# St. Louis, MO Air Quality Index (AQI) Today > Source: https://breathecheck.com/air-quality/us/missouri/st-louis/ > Live AQI for St. Louis, MO with a plain verdict — go, short and unloaded, or don't go — plus the monitor behind it and our own hourly record. # St. Louis, MO Air Quality Index (AQI) Today Readings collected automatically every hour; text written and checked by BreatheCheck Editorial . How this page is made → Right now: St. Louis is at 44 , good . ## Is the air safe to breathe in St. Louis today? In St. Louis, BreatheCheck reads 44 as Go . The cutoffs: go below 101 , keep it short and easy from 101 to 150 , move it indoors above 150 . These are our own thresholds, drawn on the EPA bands — EPA itself classifies 101–150 as Unhealthy for Sensitive Groups and does not tell you how long to stay out. Go AQI 0–100 Normal outdoor plans. No change needed. Short & unloaded AQI 101–150 A 20-minute dog walk, not a 2.5-hour run. Keep it brief and easy. Don’t go AQI 151+ Move it indoors. Above 200 that becomes everyone, not just sensitive groups. Above 200 this stops being a personal call: everyone indoors. Duration and intensity are two separate dials | Under 20 min | 20–60 min | Over an hour | | Easy | Go | Go | Go | | Moderate | Go | Go | Go | | Hard | Go | Go | Go | Go Every combination lands the same way at this reading — a hard hour is no different from an easy twenty minutes. The two dials separate above 80 . Where people actually draw the line **All the thresholds people reported, and who reported them** - 60 Asthmatic runner one runner’s own 156-run log - 75–100 Daycare / school parents and staff in one r/toddlers thread, Aug 2025 — not a published rule - 100 Parent community-reported, escalating at 150 and 200 - 110 Runner community-reported EPA classifies 101–150 as Unhealthy for Sensitive Groups. That is an attributed classification, not a verdict on your lungs specifically. If you’re staying in, filter the room you’re actually in A portable HEPA unit or a Corsi-Rosenthal box — one builder put theirs at about $130 for a 12×14 bedroom. We don’t default to a MERV-13 swap in whole-house HVAC: it can restrict airflow, and the safe version needs a static-pressure check first. Dominant pollutant O3 UV index 1.2 low · modelled UV hour by hour in St. Louis → Days above AQI 100 1 of 30 days we have recorded The record, day by day → 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. BreatheCheck tracks the air quality St. Louis has actually breathed since we began measuring here, not just the single figure sitting above. Pick your own number and get a message when the river valley crosses it . ## 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, and 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, against 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. BreatheCheck rebuilds the air quality St. Louis actually breathed since we began measuring here, from the same hourly feed as the reading above. It also carries a running count of days above AQI 100 since we began counting — the shape IQAir, AccuWeather and Missouri’s own monitor grid never show. Since 16 August 2026 we have recorded 30 days for St. Louis. 1 went above AQI 100, and the highest daily maximum was 101 , on 26 August 2026. The day-by-day record is further down. ## 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 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 smoothing across recent hours holds back 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 goes through them all. ## 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. ## What makes the call different in St. Louis? 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 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 , 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 covers the rest. ## 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 smoothing across recent hours, 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. Today’s reading for St. Louis comes from AirNow’s St. Louis (Metro-east) reporting area, whose reference point sits about 12.8 miles from the metro centre. As of 14 September 2026 at 22:12 UTC . 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 is written up in full. ## Where this number sits Of the 121 US metros reporting to us right now, St. Louis is number 47 and number 2 of the 2 we track in Missouri. The pollutant driving it today is O3 . Nearby metros we also measure, and what they read at the same hour: - Peoria, IL — 146 miles away, AQI 31 - Indianapolis, IN — 230 miles away, AQI 35 - Kansas City, MO — 238 miles away, AQI 53 - Memphis, TN — 241 miles away, AQI 52 Why a metro a short drive away reads differently → Where this reading comes from AirNow’s St. Louis (Metro-east) reporting area, about 12.8 miles from the metro centre. ## What has air quality in St. Louis actually been? Daily maximum, our own readings. 30 days since 2026-08-16; nothing back-filled. Why we keep our own → 1 of the 30 days we have recorded so far went above AQI 100. | Date | Daily max AQI | EPA band | Hours read | | 2026-09-14 | 44 | Good | 23 | | 2026-09-13 | 71 | Moderate | 23 | | 2026-09-12 | 74 | Moderate | 24 | | 2026-09-11 | 53 | Moderate | 23 | | 2026-09-10 | 52 | Moderate | 23 | | 2026-09-09 | 72 | Moderate | 23 | | 2026-09-08 | 90 | Moderate | 24 | | 2026-09-07 | 44 | Good | 23 | | 2026-09-06 | 55 | Moderate | 23 | | 2026-09-05 | 67 | Moderate | 23 | | 2026-09-04 | 75 | Moderate | 24 | | 2026-09-03 | 76 | Moderate | 24 | | 2026-09-02 | 68 | Moderate | 23 | | 2026-09-01 | 100 | Moderate | 23 | | 2026-08-31 | 93 | Moderate | 23 | | 2026-08-30 | 77 | Moderate | 23 | | 2026-08-29 | 87 | Moderate | 23 | | 2026-08-28 | 62 | Moderate | 23 | | 2026-08-27 | 80 | Moderate | 23 | | 2026-08-26 | 101 | Unhealthy for Sensitive Groups | 23 | | 2026-08-25 | 74 | Moderate | 23 | | 2026-08-24 | 84 | Moderate | 24 | | 2026-08-23 | 58 | Moderate | 23 | | 2026-08-22 | 93 | Moderate | 24 | | 2026-08-21 | 87 | Moderate | 24 | | 2026-08-20 | 52 | Moderate | 24 | | 2026-08-19 | 77 | Moderate | 23 | | 2026-08-18 | 100 | Moderate | 23 | | 2026-08-17 | 52 | Moderate | 26 | | 2026-08-16 | 66 | Moderate | 13 — partial day | ## Questions about St. Louis **What is the air quality in St. Louis right now?** AQI 44 — Good. That is number 47 of the 121 US metros reporting to us at this hour. **Which monitor does this number come from?** St. Louis (Metro-east), about 12.8 miles from the metro centre. One instrument reports for the whole metro, so the side farthest from it is the least well described. **How does St. Louis compare with metros nearby?** At this hour: Peoria 31, Indianapolis 35, Kansas City 53, Memphis 52. Readings that close together suggest one airshed rather than local sources. **Does this include pollen?** No. The AQI covers PM2.5 , PM10 , ozone, NO₂, SO₂ and CO. A high-pollen day can read clean here and still put someone in bed. What the number means, band by band → · UV in St. Louis → · Why apps disagree → ## Put this number in context On this page Alert me when St. Louis turns One email when it crosses your threshold. Updated 14 Sep 2026, 22:12 UTC