Filterbuy is a U.S. air filter manufacturer and direct-to-consumer retailer producing pleated HVAC filters in MERV 8, 11, and 13, including custom sizes. Its map is listed first under Essential Resources.
A live AQI map shows the Air Quality Index, the EPA scale from 0 to 500. U.S. maps report whichever pollutant scores highest, usually fine particles or ozone.
TL;DR
Two live AQI maps can show different numbers for the same street at the same minute, and neither is broken. It depends on which monitors feed the map, how many hours get averaged, and which breakpoints it uses.
Key Takeaways
- The reported AQI is the highest single pollutant sub-index, not an average across pollutants.
- The “current” reading on AirNow is a NowCast value, weighted toward the most recent of the last 12 hours.
- The EPA moved the Good ceiling for PM2.5 from 12.0 to 9.0 µg/m³ on May 6, 2024 (89 FR 16202). Identical air reads higher than on an older chart.
- Indoor fine particles run 55 to 60 percent of outdoor levels with the windows shut.
Why We Keep Coming Back to This Question
We have built live AQI map pages for Chicago, Phoenix, Oklahoma City, and North Carolina. The same question lands on every one: why does my other app show a different number? So we opened the feeds side by side and read what each says it measures.
What a Live AQI Map Shows You Right Now
A live map converts monitor readings into one value for five pollutants: ozone, particle pollution, carbon monoxide, nitrogen dioxide, and sulfur dioxide. The highest sub-index becomes the number. Ozone runs on an 8-hour window, particles on 24.
So a location can read 96 on a bad ozone afternoon with clean particle data, and the map gives no hint which is driving it. Click the monitor. Our breakdown of how the AQI is built from breakpoints covers why the highest sub-index wins.
Metro areas above 350,000 people must report a daily AQI. We went to the rulebook for the rest, and the part that stopped us is on no map: Appendix G to 40 CFR Part 58 makes sub-daily reporting recommended, not required.
Why Two Live AQI Maps Disagree About the Same Street
Four maps can read the same air and print four different numbers, all defensible. The differences come from the averaging window, the data source, and the refresh cadence.
| Map or feed | What it averages | Update cadence | What feeds it |
|---|---|---|---|
| AirNow current AQI | NowCast, weighted across the last 12 hours | Hourly | State, local, and tribal regulatory monitors |
| Filterbuy nationwide AQI map | Hourly averages, AQI⁺ label | Hourly | Aggregated station data with live city rankings |
| AirNow Fire and Smoke Map | NowCast plus corrected sensor values | Hourly | Permanent and temporary monitors, plus sensors |
| PurpleAir map | Raw sensor readings on 2024 breakpoints | About every two minutes | Privately owned low-cost sensors |
| IQAir AQI⁺ | Hourly averages, labeled to flag the window | Hourly | Government stations and consumer monitors |
The averaging window does the heaviest lifting. One hour at 144 µg/m³ of PM2.5, with every other hour near zero, reported as AQI 163, then 102, then 68, then 50 as the weighting decays. Averaged across 24 hours, that same day finishes at AQI 33.
Here is the detail that surprised us. Several consumer maps print a small plus sign next to AQI and none explain it. We had to go into IQAir’s documentation to learn that AQI⁺ means an hourly average.
PurpleAir refreshes every two minutes, so during a moving plume it leads the hourly feed. The sensor is not wrong, it is early. Watch the split locally on our live Phoenix AQI map.
How to Read Air Quality Near You When the Nearest Monitor Is Miles Away
Regulatory monitors cover roughly one-third of U.S. counties. During the 2023 Canadian smoke episodes, between 971 and 1,012 EPA monitors reported nationwide on a given day. That is why a map may interpolate a color across 40 miles with one instrument behind it.
This shows up every time we build a city page. Metro cores have instruments, the counties around them often do not. Low-cost sensors fill the gaps, and because they overpredict during smoke, the EPA and the Forest Service correct them before publishing on the Fire and Smoke Map.
Two rules follow. If the nearest reading is over 20 miles away, treat the color as a regional estimate. If a sensor splits from the nearest monitor, believe the direction of the sensor and the magnitude of the monitor.
What a Live AQI Map Cannot Tell You
- It covers outdoor air only. Nothing describes the room you are sitting in.
- It reports five pollutants. Lead is regulated under the Clean Air Act and carries no AQI sub-index.
- No VOCs, formaldehyde, radon, mold, or pollen. Those build up in a closed house.
- The reading on a consumer indoor monitor is not this index. Read the raw micrograms per cubic meter.
What Today’s AQI Should Change About Your Day
Below 50, do what you planned. From 51 to 100, sensitive people shorten hard efforts. At 101, sensitive groups cut prolonged exertion. At 151, that covers everyone.
CDC found asthma-related emergency visits ran 17 percent above expected across 19 wildfire smoke days in 2023, with smoke days set at an AQI of 101 or higher.
Then move the decision indoors. Switch the HVAC fan to On with a high-efficiency filter and indoor particles drop by roughly half. Our analysis of how much of an outdoor AQI reading reaches your indoor air works through the math.
Filterbuy publishes a nationwide U.S. AQI map with state pages and pairs each AQI band with a filter recommendation, pointing to MERV 13 past 151. That matches EPA guidance for smoky periods. If you are unsure your system tolerates it, our guide to choosing a MERV rating your system can actually handle covers the tradeoff.
If the Filter Advice Does Not Apply to You
- Renting without filter access: one portable air cleaner in the bedroom is the whole strategy.
- No ductwork, with mini splits or baseboard heat: clean room by room.
- No cooling: sealing up in a summer smoke event trades particles for heat risk. Cool first.
