Calculate Air Quality Index
Enter pollutant concentrations to calculate the AQI using official EPA breakpoints. Select a single pollutant or compare all six at once.
About the Air Quality Index
The Air Quality Index (AQI) is a standardized system developed by the U.S. Environmental Protection Agency (EPA) to communicate daily air quality. It transforms pollutant concentration measurements into a single number on a scale from 0 to 500, making it easy to understand how clean or polluted the air is and what health effects might be a concern.
The AQI focuses on health effects that may be experienced within a few hours or days of breathing polluted air. An AQI value of 100 generally corresponds to the national air quality standard for that pollutant -- the level the EPA has set to protect public health. Values below 100 are generally satisfactory; values above 100 are considered increasingly unhealthy.
The AQI Formula
The AQI for a given pollutant is calculated using linear interpolation between EPA breakpoints:
AQI = ((IHi - ILo) / (BPHi - BPLo)) × (Cp - BPLo) + ILo
Where:
- Cp = the truncated pollutant concentration
- BPHi = the breakpoint greater than or equal to Cp
- BPLo = the breakpoint less than or equal to Cp
- IHi = the AQI value corresponding to BPHi
- ILo = the AQI value corresponding to BPLo
When multiple pollutants are measured, the overall AQI is the highest individual pollutant AQI. That pollutant is called the "dominant" or "responsible" pollutant.
EPA Breakpoint Reference Tables
These are the official EPA breakpoint concentrations used to calculate AQI for each pollutant.
PM2.5 (Fine Particulate Matter, µg/m³, 24-hr avg)
| AQI Category | AQI Range | PM2.5 (µg/m³) |
|---|---|---|
| Good | 0 -- 50 | 0.0 -- 9.0 |
| Moderate | 51 -- 100 | 9.1 -- 35.4 |
| USG | 101 -- 150 | 35.5 -- 55.4 |
| Unhealthy | 151 -- 200 | 55.5 -- 125.4 |
| Very Unhealthy | 201 -- 300 | 125.5 -- 225.4 |
| Hazardous | 301 -- 500 | 225.5 -- 325.4 |
PM10 (Coarse Particulate Matter, µg/m³, 24-hr avg)
| AQI Category | AQI Range | PM10 (µg/m³) |
|---|---|---|
| Good | 0 -- 50 | 0 -- 54 |
| Moderate | 51 -- 100 | 55 -- 154 |
| USG | 101 -- 150 | 155 -- 254 |
| Unhealthy | 151 -- 200 | 255 -- 354 |
| Very Unhealthy | 201 -- 300 | 355 -- 424 |
| Hazardous | 301 -- 500 | 425 -- 604 |
O3 (Ozone, ppb, 8-hr avg)
| AQI Category | AQI Range | O3 (ppb) |
|---|---|---|
| Good | 0 -- 50 | 0 -- 54 |
| Moderate | 51 -- 100 | 55 -- 70 |
| USG | 101 -- 150 | 71 -- 85 |
| Unhealthy | 151 -- 200 | 86 -- 105 |
| Very Unhealthy | 201 -- 300 | 106 -- 200 |
CO (Carbon Monoxide, ppm, 8-hr avg)
| AQI Category | AQI Range | CO (ppm) |
|---|---|---|
| Good | 0 -- 50 | 0.0 -- 4.4 |
| Moderate | 51 -- 100 | 4.5 -- 9.4 |
| USG | 101 -- 150 | 9.5 -- 12.4 |
| Unhealthy | 151 -- 200 | 12.5 -- 15.4 |
| Very Unhealthy | 201 -- 300 | 15.5 -- 30.4 |
| Hazardous | 301 -- 500 | 30.5 -- 50.4 |
SO2 (Sulfur Dioxide, ppb, 1-hr avg)
| AQI Category | AQI Range | SO2 (ppb) |
|---|---|---|
| Good | 0 -- 50 | 0 -- 35 |
| Moderate | 51 -- 100 | 36 -- 75 |
| USG | 101 -- 150 | 76 -- 185 |
| Unhealthy | 151 -- 200 | 186 -- 304 |
| Very Unhealthy | 201 -- 300 | 305 -- 604 |
| Hazardous | 301 -- 500 | 605 -- 1004 |
NO2 (Nitrogen Dioxide, ppb, 1-hr avg)
| AQI Category | AQI Range | NO2 (ppb) |
|---|---|---|
| Good | 0 -- 50 | 0 -- 53 |
| Moderate | 51 -- 100 | 54 -- 100 |
| USG | 101 -- 150 | 101 -- 360 |
| Unhealthy | 151 -- 200 | 361 -- 649 |
| Very Unhealthy | 201 -- 300 | 650 -- 1249 |
| Hazardous | 301 -- 500 | 1250 -- 2049 |
Understanding the Six Pollutants
PM2.5 -- Fine Particulate Matter
Particles smaller than 2.5 micrometers in diameter. Sources include vehicle exhaust, power plants, wildfires, and industrial processes. PM2.5 can penetrate deep into the lungs and enter the bloodstream, causing respiratory and cardiovascular problems. It is often the dominant pollutant in AQI readings worldwide.
