Air Quality Index (AQI) Explained: Categories, Colors and Ranges

The Air Quality Index (AQI) is the U.S. Environmental Protection Agency’s color-coded scale for reporting outdoor air quality. It has six categories: Good (0–50, green), Moderate (51–100, yellow), Unhealthy for Sensitive Groups (101–150, orange), Unhealthy (151–200, red), Very Unhealthy (201–300, purple) and Hazardous (301 and higher, maroon). EPA says an AQI value of 100 generally corresponds to the level of the short-term national air quality standard, so values above 100 mean unhealthy air, first for sensitive groups and then for everyone.

The scale is the same across the country. EPA calls the U.S. AQI the official Air Quality Index for the United States. It is the index shown on AirNow.gov, in the AirNow app and on the AirNow Fire and Smoke Map.

What do the AQI colors and numbers mean?

Each category has a fixed name, number range and color set by EPA. The descriptions below are EPA’s wording from AirNow’s AQI Basics page. The hex codes match the standard RGB values in EPA’s AQI Technical Assistance Document (May 2026 edition).

Category	AQI range	Color (hex)	EPA description
Good	0–50	Green (#00E400)	“Air quality is satisfactory, and air pollution poses little or no risk.”
Moderate	51–100	Yellow (#FFFF00)	“Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.”
Unhealthy for Sensitive Groups	101–150	Orange (#FF7E00)	“Members of sensitive groups may experience health effects. The general public is less likely to be affected.”
Unhealthy	151–200	Red (#FF0000)	“Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.”
Very Unhealthy	201–300	Purple (#8F3F97)	“Health alert: The risk of health effects is increased for everyone.”
Hazardous	301 and higher	Maroon (#7E0023)	“Health warning of emergency conditions: everyone is more likely to be affected.”

EPA also lets reporting agencies use an alternative “ColorVision Assist” palette with stronger contrast for people with color vision deficiencies. The category names and the six basic colors stay the same.

What is the AQI based on?

EPA calculates an AQI for five pollutants regulated under the Clean Air Act: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide and nitrogen dioxide. Each pollutant has its own national ambient air quality standard. For each one, the scale is built so that 100 generally matches the short-term standard.

The AQI reported for an area is the highest of the individual pollutant values. EPA calls that pollutant the main pollutant. In most places it is ozone or fine particles. EPA’s guidance describes those two as the major sources of unhealthy air quality around 99% of the time.

A daily AQI needs a full day of data. EPA’s reporting guidance says it takes 24 hourly values for particle pollution, or the highest 1-hour or 8-hour value in a 24-hour period for the other pollutants. That is why official daily reports describe yesterday’s air.

The AQI has limits. EPA states that it does not apply to hazardous air pollutants (air toxics), extreme heat or other aspects of outdoor air. EPA counts every value above 300 as Hazardous, including values above 500.

What is the NowCast, and how is it different from the daily AQI?

The NowCast is EPA’s method for turning hourly monitor readings into a current value on the AQI scale. AirNow uses it for current conditions on its maps and app. The AQI itself is based on daily averages or daily maximums, and EPA says it is not valid to calculate an AQI value from hourly data alone.

For PM2.5 and PM10, the NowCast is a weighted average of the previous 12 hours. When air quality is stable, the hours count almost equally, close to a 12-hour average. When readings change quickly, as during a wildfire, the most recent hours get more weight, close to a 3-hour average. EPA requires valid data for 2 of the last 3 hours.

The ozone NowCast uses a method EPA adopted on August 1, 2019. It is based on the relationship between 1-hour and 8-hour ozone data over a rolling two-week window. It rises and falls with the hourly reading while still representing an 8-hour average. Sulfur dioxide and nitrogen dioxide need no NowCast because their AQI already uses 1-hour values.

A NowCast value can change several times a day. The forecast or yesterday’s daily AQI may show one category while the current NowCast shows another.

What health messages does EPA give for each category?

