't agree with each other either. On r/Marathon_Training, one runner wrote: "I'll run in up to 110 because...error bars." Parents online tend to land lower, around 100. One asthmatic runner logged 156 runs against AQI and set a personal cutoff at 60. Above it, "my lungs struggle regardless of how well I've trained" — even though the EPA calls nothing under 100 unhealthy. The same log: 87% of runs under AQI 50 went fine, 42% between 50 and 70, just 12% above 70. That's a 1.8x spread, runner to asthmatic, for identical air. Wildfire smoke pulls the whole table down roughly one EPA band. "Smoke at AQI 50 feels way worse than pollution at AQI 50," one commenter wrote. 101–150 is what the EPA files as [code orange, Unhealthy for Sensitive Groups](/aqi/), a classification and not a call on your lungs. Indoors, default to a portable HEPA purifier or a Corsi-Rosenthal box. One builder in the community priced a 12x14 bedroom's worth at around $130. Skip the whole-house MERV-13 swap unless a licensed tech has checked your system's static pressure first — the wrong system chokes on it. [The three levers, in the order they're worth pulling](/indoor/) covers the rest. ![Two EPA AQI colour scales for reading St. Louis's number. The upper strip runs the whole index 0 to 500 in six bands, each segment as wide as the range it covers: Good 0-50 green, Moderate 51-100 yellow, Unhealthy for Sensitive Groups 101-150 orange, Unhealthy 151-200 red, Very Unhealthy 201-300 purple, Hazardous 301-500 maroon. The lower strip expands the 0-150 decision zone and marks three thresholds real readers set for themselves: AQI 60 (asthmatic runner), AQI 100 (parent / MN school), AQI 110 (runner).](/img/st-louis/inline-1.webp) *EPA AirNow's own colours and breakpoints. The three marked cutoffs are not EPA guidance and not St. Louis rules — each is one reader's or one institution's own line.* ## Why our number may not match what you are seeing and smelling The mismatch runs in both directions here. A clear sky with a bad number is usually ground-level ozone — invisible and odorless. A hazy-looking sky with a calm official reading is usually a 24-hour average, or smoke sitting aloft above the monitor. One St. Louis resident described a third experience in February 2025, closer to a general toll than one mismatched reading: "I need to get out of St. Louis. The air quality is terrible... my quality of life sucks here." They named something neither mechanism covers — the air felt heavier "because the air is heavy from rain," running against the usual assumption that rain clears the air. Neither official direction means the number is wrong. It means the number, your senses, and sometimes the weather itself are reading different things. That's the station behind today's reading — its distance from the metro centroid, and the nearby PurpleAir sensor count. We haven't yet confirmed which side of the river it sits on, Missouri or Illinois, which matters more here than in most metros this project tracks. [Why which side of the river a station sits on matters](/methodology/) is written up in full. ## FAQ ### What's the air quality forecast for St. Louis tomorrow? Wind direction and any incoming plume can shift this within hours. Treat it as a direction, not a lock — recheck the morning of. ### What's the UV index in St. Louis today? UV runs on its own scale, separate from AQI. A clear, low-AQI day can still be the highest-UV day of the week, so a good air quality reading isn't a reason to skip sunscreen. ### Who has the worst air quality in the US right now? See [how St. Louis ranks today](/) against every tracked city. Its rank moves hardest during wildfire season, when the Canadian plume corridor reaches this far south. ### What is in the air in St. Louis right now? That's today's specific reading. Ground-level ozone typically dominates summer afternoons here; PM2.5 from traffic, regional industry or an active wildfire plume dominates the rest of the year. ### Why does it smell like smoke outside in St. Louis? Wildfire smoke is burning vegetation, sometimes carried in from Canada at the southern edge of the same corridor that reaches Chicago and the Upper Midwest. It's a different particulate mix from ordinary traffic exhaust at the same AQI number. If a plume is active, [How far a Canadian plume travels before it lands](/wildfire-smoke/) covers the mechanics.