Air Quality Index: How to Calculate AQI From Pollutant Concentration
Use this interactive AQI calculator to estimate the U.S. Air Quality Index from measured pollutant concentration. Select a pollutant, enter the concentration, and instantly see the AQI value, category, health meaning, and a visual chart.
AQI Calculator
Enter a pollutant concentration and click Calculate AQI to see your result.
Air Quality Index: how to calculate it correctly
The Air Quality Index, usually called AQI, is a standardized way to convert raw air pollution measurements into a simple public health number. Instead of expecting the public to interpret a concentration like 42.7 micrograms per cubic meter of PM2.5 or 0.081 parts per million of ozone, AQI translates that number into a scale with categories such as Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous. If you are searching for air quality index how to calculate, the core idea is straightforward: you identify the pollutant, find the concentration breakpoints that bracket the measured value, and interpolate the AQI within that range.
In the United States, the AQI system is managed by the U.S. Environmental Protection Agency. The AQI generally runs from 0 to 500, though most daily values are much lower. The highest pollutant-specific AQI among the measured pollutants becomes the “overall” AQI reported for a location. This means that even if multiple pollutants are present, the worst one for that time period usually determines the headline AQI.
The AQI formula
The AQI calculation uses a linear interpolation equation. Once you know the concentration breakpoints that surround the measured concentration, you calculate the index value using this formula:
AQI = ((Ihigh – Ilow) / (BPhigh – BPlow)) x (Cp – BPlow) + Ilow
- Cp = the truncated pollutant concentration
- BPlow = the lower concentration breakpoint that contains Cp
- BPhigh = the upper concentration breakpoint that contains Cp
- Ilow = the AQI value associated with BPlow
- Ihigh = the AQI value associated with BPhigh
After interpolation, the AQI is rounded to the nearest whole number. This is why raw concentration values do not map directly to AQI values on a one-to-one basis. The relationship changes from one pollutant range to another because each category has its own concentration boundaries.
Why truncation matters
One of the most overlooked parts of AQI calculation is pollutant-specific truncation. Before the formula is applied, the measured concentration is not always used exactly as recorded. Under U.S. EPA conventions:
- PM2.5 is truncated to one decimal place.
- PM10 is truncated to a whole number.
- 8-hour ozone is truncated to three decimal places.
This step matters because a concentration of 35.49 for PM2.5 is treated as 35.4 for AQI purposes, not 35.5. That can move the reading to a slightly different point within a breakpoint range and change the final AQI by a point or two.
EPA AQI categories and what they mean
The AQI categories are tied to health messaging. As the AQI number rises, the public health concern becomes more serious.
| AQI Range | Category | Color | Health Meaning |
|---|---|---|---|
| 0 to 50 | Good | Green | Air quality is satisfactory, and air pollution poses little or no risk. |
| 51 to 100 | Moderate | Yellow | Air quality is acceptable, but there may be a moderate concern for an unusually sensitive few. |
| 101 to 150 | Unhealthy for Sensitive Groups | Orange | Children, older adults, and people with heart or lung disease may be affected more strongly. |
| 151 to 200 | Unhealthy | Red | Some members of the general public may experience health effects; sensitive groups face greater risk. |
| 201 to 300 | Very Unhealthy | Purple | Health alert conditions. The risk of health effects increases for everyone. |
| 301 to 500 | Hazardous | Maroon | Emergency conditions. The entire population is more likely to be affected. |
Common pollutant breakpoints used in AQI calculations
Different pollutants have different health-based concentration ranges. For the calculator on this page, the selected pollutants use the following standard AQI breakpoint structure.
| Pollutant | Units | Good | Moderate | USG | Unhealthy | Very Unhealthy | Hazardous |
|---|---|---|---|---|---|---|---|
| PM2.5 (24-hour) | micrograms/m3 | 0.0 to 12.0 | 12.1 to 35.4 | 35.5 to 55.4 | 55.5 to 150.4 | 150.5 to 250.4 | 250.5 to 500.4 |
| PM10 (24-hour) | micrograms/m3 | 0 to 54 | 55 to 154 | 155 to 254 | 255 to 354 | 355 to 424 | 425 to 604 |
| Ozone (8-hour) | ppm | 0.000 to 0.054 | 0.055 to 0.070 | 0.071 to 0.085 | 0.086 to 0.105 | 0.106 to 0.200 | Not typically reported in the same way above this range |
These ranges reveal something important: AQI is not a simple percentage scale. The same numerical AQI increase does not always correspond to the same increase in pollutant concentration. For example, moving from PM2.5 concentration 10 to 20 micrograms/m3 changes AQI meaningfully, but moving from 100 to 110 micrograms/m3 occurs in a very different health context.
Step-by-step example: calculating AQI for PM2.5
Suppose a monitor reports a 24-hour PM2.5 concentration of 35.9 micrograms/m3.
- First, truncate to one decimal place. In this case, 35.9 stays 35.9.
- Find the PM2.5 breakpoints that contain 35.9. That falls in the 35.5 to 55.4 range, which corresponds to AQI 101 to 150.
- Plug the values into the equation:
- Cp = 35.9
- BPlow = 35.5
- BPhigh = 55.4
- Ilow = 101
- Ihigh = 150
- Compute the linear interpolation and round to the nearest whole number.
