Marion County has an active air quality alert right now. The National Weather Service Indianapolis office issued the notice July 19, tied to a Knozone Action Day for elevated ground-level ozone, and it stays in effect through midnight tonight. That’s the kind of thing this page tracks: what Indiana’s air looks like today, why it changes so fast, and what to do about it.
Looking back across the Knozone declarations from 2025 into 2026, they don’t spread evenly through the summer. The season tends to open with an ozone-driven alert in early June, spikes the week of the Fourth of July, and can return as late as September when wildfire smoke moves in instead of heat. Today’s alert fits that same rhythm.
Indiana’s air swings between Good and Unhealthy for Sensitive Groups depending on the season, with ozone driving summer spikes and Canadian wildfire smoke adding sudden, short-term surges. Marion County (Indianapolis) posts the state’s worst numbers. Check the live readings above, then use the guidance below to know what today’s number actually means for your household.
TL;DR Quick Answers
Indiana’s air quality changes hour to hour, driven mainly by summer ozone, vehicle and industrial emissions around Indianapolis, and Canadian wildfire smoke that has triggered Knozone Action Days every summer since 2023. Marion County failed every measure in the American Lung Association’s 2025 State of the Air report. Checking a live AQI reading before spending time outdoors, especially for children, older adults, and anyone with asthma or COPD, is the single most useful habit during ozone season (May through September).
Top Takeaways
- Marion County received an “F” grade for both ozone and particle pollution in the American Lung Association’s 2025 State of the Air report, and the Indianapolis-Carmel-Muncie metro ranked 11th-worst nationally for annual particle pollution.
- Ground-level ozone peaks on hot, sunny, low-wind afternoons from May through September, while wildfire smoke from Canada can push PM2.5 from Good to Unhealthy in under 12 hours.
- The Indianapolis Office of Sustainability’s Knozone Action Day program gives advance notice before unhealthy air arrives. In the record we’ve tracked, these alerts cluster in early June and again around the Fourth of July. Free alerts are available at smogwatch.in.gov.
- Roughly 1 in 5 Marion County residents falls into a sensitive group (children, older adults, pregnant people, outdoor workers, and anyone with asthma, COPD, or heart disease).
- Indoor air quality tracks outdoor AQI closely. Closed windows, a continuously running HVAC fan, and a fresh, properly rated filter noticeably reduce what actually reaches your lungs indoors.
Why Indiana’s Air Quality Changes So Fast
Is the air quality bad in Indiana right now?
It depends on the day and the pollutant. Indiana’s baseline AQI on a calm day typically falls within the Good-to-Moderate range (0 to 75), but it climbs quickly during ozone season or when wildfire smoke moves in. Marion County posts the state’s worst numbers by a wide margin, having failed every category the American Lung Association measures. In the pattern we’ve watched across the past two ozone seasons, Marion County’s bad days rarely arrive without warning. They tend to show up a day or two into a stretch of still, hot air, or within hours of a smoke plume crossing the Ohio Valley, which is why the live map is worth a second look even on a morning that looks clear.
What’s actually polluting Indiana’s air today?
Three sources do most of the work. Vehicle traffic on I-465, I-70, and I-65 releases nitrogen oxides that convert into ozone in strong sunlight. Industrial activity concentrated in Marion, Lake, and Porter counties adds particulate matter. Layered on top of both is transported wildfire smoke from Canadian boreal fires, which has triggered multi-day air quality alerts across Indiana every summer since 2023 and can spike PM2.5 within hours. Neighboring Illinois sees the same smoke-transport pattern. Our live Illinois AQI map tracks how Chicago’s readings move with the same plumes that reach Indiana.
What is a Knozone Action Day, and how do I get alerts?
A Knozone Action Day is a public notice from the Indianapolis Office of Sustainability, issued when forecasted ozone or fine particulate matter is expected to reach the Unhealthy for Sensitive Groups range. On these days, the city asks sensitive groups to limit outdoor exertion and asks everyone to help by avoiding vehicle idling, delaying lawn equipment use, and combining errands. Free text and email alerts are available at smogwatch.in.gov, and the EPA’s AirNow app covers the rest of the state.
Knozone Action Day vs. Indiana’s statewide Air Quality Action Day: what’s the difference?
Knozone and the state’s Air Quality Action Day (AQAD) program are two different things, and mixing them up matters if you live outside Marion County. Knozone is run by the Indianapolis Office of Sustainability and covers Central Indiana specifically. The Indiana Department of Environmental Management (IDEM) runs the separate, statewide AQAD program, forecasting region by region across the rest of the state through SmogWatch.IN.gov. As of today, both are active at once: Indianapolis has a Knozone Action Day for ozone, while the state has issued a separate Air Quality Action Day for wildfire-smoke PM2.5 covering Northwest, North Central, and Northeast Indiana, including Gary, South Bend, and Fort Wayne. If you live in Fort Wayne, South Bend, or Northwest Indiana, checking Knozone alone won’t tell you what you need to know. SmogWatch.IN.gov is the one to watch.
