The Air Quality Index turns pollution data into a simple number and color that can help you decide when to adjust outdoor plans.

Quick Breakdown

• What does AQI stand for? Air Quality Index.

• What is a good AQI? An AQI from 0 to 50 is considered good.

• When does AQI become unhealthy? Levels above 100 can affect sensitive groups, while levels above 150 may affect anyone.

• What does AQI measure? Particle pollution, ground-level ozone, carbon monoxide, sulfur dioxide and nitrogen dioxide.

• Is a higher AQI worse? Yes. The scale runs from 0 to 500, and higher numbers mean greater health concerns.

• Does AQI include pollen? No. Pollen forecasts and air-quality forecasts measure different things.

AQI is shorthand for Air Quality Index.

It takes complicated pollution measurements and turns them into a number and color you can use to understand how clean or polluted the outdoor air is.

The AQI scale runs from 0 to 500. The higher the number, the greater the level of pollution and the potential health concern.

What Does AQI Measure?

The Air Quality Index tracks five major types of outdoor air pollution:

• Particle pollution

• Ground-level ozone

• Carbon monoxide

• Sulfur dioxide

• Nitrogen dioxide

These pollutants can come from vehicles, power plants, factories, wildfires, construction, dust and other sources.

Weather also matters. Light winds, temperature inversions and stagnant air can trap pollution near the ground, while stronger winds or an incoming front may help disperse it.

How Is AQI Calculated?

AQI is not an average of every pollutant in the air.

Each pollutant receives its own AQI value. The highest of those values becomes the AQI reported for that location.

For example, if ozone levels are moderate but wildfire smoke pushes particle pollution into the unhealthy range, the overall AQI will be reported as unhealthy and particle pollution will be identified as the main pollutant.

This is why the cause of a high AQI can change from one day to the next.

What Do the AQI Colors and Numbers Mean?

The AQI uses six color-coded categories.

AQI 0–50: Good

Air quality is considered satisfactory, and pollution poses little or no risk for most people.

Color: Green

AQI 51–100: Moderate

Air quality is acceptable for most people. However, a small number of people who are unusually sensitive to pollution may experience symptoms.

Color: Yellow

AQI 101–150: Unhealthy for Sensitive Groups

Children, older adults and people with heart or lung conditions may be more likely to experience health effects. People who are active outdoors may also receive a higher dose of pollution because they breathe more deeply.

Color: Orange

AQI 151–200: Unhealthy

Some members of the general public may begin to experience health effects. Sensitive groups may experience more serious symptoms.

Color: Red

AQI 201–300: Very Unhealthy

The risk of health effects increases for everyone. Outdoor activity may need to be reduced or rescheduled.

Color: Purple

AQI 301–500: Hazardous

This is the highest health-warning category. Everyone is more likely to be affected, and local officials may recommend avoiding outdoor activity.

Color: Maroon

What Is Particle Pollution?

Particle pollution, also called particulate matter, is a mixture of tiny solids and liquid droplets floating in the air.

It is usually divided into two sizes:

• PM10: Particles smaller than 10 micrometers

• PM2.5: Fine particles smaller than 2.5 micrometers

PM2.5 is especially concerning because the particles are small enough to travel deep into the lungs. Wildfire smoke is a major source of fine-particle pollution, but PM2.5 can also come from vehicles, factories, power plants and anything that burns fuel.

Larger particles may come from dust, roads, fields and construction.

What Is Ground-Level Ozone?

High in the atmosphere, ozone helps protect Earth from some of the sun’s harmful ultraviolet radiation.

Near the ground, ozone is pollution.

Ground-level ozone forms when pollutants from vehicles, power plants and other sources react in sunlight. Ozone levels can become especially high during hot, sunny weather with light winds.

Breathing ozone can irritate the airways and make outdoor activity more difficult, particularly for people with asthma or other lung conditions.

What Are the Other AQI Pollutants?

Carbon Monoxide

Carbon monoxide is produced when fuel burns. Vehicles, generators, heating systems and industrial facilities can all release it.

Outdoors, pollution can build near the ground when the atmosphere is stagnant. Carbon monoxide detected inside a home is a separate emergency and should be taken seriously, regardless of the outdoor AQI.

Sulfur Dioxide

Sulfur dioxide can come from volcanoes, but it is also released when fuels containing sulfur are burned at power plants and industrial facilities.

It can irritate the respiratory system and contribute to particle pollution and acid rain.

Nitrogen Dioxide

Nitrogen dioxide is produced primarily by burning fuel.

Cars, trucks, buses, power plants and heavy equipment can all release it. Nitrogen dioxide can also contribute to the formation of ground-level ozone and particle pollution.

Does AQI Include Pollen or Indoor Air Quality?

No. AQI describes specific types of outdoor air pollution.

It generally does not tell you:

• How much pollen is in the air

• Whether mold spores are elevated

• The carbon dioxide level inside a building

• Whether smoke, chemicals or other pollutants are accumulating indoors

• Whether carbon monoxide is present inside your home

You can have a low outdoor AQI and still experience allergy symptoms or poor indoor air quality. AQI, pollen forecasts and indoor-air monitors answer different questions.

What Should You Do When AQI Is High?

The right response depends on the AQI level, the main pollutant and your health.

On unhealthy-air days, you may want to:

• Shorten or reschedule strenuous outdoor activity.

• Take more breaks when working or exercising outside.

• Keep windows closed when smoke or outdoor pollution is elevated.

• Run an appropriately sized air purifier indoors if available.

• Follow local air-quality alerts and health guidance.

• Follow your asthma or respiratory action plan.

• Contact a healthcare professional if symptoms worsen.

Sensitive groups may need to make changes at lower AQI levels than the general public.

Before exercising, commuting or making outdoor plans, check the latest air quality near you with WeatherBug. Conditions can change quickly when smoke, ozone or stagnant weather is involved.

The Bottom Line

AQI is a quick way to understand outdoor air pollution and its potential health effects.

A lower number means cleaner air. As the AQI rises above 100, sensitive groups should begin paying closer attention. Once it rises above 150, the air may be unhealthy for anyone.

The number matters, but so does the pollutant behind it. Checking both can help you make a more informed decision about working, exercising or spending time outside.

Sources: AirNow and the U.S. Environmental Protection Agency.