Air Quality Index (AQI) Calculator

Select a pollutant type, enter the measured concentration, and the calculator automatically looks up EPA breakpoints to give you the estimated AQI, health category, and risk level.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Select the Pollutant Type
Choose the pollutant you're measuring from the dropdown (PM2.5, PM10, Ozone, NO2, SO2, or CO). The calculator will automatically use the correct EPA breakpoints. Select 'Custom / Other' to enter breakpoints manually.
2
Enter the Pollutant Concentration
Input the measured concentration of the pollutant. Units depend on the pollutant (e.g., µg/m3 for PM2.5, ppm for CO).
3
Review your results
The calculator displays the estimated AQI, health category, risk level, and your position within the AQI scale.

Example Calculation

An environmental scientist measures a PM2.5 concentration of 45 µg/m3 and wants to determine the corresponding AQI and health category.

Pollutant Type

PM2.5 (Fine Particulate Matter)

Pollutant Concentration

45 µg/m3

Results

124

Tips

Breakpoints Are Auto-Selected

For the six standard EPA pollutants (PM2.5, PM10, O3, NO2, SO2, CO), the calculator automatically looks up the correct breakpoint range based on your concentration. You only need to enter breakpoints manually when using 'Custom / Other'.

Monitor for Sensitive Groups

If the AQI approaches or exceeds 101 ('Unhealthy for Sensitive Groups'), individuals with respiratory or heart conditions, children, and the elderly should limit prolonged outdoor exertion. Even moderate AQI (51-100) can affect unusually sensitive people.

Different Pollutants, Different Scales

A concentration of 45 means very different things for PM2.5 vs. CO vs. Ozone. Each pollutant has its own breakpoint table because their health impacts occur at different concentration levels. Always select the correct pollutant type for accurate results.

Calculating Your Local Air Quality Index (AQI) from Pollutant Data

The Air Quality Index (AQI) Calculator converts raw pollutant concentration data into a standardized health risk score using the EPA's linear interpolation formula. Select your pollutant type, enter the measured concentration, and the calculator automatically looks up the correct EPA breakpoints to produce an accurate AQI value. For example, a PM2.5 concentration of 45 µg/m3 produces an AQI of 124, categorizing it as 'Unhealthy for Sensitive Groups'.

Why Knowing Your AQI Category is Crucial

Knowing your Air Quality Index (AQI) category is crucial because it directly translates complex environmental data into actionable health guidance. An AQI of 50 ("Good") signals minimal health risk, allowing unrestricted outdoor activity. However, an AQI of 101 ("Unhealthy for Sensitive Groups") immediately alerts individuals with respiratory issues, children, and the elderly to limit prolonged outdoor exertion. As the AQI rises to 151 ("Unhealthy") or higher, the recommendations become more stringent, advising everyone to reduce or avoid outdoor activities. This categorization empowers individuals to protect their health and make informed decisions about daily life, preventing potential exposure to harmful levels of air pollution.

Understanding the EPA's Linear AQI Conversion

The EPA utilizes a linear interpolation method to convert pollutant concentrations into the Air Quality Index (AQI). This approach ensures a consistent and standardized way to report air quality across various pollutants and concentration ranges.

The formula is:

AQI = [(Ip High - Ip Low) / (Cp High - Cp Low)] x (Concentration - Cp Low) + Ip Low

Where:

Concentration is the measured pollutant concentration (e.g., µg/m3).
Cp Low is the concentration breakpoint that is less than or equal to the Concentration.
Cp High is the concentration breakpoint that is greater than or equal to the Concentration.
Ip Low is the AQI value corresponding to Cp Low.
Ip High is the AQI value corresponding to Cp High.

This calculator automatically selects the correct Cp and Ip values from EPA tables when you choose a standard pollutant. For custom pollutants, you enter these breakpoints manually.

💡 For broader atmospheric analysis, tools like the Lifting Condensation Level (LCL) Calculator can help understand cloud formation and atmospheric stability.