PM10 -- Coarse Particulate Matter
Particles smaller than 10 micrometers, including dust, pollen, and mold. Sources include construction sites, unpaved roads, and wind-blown dust. While larger than PM2.5, these particles can still irritate the respiratory system and aggravate conditions like asthma.
O3 -- Ground-Level Ozone
Unlike stratospheric ozone that protects from UV radiation, ground-level ozone is harmful. It forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in sunlight. Ozone irritates the respiratory system, reduces lung function, and aggravates asthma. Levels are typically highest on hot, sunny afternoons.
CO -- Carbon Monoxide
A colorless, odorless gas produced by incomplete combustion of fossil fuels. Major sources are vehicles and industrial processes. CO reduces the blood's ability to carry oxygen. High concentrations are most dangerous in enclosed spaces but outdoor levels near heavy traffic can also be concerning.
SO2 -- Sulfur Dioxide
Produced primarily by burning sulfur-containing fuels in power plants and industrial facilities. SO2 irritates the respiratory system and can cause difficulty breathing, particularly for people with asthma. It also contributes to the formation of particulate matter and acid rain.
NO2 -- Nitrogen Dioxide
A reddish-brown gas produced by vehicle engines, power plants, and other combustion sources. NO2 inflames airways, reduces lung function, and increases susceptibility to respiratory infections. It also contributes to ozone and particulate matter formation.
Health Recommendations by AQI Level
| AQI Level | Concern | Guidance |
|---|---|---|
| Good (0--50) | None | Air quality is satisfactory. Enjoy outdoor activities. |
| Moderate (51--100) | Low | Acceptable. Unusually sensitive individuals should consider limiting prolonged outdoor exertion. |
| USG (101--150) | Moderate | Sensitive groups (asthma, heart/lung disease, elderly, children) should reduce prolonged outdoor exertion. |
| Unhealthy (151--200) | High | Everyone may begin to experience effects. Sensitive groups should avoid prolonged outdoor exertion. |
| Very Unhealthy (201--300) | Very High | Health alert: everyone should reduce outdoor exertion. Sensitive groups should avoid all outdoor activity. |
| Hazardous (301--500) | Emergency | Health warning: everyone should avoid all outdoor physical activity. Stay indoors with windows closed. |
Frequently Asked Questions
What is the Air Quality Index (AQI)?
The AQI is a standardized scale from 0 to 500 developed by the EPA to communicate daily air quality. It translates pollutant concentrations into a single number divided into six color-coded categories, from Good (green) to Hazardous (maroon). The higher the AQI, the greater the health concern.
How is AQI calculated?
AQI is calculated using EPA breakpoint tables and linear interpolation. For each pollutant, the measured concentration is mapped to the corresponding AQI range using the formula: AQI = ((I_Hi - I_Lo) / (BP_Hi - BP_Lo)) * (Cp - BP_Lo) + I_Lo. When multiple pollutants are measured, the overall AQI is the highest individual value.
