Understanding the Air Quality Index (AQI): A Complete Guide to the Numbers
Learn what AQI means, how the 0-500 scale works, what each color code signals, and what to do at every level to protect your health.
Table of Contents
- What the Air Quality Index Is
- The AQI Scale: 0 to 500
- How PM2.5 Maps to AQI
- What Each AQI Level Means for You
- Green (0-50): Business as Usual
- Yellow (51-100): Most People Are Fine
- Orange (101-150): Sensitive Groups Take Notice
- Red (151-200): Everyone Should Adjust
- Purple (201-300): Avoid Outdoor Exertion
- Maroon (301-500): Stay Indoors
- Ozone: The Other Key Pollutant
- How Home Air Quality Monitors Use AQI
- AQI vs. International Standards
- Checking AQI: Where to Look
- Key Takeaways
TL;DR
The Air Quality Index (AQI) is a 0-500 scale the EPA uses to communicate how polluted the air is. It tracks five major pollutants: PM2.5, PM10, ozone, carbon monoxide, nitrogen dioxide, and sulfur dioxide. Green (0-50) means good air, yellow (51-100) is moderate, orange (101-150) affects sensitive groups, red (151-200) is unhealthy for everyone, purple (201-300) is very unhealthy, and maroon (301-500) is hazardous. Home air quality monitors measure PM2.5 and sometimes VOCs, then convert those readings into an AQI number you can act on.
You have probably seen AQI numbers on your weather app or heard them mentioned during wildfire season. Maybe the number was 42 one day and 165 the next, with a color change from green to red. But what do those numbers actually mean, and when should you change your behavior based on them?
The Air Quality Index is simpler than it looks once you understand the scale. This guide breaks down every piece of it.
What the Air Quality Index Is
The Air Quality Index (AQI) is a standardized scale the U.S. Environmental Protection Agency (EPA) uses to report how clean or polluted the air is. It runs from 0 to 500, where lower numbers mean cleaner air.
The AQI is not measuring just one thing. It tracks six major pollutants regulated under the Clean Air Act:
- PM2.5 (fine particulate matter): tiny particles from combustion, cooking, and wildfires
- PM10 (coarse particulate matter): dust, pollen, and construction debris
- Ground-level ozone (O₃): formed when vehicle exhaust and industrial emissions react with sunlight
- Carbon monoxide (CO): produced by burning fuel, especially from vehicles and gas appliances
- Nitrogen dioxide (NO₂): comes from vehicle engines and power plants
- Sulfur dioxide (SO₂): released by coal-burning power plants and industrial processes
The EPA calculates a separate AQI value for each pollutant based on its concentration. The highest individual AQI becomes the overall AQI for that hour or day. Your weather app or AirNow report usually shows this overall number along with which pollutant is driving it.
The AQI Scale: 0 to 500
The AQI divides into six categories, each with a color code designed to communicate risk at a glance.
| AQI Range | Color | Category | What It Means |
|---|---|---|---|
| 0-50 | Green | Good | Air quality is satisfactory. No health risk. |
| 51-100 | Yellow | Moderate | Acceptable for most. Unusually sensitive people may notice effects. |
| 101-150 | Orange | Unhealthy for Sensitive Groups | People with asthma, heart disease, children, and older adults should reduce prolonged outdoor exertion. |
| 151-200 | Red | Unhealthy | Everyone may begin to experience health effects. Sensitive groups face more serious risk. |
| 201-300 | Purple | Very Unhealthy | Health alert. Everyone faces increased health risk. |
| 301-500 | Maroon | Hazardous | Emergency conditions. The entire population is likely affected. |
A few things to notice about this scale. The jump from green to yellow at 50 is not particularly concerning for most people. The critical threshold is 100: below it, air quality is generally fine for normal activities. Above it, vulnerable groups need to start paying attention.
The scale is also non-linear. The difference between AQI 25 and AQI 50 represents a much smaller change in actual pollution concentration than the difference between AQI 200 and AQI 300. This compression at the lower end means the scale amplifies small changes at dangerous levels, which is by design.
How PM2.5 Maps to AQI
For most people reading this, PM2.5 is the pollutant that matters most for everyday air quality. It drives the AQI on the majority of days in most U.S. cities, and it is what home air quality monitors primarily measure.