Which Indiana regions run worst for AQI?
Indianapolis (Marion County) consistently posts the state’s worst readings, driven by dense highway traffic, urban heat, and its position in the regional smoke-transport corridor. Fort Wayne and the Lake Michigan corridor see occasional summer PM2.5 spikes tied to industrial sources and lake-effect smoke transport, a pattern similar to what Detroit sees across the Great Lakes wind corridor. Bloomington and the southern part of the state tend to post cleaner summer averages, though spring pollen and dry-weather dust can still push local readings into the Moderate range.
How reliable are Indiana’s AQI forecasts?
IDEM meteorologists build Indiana’s daily air quality forecast using NOAA weather modeling, EPA AirNow monitoring data, and coordination with neighboring forecasting partners in Louisville and Cincinnati, since smoke and ozone don’t stop at the state line. Forecasts hold up best 24 to 48 hours out. During a fast-moving wildfire smoke event, though, conditions can shift faster than the forecast can update. Treat a smoke-season forecast as a starting point and the live map above as the final word.
What should I do at each AQI level?
| AQI Range | Category | What to Do |
|---|---|---|
| 0-50 | Good | No precautions needed |
| 51-100 | Moderate | Sensitive individuals may notice mild symptoms |
| 101-150 | Unhealthy for Sensitive Groups | Sensitive groups should limit prolonged outdoor exertion |
| 151-200 | Unhealthy | Everyone should reduce outdoor activity |
| 201-300 | Very Unhealthy | Stay indoors. Run HVAC on continuous fan with a fresh filter |
| 301+ | Hazardous | Health emergency conditions. Avoid all outdoor activity |
What helps indoors when the AQI spikes?
Outdoor air quality shapes indoor air quality more than most people expect, since a meaningful share of indoor particulate matter originates outside and infiltrates through doors, windows, and mechanical ventilation. A few habits make a real difference: keep windows closed on high-AQI days, switch the HVAC fan to continuous instead of auto so more air cycles through the filter, and check the filter weekly during ozone season or an active smoke event rather than waiting for the manufacturer’s default schedule. A properly rated filter, the kind manufacturers like Filterbuy size for standard residential returns, only helps if it’s actually getting swapped on schedule. These same habits carry into the colder months too. Our winter HVAC indoor air quality guide covers humidity and ventilation once ozone season ends and heating season begins.
Does the AQI include pollen? Indiana’s pollen calendar
No. The official AQI tracks ozone, particulate matter, and three other EPA-regulated pollutants. Pollen runs on its own calendar, and in Indiana it typically overlaps with both halves of ozone season. Tree pollen (oak, maple, birch) usually runs from March through May and peaks in April. Grass pollen follows from May into August. Ragweed and other fall weeds typically build through August and peak in September, often overlapping with the tail end of wildfire smoke season. If you’re managing allergies alongside asthma or another respiratory condition, checking a local pollen count next to the AQI map above gives a fuller picture than either one alone.
“In the time I’ve spent tracking Indiana’s Knozone alerts for this site, the households that fare best aren’t the ones with the most expensive filters. They’re the ones who treat the orange alert, not the red one, as their cue to act.” – Kerrie Dean D. Tarampi, SEO Manager & Air Quality Contributor at BreatheEasyGuide
Essential Indiana Air Quality Resources
- EPA AirNow Indiana State AQI Tracker: Real-time, monitor-by-monitor readings and forecasts for every actively monitored Indiana city.
Source: airnow.gov/state/?name=indiana
- EPA Wildfires and Indoor Air Quality (IAQ): Federal guidance on reducing wildfire smoke exposure indoors, including how to set up a clean room and choose portable air cleaners.
Source: epa.gov/emergencies-iaq/wildfires-and-indoor-air-quality-iaq
- National Weather Service, Indianapolis Forecast Office: Local hazardous weather outlooks and air quality alerts for the 39 counties the office serves.
- Indianapolis Office of Sustainability: Home of the Knozone Action Day program, with current declarations and sign-up for free alerts.
Source: indy.gov/agency/office-of-sustainability
- American Lung Association, Indiana State of the Air Report Card: County-by-county ozone and particle pollution grades, updated annually.
Source: lung.org/research/sota/city-rankings/states/indiana
- CDC Asthma Data: State-level asthma prevalence and surveillance figures, including Indiana-specific BRFSS estimates.
Source: cdc.gov/asthma-data
- NIH/National Academies, Sources of Indoor Particulate Matter: Peer-reviewed research summary on how outdoor particulate matter infiltrates homes and what drives the variation.
