Air Quality Index (AQI) Explained: What 50, 100, 150 and 200 Mean for Your Health

The Air Quality Index (AQI) is a U.S. scale that explains how outdoor air pollution may affect your health. A higher AQI means greater health concern: 0–50 is Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, 151–200 Unhealthy, 201–300 Very Unhealthy, and 301 or higher Hazardous. To decide whether to exercise, open windows or avoid smoke, check your location’s current air quality at AirNow.gov and read the guidance for the pollutant driving the reading.

AQI color chart: what each number means

U.S. AQI	Official color	Level of concern	General interpretation
0–50	Green	Good	Air pollution poses little or no risk for most people
51–100	Yellow	Moderate	Generally acceptable, though unusually sensitive individuals may be affected
101–150	Orange	Unhealthy for Sensitive Groups	Sensitive groups may experience effects; most of the public is less likely to be affected
151–200	Red	Unhealthy	Some effects may occur in the general population; higher concern for sensitive groups
201–300	Purple	Very Unhealthy	Health alert, with increased risk for everyone
301+	Maroon	Hazardous	Emergency-level health warning; everyone is more likely to be affected

These ranges and colors are defined in the official AirNow AQI Basics table. The values are index numbers, not direct measurements in micrograms per cubic meter, and the same AQI number can arise from different pollutants or averaging periods.

What do AQI 50, 100, 150 and 200 mean?

AQI 50: Good

An AQI of 50 is still in the green Good category. It indicates a comparatively low level of concern from the pollutant being reported. However, an outdoor AQI reading does not test the air in a specific room, building or vehicle.

AQI 100: Moderate

An AQI of 100 is yellow Moderate, not orange. AirNow describes this as generally acceptable air quality, although people who are unusually sensitive to pollution may be at some risk. For those individuals, the activity advice can depend on whether ozone or particle pollution is driving the number.

AQI 150: Unhealthy for Sensitive Groups

An AQI of 150 is at the top of orange Unhealthy for Sensitive Groups. People with relevant lung or heart conditions, some children and older adults, and other groups identified in the local pollutant-specific guidance should consider reducing prolonged or strenuous outdoor exertion.

AQI 200: Unhealthy

An AQI of 200 remains in red Unhealthy; 201 begins purple Very Unhealthy. At a red reading, some people in the general population may also experience effects, and people in sensitive groups can be at higher risk. Limit exposure and check the local AirNow recommendations rather than assuming a short walk or a hard outdoor workout carries the same level of concern.

How to check today’s AQI by ZIP code

1. Go to AirNow.gov and enter your U.S. ZIP code or city.
2. Read the displayed AQI number, color and category, and check the timestamp.
3. Look for the primary pollutant, often ozone or fine particulate matter (PM2.5), where displayed.
4. Compare the current conditions with the forecast for the day and following day, if provided.
5. Read the activity advice for that pollutant and for any relevant sensitive group.
6. Recheck if nearby fires, winds or weather conditions are changing rapidly.

AirNow’s official guide to using the AQI distinguishes current NowCast data from daily reports and forecasts. NowCast uses recent readings and adapts to changing conditions; a next-day forecast is a prediction, not the same as a live monitoring result.

What pollutants are included in the Air Quality Index?

EPA establishes AQI calculations for five major outdoor pollutants: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Different pollution types affect health differently; the color is a quick overview, not a diagnosis.

PM2.5: fine particles and wildfire smoke

PM2.5 refers to particles with aerodynamic diameters of 2.5 micrometers or less. Fine particles can occur in smoke from wildfires, vehicle exhaust and other combustion sources. On smoky days, PM2.5 levels can change quickly with wind and fire conditions. The AirNow Fire and Smoke Map offers additional location-specific smoke and fine-particle information, but it is not a guarantee that an individual indoor location has clean air.

Ground-level ozone: often a warm-season concern

Ground-level ozone forms through atmospheric reactions involving pollutants and sunlight. Ozone alerts may be particularly relevant for outdoor exercise because increased breathing during exertion can increase exposure. A filter mask used against smoke particles does not automatically protect against ozone gas.

Who should pay extra attention to an AQI alert?

