The Air Quality Index — AQI — is a standardized scale that translates complex air pollution data into a single number you can understand at a glance. Developed by the U.S. Environmental Protection Agency (EPA), the AQI runs from 0 to 500 and is color-coded from green to maroon. The higher the number, the worse the air quality and the greater the health risk. Whether you’re checking a weather app, looking at an outdoor air quality monitor, or trying to decide if it’s safe to exercise outside, understanding what AQI numbers actually mean helps you make informed decisions about your health — especially if you have asthma, heart disease, or other conditions that make you sensitive to air pollution.
How the AQI Works
The AQI isn’t a single measurement — it’s a composite index calculated from the concentrations of five major air pollutants regulated under the Clean Air Act:
- Ground-level ozone (O₃)
- Particulate matter — PM2.5 (fine particles, 2.5 microns and smaller)
- Particulate matter — PM10 (inhalable particles, 10 microns and smaller)
- Carbon monoxide (CO)
- Sulfur dioxide (SO₂)
- Nitrogen dioxide (NO₂)
Each pollutant is measured independently, and its concentration is converted to an AQI value using EPA-defined breakpoints (specific concentration ranges that correspond to each AQI category). The overall AQI for a given location is the highest individual pollutant AQI — so if PM2.5 has an AQI of 85 and ozone has an AQI of 45, the reported AQI is 85, and PM2.5 is identified as the “primary pollutant.”
This is an important detail: the AQI number tells you how bad the worst pollutant is, not an average of all pollutants. Knowing which pollutant is driving the AQI helps you understand the specific health risk and take appropriate action.
2024 AQI Update: Stricter PM2.5 Standards
In May 2024, the EPA revised the AQI breakpoints for PM2.5 to reflect updated health science. The key change: the “Good” category threshold for PM2.5 was lowered from 12 µg/m³ to 9 µg/m³, and breakpoints for the “Very Unhealthy” and “Hazardous” categories were also tightened. This means that the same PM2.5 concentration that previously registered as “Good” may now show as “Moderate” under the updated scale. The revision reflects growing evidence that PM2.5 causes health effects at lower concentrations than previously recognized.
The Six AQI Categories
The AQI is divided into six categories, each with a color code, a health concern level, and specific guidance:
| AQI Range | Color | Category | Health Concern |
|---|---|---|---|
| 0-50 | 🟢 Green | Good | Air quality is satisfactory. Little or no risk. |
| 51-100 | 🟡 Yellow | Moderate | Acceptable. Some pollutants may pose a moderate concern for a very small number of unusually sensitive individuals. |
| 101-150 | 🟠 Orange | Unhealthy for Sensitive Groups | Members of sensitive groups (people with asthma, heart disease, children, older adults) may experience health effects. General public is less likely to be affected. |
| 151-200 | 🔴 Red | Unhealthy | Everyone may begin to experience health effects. Sensitive groups may experience more serious effects. |
| 201-300 | 🟣 Purple | Very Unhealthy | Health alert: everyone may experience more serious health effects. |
| 301-500 | 🟤 Maroon | Hazardous | Health emergency. The entire population is likely to be affected. |
What Each Level Means in Practice
Good (0-50): Green Light
Air quality is considered satisfactory, and air pollution poses little or no risk. This is the ideal range. You can exercise outdoors, open windows for ventilation, and go about your day without concern. Most rural areas and many suburban areas regularly fall in this range.
Moderate (51-100): Proceed with Awareness
Air quality is acceptable for most people. However, individuals who are unusually sensitive to air pollution — particularly ozone — may experience mild respiratory symptoms. If you notice that you cough or feel slight throat irritation on “moderate” days but not on “good” days, you may be in this sensitive category. For most people, no action is needed.
Unhealthy for Sensitive Groups (101-150): Caution for Some
This is where the AQI starts to matter for a significant portion of the population. “Sensitive groups” include:
If you’re in a sensitive group, reduce prolonged or heavy outdoor exertion. The general public is unlikely to be affected at this level.
Unhealthy (151-200): Everyone Should Take Notice
At this level, everyone may begin to experience health effects — not just sensitive groups. You might notice eye irritation, coughing, throat discomfort, or shortness of breath during exertion. Sensitive groups may experience more serious effects. Reduce prolonged outdoor exertion. Consider moving exercise indoors. Keep windows closed and run air purifiers if you have them.