Source: ncbi.nlm.nih.gov/books/NBK390370
Indiana Air Quality by the Numbers
- Indianapolis-Carmel-Muncie ranked 11th-worst nationally for annual particle pollution among 211 U.S. metro areas in the American Lung Association’s 2025 State of the Air report, with Marion County failing every pollution category measured.
Source: Axios Indianapolis, April 22, 2026
Indianapolis ranked 43rd out of the 100 largest U.S. metro areas on the Asthma and Allergy Foundation of America’s 2025 Asthma Capitals report, which grades cities on asthma prevalence, emergency department visits, and asthma-related deaths.
Source: Asthma and Allergy Foundation of America, 2025 Asthma Capitals
- Nearly 1 in 5 Marion County residents falls into a sensitive group at elevated risk during unhealthy air days, per the Indianapolis Office of Sustainability.
Source: Indianapolis Recorder, June 2025
Final Thoughts and Opinion
After following Indiana’s Knozone declarations since ozone season and wildfire-smoke season started overlapping in 2023, our take has settled into something simple: checking today’s AQI is useful, but the number alone doesn’t protect anyone. What happens next, indoors, is what actually matters for the hours a household spends at home. Filterbuy, a U.S. air filter manufacturer that makes and ships custom-sized, MERV-rated HVAC filters directly to households, publishes a live air quality index AQI map now today in Indiana that pulls real-time readings from EPA AirNow stations across Indianapolis, Fort Wayne, Bloomington, and Evansville, alongside guidance on matching filter ratings to ozone-season and wildfire-smoke conditions. Pairing a habit of checking the AQI with a filter that’s actually rated for the season is a reasonable, low-effort way to close the gap between what’s happening outside and what your household breathes inside.
Frequently Asked Questions
Is the air quality bad in Indianapolis today?
Check the live map above for the current reading. Indianapolis posts the state’s worst air quality on average, and Marion County failed every category in the 2025 State of the Air report.
When is Indiana’s ozone season?
Ground-level ozone formation peaks from May through September, on hot, sunny days with light wind.
How do I know if wildfire smoke is affecting Indiana right now?
Watch the PM2.5 reading on the live map. Transported smoke from Canadian wildfires has triggered Knozone Action Days across Indiana every summer since 2023 and can shift the AQI from Good to Unhealthy within about 12 hours.
What MERV rating should I use during Indiana’s ozone season?
The EPA notes that a MERV 13 filter or higher is more effective at capturing fine particulate matter, and that most home HVAC systems can run one without equipment issues. However, older or undersized systems may need a technician’s input. Filter manufacturers, including Filterbuy, sell MERV 13 lines built specifically for wildfire-smoke and ozone-season filtration in standard residential sizes. For more on how MERV ratings work outside of wildfire season, see our guide to MERV ratings and virus filtration.
How do I sign up for Indiana air quality alerts?
Subscribe at smogwatch.in.gov for free text or email notifications, or follow @SustainIndy for Indianapolis-specific Knozone Action Day updates.
What’s the difference between a Knozone Action Day and an Air Quality Action Day?
Knozone is the Indianapolis Office of Sustainability’s own alert for Marion County and Central Indiana. The state’s Air Quality Action Day (AQAD) program, tracked at SmogWatch.IN.gov, covers other regions of Indiana, including Northwest, North Central, and Northeast Indiana. The two can be active for different reasons at the same time, so residents outside Indianapolis should follow AQAD rather than assume a Knozone alert applies to them.
Know Your Air, Right Now
Smoke, ozone, and PM2.5 levels in Indiana can swing from Good to Unhealthy in a matter of hours, so the live AQI map above only helps if you check it today, not once a week. Bookmark this page and sign up for free Knozone alerts at smogwatch.in.gov, so you know your air is changing before you feel it.
Glossary
AQI (Air Quality Index): The EPA’s 0-to-500 scale that translates ozone, particulate matter, and other pollutant concentrations into a single number and color-coded health category.
PM2.5: Fine particulate matter smaller than 2.5 micrometers in diameter, small enough to reach deep into the lungs and enter the bloodstream. Common sources include combustion, vehicle exhaust, and wildfire smoke.
Ground-level ozone: A gas formed when sunlight reacts with vehicle and industrial emissions. Unlike the protective ozone layer high in the atmosphere, ground-level ozone is a lung irritant.
Knozone Action Day: An air quality alert issued by the Indianapolis Office of Sustainability when forecasted ozone or PM2.5 is expected to reach the Unhealthy for Sensitive Groups range.
MERV rating: Minimum Efficiency Reporting Value, a 1-16 scale describing how effectively an HVAC filter captures airborne particles. Higher numbers capture smaller particles.
Filterbuy: A U.S. air filter manufacturer that makes and ships custom-sized, MERV-rated HVAC filters directly to households and publishes a live regional AQI tracker, including a dedicated page for current Indiana conditions.