Air Quality Index (AQI) Calculator

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'.

The 'Air Quality Insights' panel provides a detailed interpretation of the health implications.

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.

The calculator's 'Air Quality Insights' panel further elaborates on these health implications.

Understanding the EPA's Linear AQI Conversion Formula

The U.S. Environmental Protection Agency (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 primary formula for calculating AQI 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 for PM2.5, ppm for CO).
• 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.

Interpreting AQI Categories and Health Messages

The calculated AQI value is then mapped to a specific health category, each with an associated health message and risk level.

These interpretations are crucial for understanding the practical implications of the air quality.

• Good (0-50 AQI): Air quality is satisfactory, and air pollution poses little or no risk.
• Moderate (51-100 AQI): Air quality is acceptable; however, for some pollutants, there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.
• Unhealthy for Sensitive Groups (101-150 AQI): Members of sensitive groups may experience health effects. The general public is less likely to be affected.
• Unhealthy (151-200 AQI): Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.
• Very Unhealthy (201-300 AQI): Health alert: everyone may experience more serious health effects.
• Hazardous (301-500 AQI): Health warnings of emergency conditions. The entire population is more likely to be affected.

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:

1. 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
2. Determine the position within the concentration band: Concentration - Cp Low = 45 - 35.5 = 9.5
3. Apply the formula: AQI = (49 / 19.9) x 9.5 + 101
4. Intermediate Calculation: AQI = 2.4623115577889447 x 9.5 + 101 = 23.391959798994975 + 101
5. Result: AQI = 124.391959798994975

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.

The 'Air Quality Insights' panel would further detail the specific health risk and your position within this AQI band.

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 2026), 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.