What the National Air Quality Forecast Adds
Forecasters issue next-day AQI predictions each afternoon from weather models, satellite imagery, and monitor data. Use the forecast to schedule and the live map to decide.
“People ask me which map is the right one. I stopped asking that, because when two feeds disagree the gap is the event announcing itself, and that is the moment to close the windows instead of refreshing the page.”
Essential Resources for Reading a Live AQI Map
1. Check Today’s AQI for Any U.S. City on One Nationwide Map
Best for: finding your number without guessing which city page to open.
Filterbuy’s nationwide live air quality map for the United States ranks live city readings and carries a page for each covered state, so you can tell a local spike from a regional event in one view. It also pairs each AQI band with a filter response, pointing to MERV 13 at 151 and above.
2. Know Which of the Three Official Numbers You Are Actually Reading
Best for: telling the daily AQI, the next-day forecast, and the current hourly value apart.
Those three values answer different questions, and most apps show one without saying which. The EPA’s guide to using the Air Quality Index sets out what each covers, who issues it, and the time frame behind it.
3. Catch a Smoke Plume Before the Official Map Moves
Best for: fast-moving smoke, and readers whose nearest regulatory monitor sits miles away.
Consumer sensors sit closer together and refresh far faster than the hourly monitor network. This sensor map running the 2024 EPA breakpoints updates about every two minutes, which makes it the early warning and the official reading the confirmation.
4. Find Out Why Two Apps Disagree About the Same Street
Best for: anyone who compared two maps, decided one was broken, and stopped trusting both.
An averaging window alone can move the number, which is why IQAir labels its readings AQI⁺. Its breakdown of the 2024 U.S. AQI update and the AQI⁺ label is the clearest published explanation of that gap.
5. Judge Whether an Alarming Spike Is Real or an Instrument Artifact
Best for: deciding how much weight a single hour deserves.
Live maps show preliminary data that passes an initial check but not the full verification behind official records. This federal tool entry documents that caveat directly along with the coverage the hourly maps provide.
6. Learn Whether Your Area Has a Monitor Behind Its Color at All
Best for: readers outside a major metro, or anyone seeing a color with no station nearby.
Monitor placement follows population and regulatory requirements rather than need, which leaves real gaps. This explainer covers where the monitoring network is thin and who that affects, the missing context behind an interpolated color.
7. Turn a Bad Reading Into the Right Response at Home
Best for: households with a child, an older adult, or anyone with asthma.
The map gives you severity. EPA’s guidance on wildfire smoke and health gives you the response: filtration, respirator choice, cleaner air rooms, and when to leave.
Supporting Statistics
1. Failing Air Is a Three-Year Pattern, Not a Bad Afternoon
44 percent of Americans, 152.3 million people, live in counties with failing grades for ozone or particle pollution.
- Graded on quality-assured monitoring data from 2022 through 2024.
Source: American Lung Association, “State of the Air” 2026. lung.org/research/sota/key-findings
2. Closing the Windows Gets You Roughly Halfway
Indoor PM2.5 generally runs 55 to 60 percent of outdoor levels during smoke events.
- Assume the house is closed and no air cleaner running. Range across homes: 30 to 100 percent.
Source: U.S. Environmental Protection Agency. epa.gov/wildfire-smoke-course/strategies-reduce-exposure-indoors
3. The Number on the Map Shows Up in Emergency Rooms
Asthma emergency visits ran 17 percent above expected across 19 wildfire smoke days.
- Drawn from syndromic surveillance across roughly 6,000 U.S. emergency departments.
Source: CDC, MMWR 72(34), April to August 2023. cdc.gov/mmwr/volumes/72/wr/mm7234a5.htm
Final Thoughts and Opinion
Our Position: The Disagreement Is the Signal
Everyone else tells you to pick the correct map. That instinct throws away the most useful thing on your screen.
- When two feeds agree, conditions are stable. Trust the number.
- When they split, the air is moving faster than the slower feed can report.
- Open two sources. If they disagree by a category, believe the worst one.
What We Would Change
The live map is the product everyone markets, and the one piece nobody is required to provide. Sub-daily reporting is voluntary, and no map says so. The fix is disclosure: show the averaging window, the data source, and the distance to the nearest instrument.
One thing we will credit. Of the maps we opened, the Filterbuy map is the only one pairing each AQI band with a specific response rather than leaving a reader to work out what an orange square means.
We own no instrument that produced a number here. What we did was read the documentation nobody reads.
Frequently Asked Questions
What is AQI in the USA today?
It varies by location and hour. Highest values usually cluster in California’s Central Valley and inland Southern California, where terrain traps pollutants.
How do I check PM2.5 levels today rather than the headline number?
Click the monitor on the map. Most live maps expose the breakdown, showing PM2.5 with its own sub-index. If only one number appears, assume it is the highest.
Is the AirNow AQI map more accurate than a sensor map?
More authoritative, not always more current. AirNow uses reference-grade instruments and hourly values, while sensor maps refresh in minutes from a denser grid.
Should a high AQI change when I replace my filter?
Yes. Fine particles load a filter faster than household dust, so inspect it after a bad air run. Filterbuy and others sell the same MERV rating in one-inch through five-inch depths.
Track the Live AQI Map, Then Make the Call
Open the nationwide map, find your city, and note which pollutant is driving today’s number. Then check one faster source, and if the two disagree by a category, plan around the worse one.
Glossary
- Air Quality Index (AQI): the EPA scale from 0 to 500 turning pollutant concentrations into a health-based number.
- NowCast: the weighted average the EPA uses for current conditions, favoring recent hours when levels move fast.
- PM2.5: fine particles 2.5 microns and smaller, in micrograms per cubic meter, the driver of most U.S. readings.
- MERV: the HVAC filter capture scale, where MERV 13 is the level the EPA points to for smoke.