Here is how PM2.5 concentrations (24-hour average, in µg/m³) translate to AQI categories, reflecting the EPA's 2024 updated breakpoints:
| AQI Category | AQI Range | PM2.5 (µg/m³) |
|---|---|---|
| Good | 0-50 | 0.0-9.0 |
| Moderate | 51-100 | 9.1-35.4 |
| Unhealthy for Sensitive Groups | 101-150 | 35.5-55.4 |
| Unhealthy | 151-200 | 55.5-125.4 |
| Very Unhealthy | 201-300 | 125.5-225.4 |
| Hazardous | 301-500 | 225.5-325.4 |
Notice the gap between "Good" (up to 9.0 µg/m³) and "Moderate" (up to 35.4 µg/m³). That "Moderate" range is wide, which means your air quality could quadruple in PM2.5 concentration while the AQI still reads under 100. This is why checking actual PM2.5 numbers on a home monitor gives you more precision than AQI alone.
The updated 2024 breakpoints lowered the "Good" threshold from 12.0 to 9.0 µg/m³, aligning more closely with current health research. If you have an older air quality monitor, its firmware might still use the previous breakpoints.
For a deeper look at what PM2.5 is and why it is dangerous, see our article on what PM2.5 is and where it comes from.
What Each AQI Level Means for You
Numbers are useful, but what should you actually do at each level? Here is a practical breakdown.
Green (0-50): Business as Usual
No precautions needed. Enjoy outdoor activities. This is what clean air looks like.
If you live in an area that regularly stays in the green range, count yourself fortunate. Many U.S. cities rarely see AQI below 30 during summer months due to ozone.
Yellow (51-100): Most People Are Fine
The air is acceptable. People who are unusually sensitive to pollution (severe asthma, advanced COPD) might notice mild symptoms during extended outdoor exercise. For everyone else, no changes needed.
If your home air quality monitor consistently reads in this range, check for indoor sources like cooking or candles. Your indoor air should generally be better than outdoor air if you are running an air purifier.
Orange (101-150): Sensitive Groups Take Notice
This is the first action-level threshold. If you or someone in your household has asthma, heart disease, or is a child or older adult, reduce prolonged or heavy outdoor exertion. Move workouts indoors. Keep windows closed and run your air purifier.
Healthy adults can still exercise outdoors but should consider shortening intense workouts.
Red (151-200): Everyone Should Adjust
At this level, even healthy adults may experience throat irritation, coughing, or difficulty breathing during exertion. Move exercise indoors. If you must be outside, reduce intensity and duration.
Keep windows and doors closed. If you have an air purifier, this is when it earns its keep. Check that your filter is not due for replacement. For purifier recommendations, see our roundup of the best indoor air quality monitors to track your home levels.
Purple (201-300): Avoid Outdoor Exertion
Health alert for the entire population. Everyone should avoid prolonged or heavy outdoor activity. Sensitive groups should remain indoors entirely.
Run air purifiers on their highest effective setting. If you do not own one, this is the time to consider purchasing one. Close all windows and seal any obvious gaps. Avoid activities that generate indoor particles, like frying food or burning candles.
Maroon (301-500): Stay Indoors
Emergency conditions. This level typically occurs during severe wildfire events or industrial accidents. Stay indoors. Avoid all physical exertion outdoors. Wear an N95 mask if you must go outside.
During the 2020 West Coast wildfires, several cities recorded AQI readings above 500, which is literally off the standard scale. Events like these are becoming more frequent with climate change.
Ozone: The Other Key Pollutant
While PM2.5 dominates headlines during wildfire season, ground-level ozone is the primary AQI driver during warm months in many cities.
Ozone is not emitted directly. It forms when nitrogen oxides (from vehicles and power plants) react with volatile organic compounds (VOCs) in the presence of sunlight. This is why ozone peaks on hot, sunny afternoons and drops overnight.
Key ozone facts for practical decision-making:
- Ozone concentrations are typically lowest in the early morning (before 9 AM) and after sunset.