What pollutants are used to calculate AQI?
The EPA uses six criteria pollutants: PM2.5 (fine particulate matter, 24-hour average), PM10 (coarse particulate matter, 24-hour average), ground-level ozone (8-hour average), carbon monoxide (8-hour average), sulfur dioxide (1-hour average), and nitrogen dioxide (1-hour average).
What does an AQI of 100 mean?
An AQI of 100 corresponds to the national ambient air quality standard for that pollutant. Below 100 is generally satisfactory for most people. Above 100, air quality becomes unhealthy, first for sensitive groups (101-150) and then for the general population as values increase.
Who are sensitive groups?
Sensitive groups include people with asthma, chronic obstructive pulmonary disease (COPD), heart disease, older adults, children, and people who are physically active outdoors (such as outdoor workers). These individuals are more likely to experience health effects at lower AQI levels.
What is PM2.5 and why is it important?
PM2.5 refers to fine particulate matter with an aerodynamic diameter of 2.5 micrometers or smaller -- about 30 times smaller than a human hair. These tiny particles can pass through the nose and throat, penetrate deep into the lungs, and even enter the bloodstream. PM2.5 is strongly associated with respiratory disease, cardiovascular problems, and premature mortality. It is frequently the dominant pollutant driving AQI values.
Can AQI go above 500?
The standard AQI scale runs from 0 to 500. Values above 500 are considered "Beyond the AQI" and represent extremely hazardous conditions. During severe pollution events like major wildfires, some agencies report values above 500, but these are outside the standard EPA breakpoint tables.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.
Privacy & Limitations
Privacy: This calculator runs entirely in your browser. No data is transmitted or stored anywhere.
Limitations: This tool uses EPA breakpoint tables for educational purposes. Actual air quality monitoring involves calibrated instruments with specific averaging periods. The AQI values shown here are based on the concentration you enter and assume it represents the correct averaging period for each pollutant. For official air quality data, consult your local environmental agency or AirNow.gov.
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Air Quality Index Calculator FAQ
What is the Air Quality Index (AQI)?
The Air Quality Index (AQI) is a standardized scale from 0 to 500 used by the EPA to communicate how polluted the air is. It translates pollutant concentrations into a single number with six categories: Good (0-50), Moderate (51-100), Unhealthy for Sensitive Groups (101-150), Unhealthy (151-200), Very Unhealthy (201-300), and Hazardous (301-500).
How is AQI calculated?
AQI is calculated using EPA breakpoint tables. For a given pollutant concentration, the formula is: AQI = ((IHi - ILo) / (BPHi - BPLo)) * (Cp - BPLo) + ILo, where Cp is the pollutant concentration, BPHi/BPLo are the breakpoint concentrations, and IHi/ILo are the corresponding AQI values. The overall AQI is the highest individual pollutant AQI.
What pollutants are used to calculate AQI?
The EPA uses six criteria pollutants: PM2.5 (fine particulate matter), PM10 (coarse particulate matter), ground-level ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen dioxide (NO2). Each has specific concentration breakpoints and averaging periods.
What does an AQI of 100 mean?
An AQI of 100 corresponds to the national air quality standard for that pollutant. Below 100 is generally considered satisfactory. Above 100, air quality becomes unhealthy, first for sensitive groups and then for everyone as the AQI rises.
Who are sensitive groups for air quality?
Sensitive groups include people with asthma, respiratory or heart disease, older adults, children, and people who are active outdoors. These individuals may experience health effects at lower pollution levels than the general population.
What is PM2.5 and why is it dangerous?
PM2.5 refers to fine particulate matter with a diameter of 2.5 micrometers or smaller. These particles are dangerous because they can penetrate deep into the lungs and enter the bloodstream, causing respiratory and cardiovascular problems. PM2.5 is often the dominant pollutant in AQI readings.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.