A good Air Quality Index (AQI) is any reading between 0 and 50. At this level, air pollution poses little or no health risk, and you can freely spend time outdoors without concern. Once the AQI climbs above 50, the air quality drops to “Moderate,” and by the time it passes 100, certain groups of people should start limiting outdoor activity.
How the AQI Scale Works
The AQI runs from 0 to 500 and is divided into six color-coded categories. The higher the number, the more polluted the air and the greater the health risk.
- Green (0 to 50), Good: Air quality is satisfactory. No restrictions on outdoor activity for anyone.
- Yellow (51 to 100), Moderate: Acceptable for most people, though a small number of individuals who are unusually sensitive to pollution may notice mild effects.
- Orange (101 to 150), Unhealthy for Sensitive Groups: The general public is unlikely to feel symptoms, but vulnerable individuals should reduce prolonged outdoor exertion.
- Red (151 to 200), Unhealthy: Everyone may begin to experience health effects. Sensitive groups face more serious risk.
- Purple (201 to 300), Very Unhealthy: Health alert level. The entire population is at risk of respiratory and cardiovascular effects.
- Maroon (301 and above), Hazardous: Emergency conditions. All outdoor activity should be avoided.
Your local AQI is reported for several pollutants separately, including fine particulate matter (the tiny particles that penetrate deep into your lungs), ground-level ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The highest individual pollutant reading becomes the overall AQI value you see on weather apps and sites like AirNow.gov.
What “Good” Actually Means in Practice
A “Good” AQI of 0 to 50 corresponds to very specific pollutant concentrations. For fine particulate matter, which is the pollutant most commonly responsible for elevated readings in U.S. cities, “Good” means the 24-hour average stays at or below 9.0 micrograms per cubic meter of air. The moment it ticks up to 9.1, the AQI crosses into “Moderate” territory.
That 9.0 threshold aligns with a standard the EPA finalized in 2024, when it tightened the annual limit for fine particulate matter from 12.0 down to 9.0 micrograms per cubic meter. The change reflected growing evidence that long-term exposure to even modest levels of particle pollution increases the risk of heart disease, lung disease, and premature death.
For context, the World Health Organization recommends an even stricter target: annual average fine particulate concentrations of no more than 5 micrograms per cubic meter. So even a “Good” AQI day in the U.S. can still exceed what the WHO considers ideal for long-term health. If you live in an area that regularly hovers in the 30 to 50 range, the air is safe day to day, but the cumulative exposure over years is worth paying attention to.
Who Needs to Pay Attention First
At “Good” AQI levels, nobody needs to take precautions. But once the index enters the “Moderate” range (51 to 100), and especially once it hits “Unhealthy for Sensitive Groups” (101 to 150), certain people face real risk before the general population does.
Sensitive groups include people with heart disease, lung conditions like asthma or COPD, diabetes, adults 65 and older, all children under 18, and people in lower-income communities who often face higher baseline exposure due to proximity to highways, industrial areas, or housing with poor ventilation. Children are particularly vulnerable because they breathe more air relative to their body size and their lungs are still developing.
If you fall into any of these categories, an AQI of 51 is your signal to start checking forecasts before planning long outdoor workouts or letting kids play outside for extended periods.
What to Do When the AQI Rises
In the “Moderate” range (51 to 100), most people won’t need to change their routine. If you’re unusually sensitive to air pollution, consider shortening intense outdoor exercise or moving it to early morning, when ozone levels tend to be lower.
Once the AQI enters the orange zone (101 to 150), the practical advice becomes more specific. People in sensitive groups should limit time outdoors, opt for shorter and less intense activities, and take more frequent breaks. Anyone with asthma should keep quick-relief medication accessible. People with heart disease should watch for palpitations, unusual fatigue, or shortness of breath. Schools and sports programs are advised to increase rest periods and substitutions during practices to keep athletes’ breathing rates down.
If fine particulate matter is the pollutant driving the high reading, N95 masks offer real protection because they filter out those tiny particles. One important caveat: masks do nothing against ground-level ozone, which is a gas. If ozone is the problem, the only effective strategy is reducing time and effort spent outdoors.
At “Unhealthy” (151 to 200) and above, everyone should minimize outdoor exposure. Move exercise indoors, close windows, and run air purifiers or HVAC systems with good filters if you have them. Outdoor workers should talk with employers about reducing heavy exertion and having N95 respirators available.
How to Check Your Local AQI
AirNow.gov, run by the EPA, provides real-time AQI readings and forecasts for locations across the United States. Most weather apps also display AQI data, typically sourced from the same monitoring network. You can search by ZIP code to see the current reading and a forecast for the next day or two.
Keep in mind that AQI can vary significantly within a single metro area. A monitor near a freeway may show a higher reading than one in a suburban park a few miles away. Wildfire smoke, in particular, can cause dramatic local differences, with one neighborhood reading 180 while another five miles upwind sits at 90. If you see haze or smell smoke, treat the AQI as likely higher than what a distant monitor reports.
During wildfire season or in cities with persistent smog, checking the AQI before heading out is as practical as checking the weather. A reading of 0 to 50 means you’re in the clear. Anything above that is worth a second look, especially if you or someone in your household belongs to a sensitive group.