Very Unhealthy (201-300): Health Alert
This triggers a health alert — the entire population is more likely to experience health effects. Avoid prolonged outdoor exertion. Sensitive groups should avoid all outdoor physical activity. Keep windows and doors closed. Run air purifiers indoors. This level is commonly seen during severe wildfire smoke events, heavy smog episodes, or industrial pollution events.
Hazardous (301-500): Emergency Conditions
This represents a health emergency. Everyone is likely to be affected, and serious health effects are possible for the entire population. Avoid all outdoor activity. Stay indoors with windows and doors sealed. Run air purifiers on the highest setting. If you must go outside, wear an N95 respirator mask. AQI values above 300 are rare in most U.S. cities but have occurred during extreme wildfire events (such as the 2020 West Coast fires and the 2023 Canadian wildfire smoke events that affected the eastern U.S.).
The Key Pollutants: What’s Actually in the Air
PM2.5 (Fine Particulate Matter)
PM2.5 refers to particles 2.5 microns or smaller in diameter — about 30 times smaller than a human hair. These particles are small enough to penetrate deep into the lungs and even enter the bloodstream. PM2.5 is the pollutant most commonly responsible for elevated AQI readings and is associated with the most significant health effects.
Sources include vehicle exhaust, power plants, industrial emissions, wildfires, wood-burning stoves, and cooking. PM2.5 is the primary concern during wildfire smoke events.
Health effects: Short-term exposure aggravates asthma, causes respiratory irritation, and can trigger heart attacks in vulnerable individuals. Long-term exposure is linked to reduced lung function, chronic bronchitis, cardiovascular disease, and premature death.
Ground-Level Ozone (O₃)
Unlike the protective ozone layer in the upper atmosphere, ground-level ozone is a harmful pollutant. It’s not emitted directly — it forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight. This is why ozone levels are typically highest on hot, sunny afternoons in urban areas.
Health effects: Ozone irritates the respiratory system, causing coughing, throat irritation, chest pain, and reduced lung function. It’s particularly dangerous for people with asthma. Long-term exposure may contribute to the development of asthma and other chronic respiratory diseases.
PM10 (Inhalable Particulate Matter)
PM10 includes particles up to 10 microns in diameter — dust, pollen, mold spores, and construction debris. These particles are inhaled into the upper respiratory tract and can cause irritation, coughing, and aggravation of existing respiratory conditions. PM10 is less dangerous than PM2.5 because the larger particles don’t penetrate as deeply into the lungs.
Carbon Monoxide (CO)
CO is a colorless, odorless gas produced by incomplete combustion of fossil fuels. Outdoor CO levels are primarily driven by vehicle traffic. At elevated levels, CO reduces the blood’s ability to carry oxygen, causing headaches, dizziness, and — at very high concentrations — death. Outdoor CO rarely drives AQI readings in most areas due to improved vehicle emission controls.
Sulfur Dioxide (SO₂)
SO₂ is produced by burning fossil fuels containing sulfur, particularly coal and oil. Power plants and industrial facilities are the primary sources. SO₂ irritates the respiratory system and can trigger asthma attacks. It also contributes to the formation of PM2.5 through atmospheric chemical reactions.
Nitrogen Dioxide (NO₂)
NO₂ comes primarily from vehicle exhaust and power plants. It irritates the airways, aggravates respiratory diseases, and contributes to the formation of both ground-level ozone and PM2.5. NO₂ levels are typically highest near busy roads and highways.
How to Check the AQI
AQI information is widely available and easy to access:
Websites and Apps
Personal Air Quality Monitors
If you want to monitor air quality at your specific location — including indoor air — consumer air quality monitors can measure PM2.5 and sometimes other pollutants in real time:
AQI and Indoor Air Quality
The AQI is designed for outdoor air, but outdoor air quality directly affects indoor air. Without filtration, indoor PM2.5 levels typically track outdoor levels — outdoor pollution enters through open windows, doors, HVAC systems, and building envelope leaks. During high-AQI events (wildfire smoke, smog), indoor air can become nearly as polluted as outdoor air if no protective measures are taken.