Calculating AQI for PM2.5 in a Moderate Scenario

Let's calculate the AQI for a measured PM2.5 concentration. Suppose a local air quality monitor reports a PM2.5 concentration of 45 µg/m3. The calculator automatically identifies the EPA breakpoints for the 'Unhealthy for Sensitive Groups' category:

Lower concentration breakpoint (Cp Low): 35.5 µg/m3
Upper concentration breakpoint (Cp High): 55.4 µg/m3
Corresponding AQI values: 101 (Ip Low) and 150 (Ip High)

Using the formula:

Calculate the range ratios:
- AQI Range = Ip High - Ip Low = 150 - 101 = 49
- Concentration Range = Cp High - Cp Low = 55.4 - 35.5 = 19.9
Determine the position within the concentration band: Concentration - Cp Low = 45 - 35.5 = 9.5
Apply the formula: AQI = (49 / 19.9) x 9.5 + 101
Result: AQI = 2.462 x 9.5 + 101 = 23.39 + 101 = 124.39

Rounding to the nearest whole number, the estimated AQI is 124. This falls within the 'Unhealthy for Sensitive Groups' category, indicating a moderate health concern.

💡 Understanding local air conditions is essential for planning, much like predicting storm severity with a Lifted Index Calculator can aid in weather preparedness.

EPA's Framework for Air Quality Reporting

The U.S. Environmental Protection Agency (EPA) established the Air Quality Index (AQI) as a nationally uniform index for reporting and forecasting daily air quality. This framework, formalized under the Clean Air Act, provides clear health guidance for five major air pollutants regulated by the National Ambient Air Quality Standards (NAAQS): ground-level ozone, particulate matter (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The EPA's system uses a standardized color-coded scale — green for good, yellow for moderate, orange for unhealthy for sensitive groups, red for unhealthy, purple for very unhealthy, and maroon for hazardous — to communicate the health implications of air pollution. This consistent reporting ensures that communities across the United States receive comparable, actionable information to protect public health.

Regulatory Standards and AQI Reporting

The Air Quality Index (AQI) is a critical tool mandated by regulatory bodies like the U.S. Environmental Protection Agency (EPA) to standardize air quality reporting. The EPA's National Ambient Air Quality Standards (NAAQS) define the maximum allowable concentrations for key pollutants, which directly inform the AQI breakpoints. For instance, the primary annual standard for PM2.5 is 9.0 µg/m3 (effective 2024), and the 24-hour standard is 35 µg/m3. An AQI of 50 corresponds to these NAAQS thresholds for most pollutants, representing the "Good" category. When pollutant concentrations exceed these standards, the AQI rises, triggering higher categories like "Unhealthy for Sensitive Groups" (AQI 101-150) or "Unhealthy" (AQI 151-200). These categories are linked to specific health advisories, compelling local air agencies to issue warnings and recommend precautions, ensuring public safety and compliance with federal air quality mandates.

Frequently Asked Questions

What is the Air Quality Index (AQI) and how is it used?

The Air Quality Index (AQI) is a color-coded scale used by government agencies, like the EPA, to communicate daily air quality to the public. It converts complex pollutant concentration data into a single, easy-to-understand number from 0 to 500, with higher values indicating greater air pollution and health risk. The AQI helps individuals make informed decisions about outdoor activities and health precautions based on current air conditions.

What are AQI breakpoints and why are they needed?

AQI breakpoints are specific concentration values for each pollutant that correspond to different AQI health categories, such as Good, Moderate, or Unhealthy. They are needed because each pollutant has varying health impacts and measurement units. These breakpoints establish a standardized, linear conversion between a measured pollutant concentration and its corresponding AQI value, ensuring consistent interpretation of air quality across different pollutants.

What are the common health categories for AQI?

The common health categories for AQI, as defined by the EPA, are Good (0-50), Moderate (51-100), Unhealthy for Sensitive Groups (101-150), Unhealthy (151-200), Very Unhealthy (201-300), and Hazardous (301-500). Each category is associated with specific health messages and recommended precautions, with sensitive groups (children, elderly, those with respiratory conditions) experiencing effects at lower AQI levels.