EPA attaches cautionary statements to each AQI category, and they differ by pollutant. The table quotes EPA’s statements for fine particle pollution (PM2.5) from the May 2026 Technical Assistance Document. They are EPA’s words, not our advice, and EPA’s full text adds notes for people with asthma and heart disease.

Category	EPA cautionary statement for PM2.5 (excerpt)
Good	“It’s a great day to be active outside.”
Moderate	“Unusually sensitive people: Consider making outdoor activities shorter and less intense. Go inside if you have symptoms such as coughing or shortness of breath.”
Unhealthy for Sensitive Groups	“Sensitive groups: Make outdoor activities shorter and less intense. It’s OK to be active outdoors but take more breaks.”
Unhealthy	“Sensitive groups: Consider rescheduling or moving all activities inside. Go inside if you have symptoms.” “Everyone else: Keep outdoor activities shorter and less intense.”
Very Unhealthy	“Sensitive groups: Avoid all physical activity outdoors.” “Everyone else: Limit outdoor physical activity.”
Hazardous	“Sensitive groups: Stay indoors and keep activity levels light.” “Everyone: Avoid all physical activity outdoors.”

For PM2.5, EPA lists the sensitive groups as people with heart or lung disease, older adults, and children, including teenagers. It adds that some minority groups, people with lower incomes and outdoor workers may have higher exposure. During smoke events, EPA recommends counting pregnant people as an at-risk group too.

EPA’s statements tell people with heart disease that symptoms such as palpitations, shortness of breath or unusual fatigue may indicate a serious problem, and to contact their health care provider if they have any of them. EPA also notes that without an air conditioner, staying indoors with the windows closed may be dangerous in extremely hot weather.

Ozone messages are worded differently. At Unhealthy for Sensitive Groups, EPA’s ozone statement includes “Plan outdoor activities in the morning when ozone is lower.” Our guide to PM2.5 vs ozone compares the two pollutants side by side.

Where can you check the AQI for your area?

1. Go to AirNow.gov or open the free AirNow app and enter your ZIP code or city. AirNow shows the current NowCast AQI and, where available, the local forecast.
2. Look at which pollutant is listed. The number shown is the highest pollutant AQI for that reporting area.
3. During wildfires, open the AirNow Fire and Smoke Map, which adds temporary monitors and corrected low-cost sensors. Our guide to wildfire smoke and air quality explains how to read it.
4. Check your state, Tribal or local air quality agency for alerts and air quality action days.

Our live AQI readings from EPA AirNow monitors by city collect the official monitor data in one place. EPA requires metropolitan areas with more than 350,000 people to report the AQI daily. Smaller towns may have no monitor, and EPA notes that AirNow participation by state and local agencies is voluntary.

Bottom line

The AQI turns pollution measurements into one number and one color. At or below 100, EPA generally considers air quality satisfactory. Above 100, EPA’s messages apply first to sensitive groups and then to everyone, and the wording depends on whether ozone or particles drive the number. Current values on AirNow are NowCast estimates that move during the day. For alerts where you live, follow your state or local air agency and local health officials.

FAQ

Is an AQI of 100 unhealthy?

An AQI of 100 is the top of the Moderate (yellow) category. EPA says values at or below 100 are generally thought of as satisfactory. Above 100, air quality is unhealthy, at first for sensitive groups.

What does an AQI above 500 mean?

EPA counts all values above 300 as Hazardous, including values above 500. Values above 500 are calculated on the same straight-line slope used between 301 and 500, and the Hazardous health messages apply.

Why does my weather app show a different AQI than AirNow?

Apps can use different monitors, different averaging times or their own indexes. EPA says the U.S. AQI is the official index for the United States and that it does not assess private air quality indexes. AirNow’s current values use the NowCast, which is not the same as a single 1-hour reading.

Why is the AQI green when the air looks hazy?

EPA gives two common reasons. The nearest monitors may measure only ozone, so particle haze does not register. The haze may also come mainly from high humidity rather than pollution.