Protecting Indoor Air When AQI Is High
When to Use an Air Purifier Based on AQI
| Outdoor AQI | Indoor Action |
|---|---|
| 0-50 (Good) | Open windows for ventilation. Air purifier optional. |
| 51-100 (Moderate) | Ventilation fine for most. Sensitive individuals may benefit from running a purifier. |
| 101-150 (USG) | Close windows if sensitive. Run air purifier in bedrooms and main living areas. |
| 151-200 (Unhealthy) | Close windows. Run air purifiers. Avoid indoor pollution sources. |
| 201-300 (Very Unhealthy) | Seal the house. Run purifiers on high. Create a clean air room. |
| 301+ (Hazardous) | Full lockdown. All purifiers running. Seal gaps around windows/doors. N95 if going outside. |
AQI Around the World: Different Scales
The U.S. EPA AQI is the most widely referenced, but other countries use different scales with different breakpoints:
| Country/Region | Scale Range | Key Difference |
|---|---|---|
| United States (EPA) | 0-500 | Six categories, PM2.5 breakpoints updated in 2024 |
| China (MEP) | 0-500 | Similar structure to U.S. but with different breakpoints — generally more lenient |
| India (NAQI) | 0-500 | Six categories, includes eight pollutants (adds NH₃ and Pb) |
| European Union (EAQI) | 1-5 (or 0-100) | Simpler scale, focuses on PM2.5, PM10, NO₂, O₃, SO₂ |
| Canada (AQHI) | 1-10+ | Health-risk based rather than concentration-based |
When comparing AQI values across countries, be aware that the same PM2.5 concentration can produce different AQI numbers depending on which scale is used. An AQI of 100 in the U.S. system doesn’t mean the same thing as an AQI of 100 in China’s system. Always check which scale is being used.
Frequently Asked Questions
Q: What AQI level is safe for outdoor exercise?
For healthy adults, AQI below 100 is generally safe for outdoor exercise. Between 101-150, sensitive individuals should reduce prolonged exertion. Above 150, everyone should limit outdoor exercise — move workouts indoors. Above 200, avoid outdoor exercise entirely. Keep in mind that exercise increases your breathing rate, which means you inhale more pollutants per minute during physical activity.
Q: Why does AQI change throughout the day?
AQI fluctuates based on pollution sources, weather, and atmospheric conditions. Ozone levels typically peak in the afternoon (sunlight drives ozone formation). PM2.5 can be higher in the morning and evening (temperature inversions trap pollution near the ground) or during specific events (wildfires, industrial activity). Wind disperses pollutants, while calm conditions allow them to accumulate. Checking AQI forecasts helps you plan outdoor activities for the cleanest part of the day.
Q: Can indoor AQI be worse than outdoor AQI?
Yes. Indoor air can be more polluted than outdoor air due to indoor sources: cooking (especially gas stoves), cleaning products, candles, fireplaces, off-gassing from furniture and building materials, and tobacco smoke. Without adequate ventilation, these pollutants accumulate. The EPA estimates that indoor air can be 2-5 times more polluted than outdoor air. An indoor air quality monitor can help you understand your specific situation.
Q: What’s the difference between AQI and PM2.5 readings?
PM2.5 is a specific measurement — the concentration of fine particles in micrograms per cubic meter (µg/m³). AQI is a standardized index that converts pollutant concentrations (including PM2.5) into a 0-500 scale for easier interpretation. A PM2.5 reading of 35 µg/m³ corresponds to an AQI of approximately 100 under the current EPA breakpoints. Many air quality monitors display both values.
Q: Should I trust the AQI from my weather app?
Weather app AQI data is generally reliable for regional trends — it’s sourced from government monitoring stations. However, air quality can vary significantly within a few miles due to local sources (traffic, industry, fires). For hyperlocal data, PurpleAir sensors or a personal air quality monitor provide a more accurate picture of conditions at your specific location.
The Bottom Line
The AQI is a practical tool that translates invisible air pollution into actionable information. The key takeaways: anything below 50 is good, below 100 is acceptable for most people, and above 100 is where sensitive groups need to start paying attention. Above 150, everyone should take precautions. The two pollutants that most commonly drive elevated AQI readings — PM2.5 and ozone — are also the ones with the most significant health effects. Checking the AQI before outdoor activities, especially if you have respiratory or cardiovascular conditions, is a simple habit that can meaningfully protect your health. And when outdoor air quality is poor, keeping windows closed and running a HEPA air purifier indoors is one of the most effective things you can do.
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