AQI Levels Explained

Understanding U.S. AQI levels

The U.S. Air Quality Index (AQI) is EPA's color-coded system for communicating outdoor air quality and health concern. A higher value means more pollution and a greater reason to reduce exposure.

AQI is pollutant-specific. The reported value may be driven by particle pollution, ozone, carbon monoxide, sulfur dioxide, or nitrogen dioxide. Always check the named pollutant before converting AQI to a concentration or Berkeley Earth health-impact equivalence.

AQI colors and categories

• Green · Good · 0–50: Air quality is satisfactory and pollution poses little or no risk.
• Yellow · Moderate · 51–100: Air quality is acceptable, although unusually sensitive people may be affected.
• Orange · Unhealthy for Sensitive Groups · 101–150: Sensitive groups may experience effects; the general public is less likely to be affected.
• Red · Unhealthy · 151–200: Some people in the general public may experience effects, while sensitive groups may experience more serious effects.
• Purple · Very Unhealthy · 201–300: The risk of health effects is increased for everyone.
• Maroon · Hazardous · 301–500: Emergency conditions: everyone is more likely to be affected.

These are category-level summaries, not a personal diagnosis. Local agencies may publish more specific instructions during wildfire smoke, ozone episodes, industrial incidents, or extreme heat.

Good: AQI 0–50

Most people can continue normal outdoor plans. If smoke is visible or you have symptoms despite a low reported AQI, check the pollutant, monitor location, and update time before relying on the number.

Useful next step: Check the forecast if conditions are changing, especially during wildfire season.

Moderate: AQI 51–100

Air quality is acceptable for most people. People who are unusually sensitive to air pollution can watch for symptoms and consider reducing prolonged or heavy outdoor exertion when needed.

Useful next step: Move a hard workout to a cleaner time of day if the forecast is rising.

Unhealthy for Sensitive Groups: AQI 101–150

Sensitive groups differ by pollutant. For example, EPA particle-pollution guidance names people with heart or lung disease, older adults, and children; during a smoke event, EPA also recommends including pregnant people as an at-risk group. Check the named pollutant and local guidance before deciding who should reduce prolonged or heavy outdoor activity.

Useful next step: Shorten the activity, lower its intensity, take more breaks, or use an indoor alternative.

Unhealthy: AQI 151–200

Some members of the general public may experience health effects. Sensitive groups may experience more serious effects and should take stronger steps to reduce exposure.

Useful next step: Reduce long or intense outdoor activity and follow local health guidance. During particle pollution or smoke, prioritize cleaner indoor air.

Very Unhealthy: AQI 201–300

This category represents a health alert. The risk of health effects is increased for everyone, so outdoor activity plans should be reconsidered.

Useful next step: Avoid strenuous outdoor activity, postpone nonessential exposure, and use the cleanest practical indoor space.

Hazardous: AQI 301–500

This category represents a health warning of emergency conditions. Everyone is more likely to be affected.

Useful next step: Follow local emergency and public-health instructions, avoid outdoor exertion, and reduce nonessential outdoor exposure.

Current AQI, daily AQI, and forecasts are different

• Daily AQI summarizes air quality over a pollutant-specific time frame. Particle pollution uses a 24-hour basis; ozone uses an 8-hour basis.
• AQI forecast predicts the next day's conditions and helps you plan activities in advance.
• NowCast AQI uses multiple recent monitoring hours to represent current conditions. It can change during the day, especially when wildfire smoke moves quickly.

Check both the forecast and current reading before a long outdoor activity. Also confirm the station location and update time; one monitor may not describe every neighborhood.

When an AQI can be estimated as PM2.5

This site's PM2.5 estimate is appropriate only when the AQI is specifically the PM2.5 sub-index. An ozone-driven or unspecified overall AQI cannot be treated as a PM2.5 concentration.

When PM2.5 is confirmed, the calculator reverses the current EPA breakpoint band to estimate concentration. The separate cigarette-equivalent result uses Berkeley Earth's rough population-level health-impact equivalence for risk communication. It does not measure cigarettes smoked or predict an individual's exposure, diagnosis, or health outcome.

Practical ways to reduce exposure

Depending on the pollutant, AQI category, symptoms, and local guidance, useful options can include:

• Choose a cleaner time or location for outdoor activity.
• Reduce duration or intensity and take more breaks.
• Move strenuous activity indoors when outdoor air is unhealthy.
• During wildfire smoke, close windows and doors, set HVAC to recirculate where appropriate, and use correctly sized filtration.
• Air out the home when outdoor air improves, while avoiding unsafe indoor heat.
• Seek medical care for severe symptoms and follow an existing care plan from your health professional.

Continue from the AQI category

• Use the activity decision tool to combine AQI with activity, duration, intensity, and audience.
• Use the PM2.5 ↔ AQI converter when the pollutant is confirmed as PM2.5.
• Read the calculation methodology before interpreting a cigarette-equivalent result.
• Review air-pollution protection options for filtration, masks, and cleaner-air planning.

Primary references

• AirNow: AQI Basics
• AirNow: Using the Air Quality Index
• U.S. EPA: Strategies to Reduce Exposure Indoors

This page provides general educational guidance. It does not replace local alerts, workplace rules, emergency instructions, or medical advice.