The Air Quality Index (AQI) is a 0 to 500 scale in the U.S. Environmental Protection Agency (EPA) uses to report outdoor air pollution. Under 100 is fine for most people. Above 100 it turns unhealthy for sensitive groups first, then everyone.
Most pages stop at that chart. At Clean Air Nerds we went to the rulebook: EPA’s technical guidance and Appendix G to 40 CFR Part 58, the regulation that defines the index. We rebuilt the tables below from it rather than copying a chart. No sensor testing informs this page. Three things stood out.
- The AQI is not a measurement. It is a converted index built from a breakpoint table.
- Your phone shows the past 12 hours. It lags smoke you can smell.
- It reports one pollutant, not five. Whichever scores worse becomes the number.
That third point decides what you do, and it is where this page spends its time.
TL;DR
The AQI reports outdoor air as one number and color, built from five pollutants, showing whichever scores worst. It is a planning tool measured over 8 to 24 hour windows, not a live meter, and says nothing about the air inside your house.
Key Takeaways
- It covers outdoor air only, not hazardous air pollutants or extreme heat.
- The value is the highest of five sub-indices, not an average.
- The number on your app is a NowCast, weighted across 12 hours. It lags smoke by design.
- EPA lowered the “Good” breakpoint from 12.0 to 9.0 µg/m³ on May 6, 2024. Air that scored Good in 2023 can score Moderate today.
What Is the Air Quality Index?
The AQI converts concentrations of five regulated pollutants into one value between 0 and 500 with one of six colors. Each carries a sub-index tied to its federal health standard, and the reported value is whichever scores highest.
They are ozone, particle pollution, carbon monoxide, nitrogen dioxide, and sulfur dioxide. Two drive almost every high reading.
- PM2.5 is fine particulate matter, particles 2.5 microns or smaller, in micrograms per cubic meter (µg/m³). Wildfire smoke produces it and it reaches deep lung tissue.
- Ozone forms when sunlight hits traffic and industrial pollution, in parts per million (ppm). It peaks on hot afternoons.
Notice what is missing. No volatile organic compounds (VOCs). No formaldehyde. No radon. No mold. Those dominate a closed house.
How Is the AQI Calculated?
EPA runs each concentration through a piecewise linear formula anchored to a breakpoint table, producing a sub-index. The AQI is the highest of them, and the pollutant behind it is the main pollutant.
We worked through the example EPA publishes in its technical assistance document for AQI reporting. Ozone at 0.078 ppm, PM2.5 at 35.9 µg/m³, and carbon monoxide at 8.4 ppm convert to 126, 102, and 90. The AQI is 126, driven by ozone.
Here is what changed how we read the number. Every consumer guide we checked calls it the pollutant with the highest concentration. The regulation says otherwise: it is the highest sub-index, because each pollutant is scored against its health standard first. Carbon monoxide at 8.4 ppm dwarfs ozone at 0.078, and ozone still wins.
The windows differ too. Ozone is judged over 8 hours, PM2.5 over 24, nitrogen dioxide over 1. Five clocks, one dial.
What Do the Six AQI Categories Mean?
The PM2.5 figures below reflect the breakpoints in force since May 2024, where many older articles are wrong.
| AQI | Category | PM2.5 (µg/m³) | What to do |
|---|---|---|---|
| 0–50 | Good | 0.0 – 9.0 | Open the windows. |
| 51–100 | Moderate | 9.1 – 35.4 | Fine for most. |
| 101–150 | Unhealthy for Sensitive Groups | 35.5 – 55.4 | Sensitive groups ease off. |
| 151–200 | Unhealthy | 55.5 – 125.4 | Close up and filter. |
| 201–300 | Very Unhealthy | 125.5 – 225.4 | Stay indoors. |
| 301+ | Hazardous | 225.5+ | Emergency. |
On May 6, 2024, the final rule published in the Federal Register lowered the top of Good from 12.0 to 9.0 µg/m³. Rebuilding the table above, 9.0 was the first figure that did not match charts already circulating. Any reference still showing 12.0 predates May 2024, a quick test of whether a page is maintained.
If red and green are hard to tell apart, EPA publishes a ColorVision Assist palette agencies may use.
What Your Local AQI Might Not Be Telling You
The index is national. The reporting is local and uneven.
