Understanding Air Quality Index Levels and What They Mean

What the AQI Actually Tells You

The Air Quality Index is a single number that summarizes how clean or polluted the air is. It is not a direct measurement of one substance. Instead, it is calculated separately for each of six pollutants: fine particles (PM2.5), coarse particles (PM10), ground-level ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide (CO). The reported AQI is the highest of those sub-indices, and that pollutant is called the "main" or "dominant" pollutant. So when you see an AQI of 150, it might be driven by ozone on a sunny afternoon or by PM2.5 during a wildfire event.

The index runs from 0 to 500. Higher numbers mean worse air quality and greater health concern. The six categories and their colors are:

What Happens in the Air at Each Level

Good (0-50)

At this level, the air is clean. Particle and gas concentrations are low. The sky is usually clear, and visibility is high. No health effects are expected. This is typical of remote areas or after a rainstorm that has washed pollutants out of the air.

Moderate (51-100)

The air is acceptable, but for a few people who are unusually sensitive to pollution, there may be mild effects. This level is common in many urban areas on an average day. People with asthma might notice slight irritation if they exercise outdoors for a long time. The sky may look slightly hazy.

Unhealthy for Sensitive Groups (101-150)

This is the first level where health effects become a real concern for certain people. Sensitive groups include people with asthma or other lung disease, people with heart disease, children and teenagers, older adults, pregnant people, and people who work or exercise hard outdoors. For them, prolonged outdoor exertion may cause coughing, chest tightness, or shortness of breath. The general population is unlikely to be affected. Ozone often drives this level on sunny afternoons.

Unhealthy (151-200)

At this level, everyone may begin to feel effects. The air may look hazy or smoky, although ozone itself is invisible. People in sensitive groups may experience more serious symptoms, and the general public may notice irritation of the eyes, nose, and throat. It is wise for everyone to reduce prolonged or heavy outdoor exertion. Particle pollution is often the culprit at this level, especially during wildfire smoke events or temperature inversions.

Very Unhealthy (201-300)

Health effects are more widespread. The air may smell smoky or acrid. Sensitive groups should avoid all outdoor physical activity, and everyone else should avoid prolonged or heavy exertion. Sensitive groups may experience more serious effects, including aggravation of asthma or heart problems. This level is common during severe wildfire episodes or in heavily polluted cities during winter inversions.

Hazardous (301-500)

This is an emergency condition. The air is dangerous to everyone. Health warnings are issued, and people are advised to stay indoors with windows closed. If you must go outside, a well-fitted N95 or FFP2 respirator is far more effective than a cloth mask at filtering fine particles. HEPA air purifiers can reduce indoor particle levels. This level is rare but can occur during extreme wildfire smoke or industrial accidents.

How the Single Number Is Built

The AQI is not a direct measurement. For each of the six pollutants, a sub-index is calculated by comparing the measured concentration to a breakpoint table. The sub-index that is highest becomes the reported AQI, and that pollutant is the "main" one. For example, if PM2.5 gives a sub-index of 110 and ozone gives 90, the reported AQI is 110, and PM2.5 is the main pollutant.

This means that the AQI can sometimes be driven by a pollutant that is not the most visible. On a clear day, ozone might be the main driver. On a smoky day, PM2.5 dominates.

Why Two Apps Can Show Different Numbers

If you check the AQI on two different apps or websites, you might see different numbers. This is normal and has several causes:

• Different scales: Other countries use their own scales. For example, the European Air Quality Index, the UK Daily Air Quality Index (which runs from 1 to 10), and India's National AQI all use different breakpoints and colors. The same air can get a different number and color elsewhere.
• NowCast vs. 24-hour average: Many services show a "NowCast" or hourly figure that reacts faster than a 24-hour average. The NowCast is designed to respond quickly to rapid changes, such as a sudden spike in smoke. The 24-hour average is slower to change but gives a more stable picture. An app using NowCast may show a higher number during a short pollution event than one using the 24-hour average.
• Station vs. satellite/model: Some apps use data from official monitoring stations, which are accurate but sparse. Others use satellite data or computer models that cover a wider area but may be less precise. A station reading may be higher or lower than a model estimate for the same location.
• The 2024 PM2.5 change: In 2024, the EPA tightened the PM2.5 scale. Before 2024, the top of the Good band was 12.0 ug/m3. Now it is 9.0 ug/m3. This means that a PM2.5 concentration of 10 ug/m3 was previously considered Good but is now Moderate. Older apps that have not been updated may still show the old colors, so you might see a different AQI for the same air.

How the AQI Compares with the WHO Guideline

The World Health Organization (WHO) sets health-based guidelines that are more protective than the US EPA standards. The WHO 2021 guideline for PM2.5 is an annual mean of 5 ug/m3 and a 24-hour mean of 15 ug/m3. In contrast, the US EPA's Good category for PM2.5 allows up to 9.0 ug/m3 for the 24-hour average. This means that air that meets the US EPA's Good standard may still exceed the WHO guideline. The WHO guideline is a health-based recommendation, not a legal standard.

For ozone, the WHO guideline is also stricter than the US EPA standard. Even so, the AQI remains a useful tool for daily decisions, as long as you understand its limitations.

Who Are Sensitive Groups?

Sensitive groups are people who are more likely to experience health effects from air pollution. They include:

• People with asthma or other lung disease
• People with heart disease
• Children and teenagers
• Older adults
• Pregnant people
• People who work or exercise hard outdoors

If you belong to one of these groups, pay close attention to the AQI, especially when it reaches the orange level or higher. Consider reducing outdoor activity, using a HEPA air purifier indoors, and wearing a well-fitted N95 or FFP2 respirator if you must go outside during poor air quality.

Frequently Asked Questions

What is the difference between PM2.5 and PM10?

PM2.5 refers to fine particles that are 2.5 microns or smaller in diameter. They can penetrate deep into the lungs and enter the bloodstream. PM10 refers to coarse particles that are 10 microns or smaller. They are larger and tend to irritate the upper airways. Both are included in the AQI, but PM2.5 is more harmful to health.

Why does the AQI sometimes change rapidly during the day?

The AQI can change quickly because pollution levels are not constant. Ozone peaks on sunny afternoons, while particle pollution is often higher in the morning and evening. Wind, rain, and temperature inversions can also cause rapid changes. The NowCast index is designed to reflect these short-term fluctuations.

Is the AQI the same everywhere in the world?

No. Different countries use different scales and breakpoints. The US EPA AQI is used in the United States. Europe uses the European Air Quality Index, the UK uses the Daily Air Quality Index (1 to 10), and India uses its own National AQI. The same air can get a different number and color depending on which scale is used.

Should I trust the AQI from my phone app?

It depends on the app. Apps that use official monitoring station data are generally reliable. Apps that use satellite data or models may be less precise. Also, check whether the app uses NowCast or a 24-hour average, as this can affect the number. If you are in the US, the AirNow website and app from the EPA are a trusted source.

What can I do to protect myself when the AQI is high?

When the AQI is in the orange or higher, reduce outdoor activity. If you must go outside, wear a well-fitted N95 or FFP2 respirator, which filters fine particles far better than cloth masks. Indoors, use a HEPA air purifier to reduce particle levels. Keep windows and doors closed. If you have asthma or heart disease, follow your action plan and consult your doctor if symptoms worsen.