The resulting AQI is just above 100, which places the day in the Unhealthy for Sensitive Groups category. That is exactly why AQI is useful: it transforms a concentration into a category that conveys public health implications more clearly.
Which pollutant determines the overall AQI?
Air pollution is often a mix of pollutants, not a single substance. Ground-level ozone may be elevated during hot sunny afternoons, while PM2.5 from wildfire smoke may dominate during a regional smoke event. In official AQI reporting, the overall AQI for the location is typically the highest sub-index across the monitored pollutants. That “dominant pollutant” is the one driving the health message.
For example, imagine a city with the following pollutant-specific AQIs on a given day:
- PM2.5 AQI: 128
- PM10 AQI: 72
- Ozone AQI: 94
The overall AQI would be 128, and PM2.5 would be considered the dominant pollutant.
Real-world statistics that add context
Understanding AQI is easier when it is linked to measurable health and pollution trends. According to publicly available U.S. government and university sources, PM2.5 and ozone are among the most closely watched pollutants because of their significant respiratory and cardiovascular effects. Wildfire smoke events have also pushed short-term PM2.5 values much higher in many regions, sometimes moving AQI rapidly from Moderate to Unhealthy or worse within hours.
| Pollutant | Typical Major Sources | Common Short-Term Health Concerns | Why AQI Monitoring Matters |
|---|---|---|---|
| PM2.5 | Wildfire smoke, combustion, vehicles, power plants, wood burning | Lung irritation, worsened asthma, cardiovascular stress | Fine particles can penetrate deep into the lungs and even enter the bloodstream. |
| PM10 | Dust, construction, roads, agriculture, industrial processes | Throat irritation, coughing, aggravated respiratory disease | Coarser particles still affect breathing, especially during dust events. |
| Ozone | Forms in sunlight from nitrogen oxides and volatile organic compounds | Chest tightness, reduced lung function, airway inflammation | Often rises on warm sunny days, especially in urban and suburban areas. |
Important limitations when calculating AQI
If you want an accurate answer to the question air quality index how to calculate, you also need to know what can go wrong. The biggest issues are usually not in the formula but in the inputs and interpretation.
1. Averaging time matters
PM2.5 and PM10 AQI values are usually based on 24-hour averages, while ozone often uses 8-hour averages. If you use the wrong averaging period, the AQI you compute may not match official reporting.
2. Sensor quality matters
Low-cost sensors can be useful, but they may require calibration, humidity correction, or quality control. Two devices in the same neighborhood can report noticeably different concentrations if one is poorly placed or not maintained.
3. Breakpoints vary by pollutant and regulatory framework
This page follows the U.S. EPA AQI concept for selected pollutants. Other countries and regions may use different breakpoints, health categories, scales, or naming conventions. Always match the calculation method to the jurisdiction you are analyzing.
4. AQI is a communication tool, not a full toxicology model
AQI is designed to communicate short-term public health risk in a simplified format. It is extremely valuable, but it does not replace more detailed exposure assessment, long-term epidemiology, or clinical advice for people with significant medical vulnerabilities.
How to use AQI results in practice
Once you calculate AQI, the practical question is what to do with it. The answer depends on the category and your health status.
- Good to Moderate: Most people can continue normal outdoor activities.
- Unhealthy for Sensitive Groups: Children, older adults, pregnant people, and individuals with asthma, COPD, or heart disease should consider reducing prolonged or heavy exertion outdoors.
- Unhealthy and above: More people should limit strenuous outdoor activity; cleaner indoor air strategies may be appropriate.
- Very Unhealthy or Hazardous: Public health advisories become more urgent, especially during wildfire smoke or severe urban pollution episodes.
Best practices when comparing AQI across locations
Comparing AQI between cities, schools, industrial sites, or neighborhoods can be useful, but consistency matters. Always compare measurements that use the same pollutant, the same averaging period, and a similar time window. For example, a PM2.5 AQI based on a 24-hour average should not be casually compared with a current 1-hour sensor reading without understanding the difference in methodology.
Another best practice is to look at both the AQI and the underlying pollutant. An AQI of 90 caused by ozone is not identical in exposure pattern to an AQI of 90 caused by PM2.5. They have different sources, behaviors, and mitigation strategies.
Authoritative resources for AQI and air pollution data
If you want to validate calculations, review official methodologies, or explore broader public health information, these sources are excellent starting points:
- AirNow.gov AQI Basics
- U.S. EPA: How AQI Is Calculated
- California Air Resources Board
- Harvard T.H. Chan School of Public Health: Air Pollution and Health
Final takeaway
If you are trying to understand air quality index how to calculate, remember the process in one sentence: measure the pollutant concentration, truncate it according to the pollutant’s rule, find the correct concentration breakpoint range, apply the linear AQI interpolation formula, and round the result to the nearest whole number. Then identify the matching category and health message.
The calculator on this page automates that process for PM2.5, PM10, and 8-hour ozone. It is useful for education, quick estimation, and understanding how raw pollution measurements become actionable public information. For regulatory, public reporting, or health-protection decisions, always cross-check with official local guidance and the latest EPA or state agency resources.