- Unlike PM2.5, ozone is a gas and is not effectively removed by HEPA air purifiers. Activated carbon filters can absorb some ozone, but the most effective strategy is staying indoors with windows closed during peak hours.
- Ozone irritates the airways even at moderate concentrations, making it especially problematic for runners, cyclists, and anyone exercising outdoors.
If your weather app shows ozone as the primary pollutant, schedule outdoor workouts for early morning or evening.
How Home Air Quality Monitors Use AQI
If you own a home air quality monitor, or you are considering one, here is how it connects to the AQI system.
Most consumer monitors measure PM2.5 using a laser particle sensor. They then apply the EPA's breakpoint formula to convert the raw µg/m³ reading into an AQI number. Some monitors display both values; others show only one.
A few important distinctions:
Indoor AQI is not the same as outdoor AQI. The AQI your monitor shows reflects what is happening inside your home. The official AQI on weather apps comes from outdoor monitoring stations that may be miles away. Your indoor reading could be much higher (during cooking) or much lower (with an air purifier running) than the outdoor number. For a practical walkthrough of what each number on your monitor's display means, see our guide on how to read an air quality monitor.
Not all monitors use the same breakpoints. Some monitors use the older pre-2024 EPA breakpoints. Others use the WHO's more conservative guidelines, or even China's GB standard. Check your monitor's documentation to understand which scale it applies. This can cause confusion when your monitor shows AQI 60 but your weather app says AQI 45 for the same area.
CO2 and VOCs are separate from AQI. If your monitor measures carbon dioxide or volatile organic compounds, those readings are not part of the standard AQI. They are still useful for understanding indoor air quality, but they use their own scales. For more on how to interpret all these readings together, see our guide on how to test air quality at home.
For a guide on pairing your monitor with a purifier for automated air cleaning, read how to use an air quality monitor with a purifier.
AQI vs. International Standards
The AQI scale described above is the U.S. EPA standard. Other countries use different scales:
| Country/Org | Scale Range | "Good" Threshold | Key Difference |
|---|---|---|---|
| U.S. EPA | 0-500 | 0-50 | Most widely recognized, updated 2024 |
| WHO | Guidelines only | 5 µg/m³ annual PM2.5 | No index scale; sets concentration targets |
| China MEP | 0-500 | 0-50 | Different breakpoint concentrations |
| EU CAQI | 0-100+ | 0-25 | Compressed scale, less granularity at high levels |
| India NAQI | 0-500 | 0-50 | Includes eight pollutants (adds ammonia and lead) |
If you travel internationally or use air quality apps that default to local standards, be aware that the same PM2.5 concentration can produce different AQI numbers depending on which country's scale is applied. A reading of AQI 80 in one country does not necessarily mean the same pollution level as AQI 80 in another.
Checking AQI: Where to Look
Several reliable sources provide real-time and forecast AQI data:
- AirNow.gov: The EPA's official air quality reporting site. Provides current AQI, forecasts, and fire/smoke maps.
- Weather apps: Most major weather apps (Apple Weather, Google Weather) display local AQI data sourced from government monitoring stations.
- PurpleAir map: A network of citizen-operated PM2.5 sensors that provides hyperlocal readings, often more relevant to your immediate neighborhood than the nearest EPA station.
- Your home monitor: Gives you real-time indoor air quality data specific to your living space.
For the most complete picture, cross-reference outdoor AQI from AirNow or your weather app with your indoor readings from a home monitor. This tells you whether your home is adequately protected, or whether outdoor pollution is infiltrating.
To learn the fundamentals of what you are breathing indoors, start with our article on indoor air quality basics.
Key Takeaways
The AQI is a tool designed to make air quality information actionable. Here is what to remember:
- Below 50 is clean. You do not need to worry.
- 100 is the first real threshold. Above it, sensitive groups should take precautions.
- 150+ affects everyone. Move exercise indoors and close windows.
- PM2.5 drives the AQI most days. A home air quality monitor that measures PM2.5 gives you the data you need.
- Indoor and outdoor AQI are different. Your home can be much better or worse than outside depending on your actions.
- Check the primary pollutant. Knowing whether PM2.5 or ozone is driving the number changes your response strategy.
Understanding what the numbers mean is the first step. The next step is measuring your own air and taking action based on what you find.