The specific groups depend on the pollutant, but EPA and CDC guidance highlights people with asthma or other lung conditions, some people with cardiovascular disease, children, older adults and people who spend long periods active outdoors. People who are pregnant or have other chronic medical conditions may also need extra precautions during wildfire smoke events.

Anyone with a personal asthma or other care plan should follow that plan and their healthcare professional’s advice. Serious symptoms—including chest pain, significant difficulty breathing or other medical emergencies—require urgent medical help, regardless of the AQI category.

What should you do when the AQI is orange, red or purple?

• Orange (101–150): sensitive individuals should review the pollutant-specific AirNow activity guide and consider making exercise shorter or less strenuous outdoors.
• Red (151–200): reduce prolonged or heavy outdoor exertion; sensitive groups should take additional precautions and may need to move activities indoors.
• Purple (201–300): reduce outdoor exposure broadly, move strenuous activities indoors if possible and prioritize a cleaner indoor environment.
• Maroon (301+): treat the reading as a serious public-health warning. Avoid unnecessary exposure, follow public health instructions and use the safest available cleaner-air environment.

These are general, non-individualized suggestions. Official activity recommendations differ by the pollutant and by a person’s risk factors. Outdoor safety also depends on heat, traffic, nearby fires and actual indoor conditions.

Can an N95 protect you when the AQI is high?

A properly fitted NIOSH-approved N95 can help reduce inhalation of particulate matter from wildfire smoke, but it is not protection against gases such as ozone or carbon monoxide. Fit is important; the respirator should seal around your nose and mouth. People who have difficulty breathing or certain health conditions may need medical advice before wearing a tight-fitting respirator. Avoid assuming a loose cloth face covering provides the same particle protection.

The CDC’s wildfire smoke safety guidance recommends reducing smoke exposure, protecting indoor spaces and following official emergency instructions. A good indoor air filter, closed windows during smoke events and a cleaner-air room can help lower exposure where suitable; follow HVAC manufacturer guidance and ensure safe cooling during hot weather.

Current AQI vs. forecast AQI vs. the Fire and Smoke Map

Data view	What it tells you	Important limitation
Current AQI / NowCast	A recent-conditions estimate using monitoring data	May change through the day; does not measure your indoor air
AQI forecast	Expected pollution conditions during a future period	Weather and emissions can change, so forecasts are not guarantees
AirNow Fire and Smoke Map	Extra particle and smoke information from monitoring and sensors	Some corrected low-cost sensor data has uncertainty and is not used for regulatory compliance

EPA’s Fire and Smoke Map disclaimer explains that sensor measurements can be delayed, missing or inaccurate. Satellite-detected smoke plumes are also not direct measurements of what people are breathing at street level.

Can you tell the AQI just by looking at the sky?

No. Haze, smoke, fog, humidity and lighting can affect visibility, and harmful pollutants are not always visible. The featured image shows an actual hazy Los Angeles skyline photographed in June 2005; it cannot establish an AQI reading for that day, nor does it show air conditions in November 2026. Always use current official monitoring information rather than estimating the index from a photograph.

Air quality checklist before an outdoor workout

1. Check AirNow’s current and forecast AQI for your actual outdoor location.
2. Note the main pollutant, the color category and whether you belong to a relevant sensitive group.
3. For orange or worse air, review the appropriate activity guidance before choosing intensity or duration.
4. Move strenuous exercise indoors or reduce exposure when warranted, especially during a smoke event.
5. Recheck AQI when local fire, wind or weather conditions change and pay attention to symptoms.

Further reading and official sources

Continue with our wildfire smoke and vehicle ventilation guide and home radon testing guide for other air-quality questions. Note that radon is a separate indoor hazard and is not the same as the outdoor AQI.

Official references: AirNow: AQI Basics, AirNow: Using the AQI, AirNow Fire and Smoke Map, and CDC: Wildfire Smoke Safety.

Evidence-based U.S. public-information guide checked November 9, 2026; not individual medical advice. Featured image: Los Angeles skyline in haze, June 2005, photographer Pag293, public domain via Wikimedia Commons. A photograph cannot establish a real-time AQI category.