- Your area may not report. Required only above 350,000 people. Smaller areas report voluntarily, some with no monitor nearby.
- A pollutant can be dropped. If one stays below index value 50 for a season, the agency may exclude it. Your number covers only what your agency included.
- Some areas monitor ozone only. Air can read Good while the sky looks hazy, because the haze was particles and the monitors read ozone.
Why the AQI on Your Phone Is Not a Live Reading
The daily AQI needs 24 hours of particle data, so the required report is yesterday’s number. EPA fills the gap with a NowCast, an hourly estimate weighted across the previous 12 hours.
| WHAT THIS MEANS ON A SMOKE DAYWhen a plume arrives, your app is still averaging the clean hours before it, so the number climbs behind the smoke. If you can smell smoke and the dial reads yellow, close the house. Our San Francisco smoke guide covers the 30-minute window. |
What the AQI Means Once It Crosses Your Walls
Two days at AQI 150 can produce completely different air inside the same house. The driving pollutant decides which.
Ozone is reactive and breaks down on surfaces, ductwork, and skin. A review of roughly 2,000 indoor environments found an indoor-to-outdoor ratio near 25 percent, under 10 percent with air conditioning on. So an ozone-driven 150 is a reason to keep the kids off the field, and a weak reason to worry about the living room. Cities like Phoenix during ozone season show it clearly.
PM2.5 does the opposite. It drifts through the envelope and stays suspended, so indoor tracks outdoor for days. The infiltration math is in our full analysis of outdoor AQI and indoor air quality.
What the AQI Cannot Tell You About Your House
- Air toxics. Formaldehyde, benzene, and VOCs from paint are invisible to it.
- Radon. A soil gas, measured separately. Every basement needs a test kit.
- Indoor carbon monoxide. The dangerous version comes from a faulty furnace. Only an alarm catches it.
- Humidity. Dust mites and mold respond to it, not to a regional average.
Consumer monitors also show an “AQI” for the room they sit in. That is not the EPA AQI, and EPA advises against applying AQI values to low-cost sensor data. Treat it as a trend, not a grade.
How to Turn an AQI Number Into an Action at Home
- Check the main pollutant first. AirNow lists it, as does our live Chicago AQI map.
- If ozone drove it, move activity indoors, windows closed.
- If PM2.5 drove it, close the envelope, set the HVAC to recirculate, and let it run. Filtration happens only while air moves.
- Match the filter to the particle. MERV, the Minimum Efficiency Reporting Value, grades particle capture. MERV 8 is not built for PM2.5, as our MERV 8 and MERV 13 comparison shows. MERV 13 is what EPA points homeowners toward. Winter shifts it: choosing a MERV rating for winter.
- Change it before the event, and do not let slot size stop you. A loaded filter restricts the airflow that delivers filtration. Older homes often have returns in odd sizes, and manufacturers including Filterbuy cut media to custom dimensions.
- No ductwork? Renters work a different playbook: wildfire smoke and window AC units.
“I went through EPA’s reporting guidance expecting to confirm the color chart. What I found was that the index reports one pollutant, not five, and which one decides whether your closed windows have done the work or nothing at all.”
Essential Resources
1. See What Every AQI Color Means for Your Household
Best for: Reading the six-category chart and knowing what to do at each level.
Filterbuy, a U.S. air filter manufacturer and direct-to-consumer retailer producing pleated HVAC filters in MERV 8, 11, and 13, including custom sizes, publishes a homeowner’s guide to the Air Quality Index. It walks all six colors with the recommended action at each one.
Source: Filterbuy
2. Find Out What You Are Breathing Right Now
Best for: Getting today’s number and, more importantly, the pollutant driving it.
AirNow is the official EPA reporting platform, and it publishes the main pollutant alongside the value. Look up your ZIP code before you decide whether to open the windows.
Source: AirNow, U.S. Environmental Protection Agency
3. Learn Which Pollutants Actually Set Your Number
Best for: Understanding the science behind each of the five AQI pollutants.
EPA maintains a hub for the six criteria air pollutants regulated under the Clean Air Act. One mismatch catches people out: EPA regulates six, but the AQI reports five, because lead has no sub-index.
Source: U.S. Environmental Protection Agency
4. Read the Actual Rule Behind the Breakpoints
Best for: Verifying a breakpoint or a calculation against the primary source.
