Understanding AQI: What Each Level Means for Your Health

The Air Quality Index, or AQI, is the system the Environmental Protection Agency uses to communicate how clean or polluted your outdoor air is and what health effects you might experience. Despite its importance, most people encounter AQI numbers without fully understanding what they represent. This guide explains the AQI system in plain language so you can make informed decisions about outdoor activities, ventilation, and health precautions.

What Exactly Is the AQI?

The AQI is a standardized scale from 0 to 500 that translates raw pollution measurements into a single number that indicates health risk. It is calculated separately for five major pollutants: ground-level ozone (O3), particulate matter (PM2.5 and PM10), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen dioxide (NO2). The overall AQI for a location is determined by whichever pollutant has the highest individual reading at that time.

This means an AQI of 150 could be driven by high ozone on a hot summer day or by high PM2.5 during a wildfire. The dominant pollutant matters because different pollutants affect your body differently. Check our pollutants information page for details on how each pollutant impacts health.

The EPA designed the AQI so that a value of 100 corresponds to the National Ambient Air Quality Standard for each pollutant, the level the EPA considers protective of public health with an adequate margin of safety. Values below 100 are generally considered satisfactory, while values above 100 indicate increasing health concern.

AQI Levels Explained

AQI 0-50: Good (Green)

Air quality is satisfactory and poses little or no health risk. This is the baseline that clean-air cities achieve most days. All outdoor activities, including intense exercise, are safe for everyone.

What produces these readings: Clean weather patterns with good wind dispersal, absence of nearby pollution sources, and favorable atmospheric conditions that prevent pollutant buildup.

Action: None needed. Enjoy outdoor activities freely.

AQI 51-100: Moderate (Yellow)

Air quality is acceptable, but some pollutants may be a moderate health concern for a very small number of people. Most people will not be affected at all. People who are unusually sensitive to ozone or particle pollution might notice mild symptoms during prolonged outdoor exertion.

What produces these readings: Normal urban conditions on warm days, moderate traffic emissions, or light agricultural burning in the distance. Many US cities spend a significant portion of summer days in this range.

Action: If you have severe asthma or respiratory disease, consider reducing prolonged outdoor exercise. Everyone else can continue normal activities.

AQI 101-150: Unhealthy for Sensitive Groups (Orange)

This is the first level where meaningful health precautions are warranted. "Sensitive groups" includes people with heart or lung disease, older adults, children, and pregnant women. For these groups, the risk of respiratory symptoms and aggravated conditions increases at this level.

What produces these readings: Hot stagnant summer days with elevated ozone, moderate wildfire smoke drift, temperature inversions trapping pollutants near the ground, or heavy traffic combined with unfavorable weather.

Action: Sensitive groups should reduce prolonged outdoor exertion. Move vigorous exercise indoors or schedule it for early morning when ozone levels are lower. Everyone else can continue normal activities but should watch for unusual symptoms.

AQI 151-200: Unhealthy (Red)

Everyone may begin to experience health effects. Members of sensitive groups may experience more serious effects. This is the level where healthy adults without pre-existing conditions start noticing symptoms like scratchy throat, eye irritation, and reduced exercise capacity.

What produces these readings: Significant wildfire smoke, severe ozone events, industrial emissions combined with stagnant air, or dense urban smog.

Action: Everyone should reduce prolonged outdoor exertion. Move exercise indoors. Sensitive groups should avoid all outdoor physical activity. If you must be outside, limit time and reduce intensity. Keep windows closed and run air purifiers indoors.

AQI 201-300: Very Unhealthy (Purple)

Health alert: everyone may experience more serious health effects. At this level, even short outdoor exposure can cause noticeable symptoms in healthy individuals. The risk of respiratory and cardiovascular events increases significantly.

What produces these readings: Nearby wildfire smoke, severe industrial pollution events, or extreme weather-driven pollution episodes. Most US cities experience these levels only a few times per year, if at all.