The AQI is federal regulation, not convention. Appendix G to 40 CFR Part 58 holds the binding text: the formula, the reporting duties, and the current breakpoint table that every explainer paraphrases.
Source: Electronic Code of Federal Regulations
5. Compare the U.S. Scale Against Global Health Guidance
Best for: Travelers, and anyone comparing readings across countries.
The U.S. AQI is one national index among many, and it is more permissive than international guidance. The WHO fact sheet on ambient air quality and health sets out the 2021 guideline values, several of which sit below U.S. standards.
Source: World Health Organization
6. Check Whether Your County Has a Real Problem
Best for: Telling an unlucky day apart from a chronic local pattern.
One reading says little about where you live. The American Lung Association State of the Air report grades every monitored U.S. county on ozone and particle pollution across a rolling three-year window.
Source: American Lung Association
7. Understand What Each Pollutant Does to Your Body
Best for: Households with a child, an older adult, or anyone with asthma or heart disease.
The AQI gives you a category, not a mechanism. CDC keeps a plain-language reference on the health effects of criteria air pollutants, which is the right next stop if someone at home falls into a sensitive group.
Source: Centers for Disease Control and Prevention
Supporting Statistics
- Americans spend 87% of their time inside buildings and 6% in vehicles, across 9,386 people. That leaves 7% of your day in the air the AQI measures. Source: Lawrence Berkeley National Laboratory, 2001
- Nearly 28 million people in the U.S. have asthma, close to 5 million children, the group the 101 threshold exists for. Source: Asthma and Allergy Foundation of America, 2026
- Hours of PM2.5 exposure can trigger cardiac events, including heart attack and stroke. Hence the 24-hour sub-index. Source: American Heart Association
Final Thoughts and Opinion
The AQI is a well-built instrument doing one narrow job: an index rather than a measurement, lagging 12 hours, reporting a worst case, stopping at your walls.
| OUR POSITIONRead the AQI as two values, never one. The number gives you severity. The pollutant gives you the action. An app showing a number and a color without naming the driving pollutant has handed you half an instrument, and it is the half you cannot act on. |
The problem is not the EPA. The index does its job. It is what the market built on top: monitors printing a room-level “AQI” that borrows the authority of a federal index while measuring something it was never defined over.
Frequently Asked Questions
What Is a Good Aqi Number?
0 to 50 is Good, which since May 2024 means PM2.5 at or below 9.0 µg/m³. Moderate, 51 to 100, is fine for most adults.
Why Does It Look Smoky When the Aqi Says the Air Is Fine?
The value averages the past 12 hours, so it climbs behind a plume. Some areas also monitor ozone but not particles, so smoke may not be in it at all.
Can I Use an Indoor Monitor to Get My Own Aqi?
Not accurately. EPA advises against applying AQI values to low-cost sensor readings. Use the raw µg/m³.
Check the Pollutant, Then Check Your Filter
Pull up your local AQI and find what is driving it. If ozone is the answer, your closed windows have already done most of the work. If fine particles are, check the MERV rating on your filter. Below 11 and that is the cheapest upgrade here. Filterbuy and others stock MERV 13 in custom sizes, so an odd slot is rarely the blocker.
Glossary
- Air Quality Index (AQI). A 0 to 500 scale published by the U.S. EPA that converts outdoor concentrations of five regulated pollutants into a single number and color category for daily public reporting.
- NowCast. The EPA algorithm that estimates a current-hour AQI from a weighted average of the previous 12 hours of monitoring data, used because the official particle index requires a full 24-hour average.
- Breakpoint. A pollutant concentration that marks the boundary between two AQI categories. Breakpoints are set from federal health standards and are revised as the science changes.
- PM2.5. Fine particulate matter 2.5 microns in diameter or smaller, reported in micrograms per cubic meter (µg/m³). The pollutant class most associated with cardiovascular and respiratory harm.
- Ground-level ozone. A reactive gas formed when sunlight drives reactions among traffic and industrial emissions. Reported in parts per million (ppm) and judged on an 8-hour average.
- Infiltration factor. The fraction of an outdoor pollutant that penetrates a building and remains suspended indoors. For PM2.5 in U.S. homes, the median sits near 0.25.
- MERV. Minimum Efficiency Reporting Value, the 1 to 16 residential scale rating how effectively an HVAC filter captures particles of a given size.