Action: Avoid all outdoor activity. Stay indoors with windows closed. Run HEPA air purifiers on maximum settings. Sensitive groups should consider relocating to a clean air shelter if they cannot maintain safe indoor air quality. Check our city air quality data for current readings in your area.

AQI 301-500: Hazardous (Maroon)

Health warning of emergency conditions. The entire population is likely to be affected. This level represents a genuine health emergency, comparable to being in the immediate vicinity of a major fire.

What produces these readings: Direct wildfire smoke, major industrial accidents, or extreme pollution events. These readings are rare in most US cities but occurred in multiple eastern cities during the Canadian wildfire smoke events of 2023.

Action: Remain indoors. Seal your home to prevent smoke infiltration. Run all available air purification continuously. Do not exercise, even indoors in unfiltered buildings. Monitor news for evacuation guidance. If anyone in your household experiences difficulty breathing, chest pain, or confusion, seek medical attention immediately.

The Pollutants Behind the Number

Understanding which pollutant is driving a high AQI helps you take more targeted precautions.

PM2.5 (Fine Particulate Matter) The most dangerous pollutant in terms of health impact. These microscopic particles penetrate deep into lungs and enter the bloodstream. Dominant during wildfire events, winter inversions, and in areas with significant diesel traffic. PM2.5 is responsible for the majority of air-pollution-related deaths worldwide. Learn more on our [PM2.5 pollutant page](/pollutants/particulate-matter).

Ground-Level Ozone Formed when nitrogen oxides and volatile organic compounds react in sunlight. Ozone peaks on hot, sunny afternoons and is the primary driver of summertime smog. Unlike PM2.5, ozone is a gas that irritates lung tissue directly and is not filtered by standard masks. Read about [ozone pollution](/pollutants/ozone-pollution) on our site.

PM10 (Coarse Particulate Matter) Larger particles from dust, construction, and agriculture. Less dangerous than PM2.5 because they are too large to penetrate deep lung tissue, but they still cause irritation and aggravate respiratory conditions.

Carbon Monoxide, Sulfur Dioxide, and Nitrogen Dioxide These gaseous pollutants are primarily concerns near heavy traffic, industrial facilities, and power plants. They rarely drive the overall AQI in most residential areas but can be significant locally.

How AQI Is Measured

The EPA operates a network of over 4,000 monitoring stations across the United States. These stations continuously measure pollutant concentrations using calibrated instruments. The raw measurements (in micrograms per cubic meter for particles, or parts per million for gases) are converted to AQI values using EPA breakpoint tables that map concentration ranges to AQI ranges.

It is important to understand that AQI readings are not instantaneous snapshots. PM2.5 AQI, for example, is based on a 24-hour average by default, though NowCast AQI uses a weighted average that responds more quickly to changing conditions. This means the AQI can lag behind rapidly deteriorating conditions, such as when a wildfire smoke plume first arrives. If you see or smell smoke, take precautions even if the official AQI has not yet caught up.

Using AQI Data in Daily Life

Check AQI before outdoor exercise. Make it as routine as checking the weather forecast. Our city pages provide current and forecasted AQI for every major US metro.

Set AQI alerts. Most air quality apps and our website allow you to set alerts when AQI crosses specific thresholds. A good default is alerting at AQI 100 (the boundary between moderate and unhealthy for sensitive groups).

Plan around daily patterns. Ozone-driven AQI peaks in the afternoon. PM2.5-driven AQI can peak at any time but is often worst during morning and evening commute hours in urban areas. Early morning is generally the best time for outdoor activities during high-pollution periods.

Know your local patterns. Different cities have different pollution profiles. Los Angeles has chronic ozone issues. Salt Lake City faces winter inversion events. Eastern seaboard cities are increasingly affected by wildfire smoke. Understanding your area's typical pollution drivers helps you anticipate problems before they appear on the AQI. Explore our state pages for regional air quality patterns.

Air quality is a fundamental aspect of public health that deserves the same attention we give to food safety, water quality, and weather preparedness. By understanding the AQI scale and making it part of your daily routine, you take control of one of the most important factors in your family's long-term health.