Air Quality Index: How to Calculate AQI from Pollutant Concentration
Use this interactive calculator to estimate the U.S. Air Quality Index for a specific pollutant concentration. Select a pollutant, enter the observed concentration, and instantly see the AQI value, category, health message, and a visual comparison against EPA category thresholds.
AQI Calculator
EPA-style interpolation
The calculator uses the standard linear interpolation formula between AQI breakpoint ranges to convert concentration into an AQI score.
Breakpoint-based categories
Results are assigned to Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, or Hazardous using AQI category limits.
Practical planning tool
Use the output to understand exposure severity and compare a measured pollutant value against official AQI category thresholds.
Example inputs: PM2.5 = 35.4, PM10 = 154, Ozone 8-hour = 0.070, CO = 9.4, SO2 = 75, NO2 = 100.
AQI Category Comparison Chart
Air Quality Index How to Calculate: An Expert Guide to the AQI Formula, Breakpoints, and Health Meaning
If you have ever wondered “air quality index how to calculate,” the answer begins with understanding that the Air Quality Index, or AQI, is not a direct measurement by itself. It is a normalized scale used to convert pollutant concentrations into a single public health communication tool. Instead of telling the public that particulate matter is 38.6 micrograms per cubic meter or ozone is 0.082 parts per million, the AQI translates those measurements into a simpler number and category such as Moderate or Unhealthy. That standardized approach makes it far easier to compare risk levels across places, days, and pollutants.
In the United States, AQI reporting is tied to major regulated pollutants that commonly affect public health: ground-level ozone, particle pollution or particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each pollutant has concentration breakpoints that correspond to AQI ranges. The highest pollutant-specific AQI on a given day becomes the reported overall AQI. So if particulate pollution yields an AQI of 122 but ozone yields an AQI of 74, the public AQI would be 122, and the dominant pollutant would be PM2.5.
What the AQI number means
The AQI scale is divided into health categories so the number has practical meaning. Lower values are better. The standard U.S. categories are:
- 0 to 50: Good
- 51 to 100: Moderate
- 101 to 150: Unhealthy for Sensitive Groups
- 151 to 200: Unhealthy
- 201 to 300: Very Unhealthy
- 301 to 500: Hazardous
These ranges are intentionally broad enough for public communication but precise enough for scientific conversion. That is why AQI calculators use pollutant-specific concentration breakpoints rather than simple percent scaling. PM2.5, ozone, and carbon monoxide behave differently in the atmosphere and affect health in different concentration ranges, so each pollutant needs its own table.
The AQI calculation formula
The standard interpolation formula is:
AQI = ((Ihigh – Ilow) / (BPhigh – BPlow)) × (Cp – BPlow) + Ilow
Where:
- Cp = the observed pollutant concentration after applying the official truncation or rounding rule
- BPlow = the lower concentration breakpoint for the interval containing Cp
- BPhigh = the upper concentration breakpoint for the interval containing Cp
- Ilow = the lower AQI value for that category
- Ihigh = the upper AQI value for that category
This formula is a straight-line interpolation between two known points. For example, if a PM2.5 concentration falls between 35.5 and 55.4 micrograms per cubic meter, that corresponds to an AQI category range of 101 to 150. The formula determines exactly where the concentration falls within that span.
Worked example: calculating PM2.5 AQI
Suppose the observed 24-hour PM2.5 concentration is 42.0 micrograms per cubic meter. That value falls in the breakpoint range 35.5 to 55.4, which maps to AQI 101 to 150. Plugging those numbers into the formula gives:
- Identify BPlow = 35.5 and BPhigh = 55.4
- Identify Ilow = 101 and Ihigh = 150
- Use Cp = 42.0
- Compute the ratio: (150 – 101) / (55.4 – 35.5) = 49 / 19.9
- Compute concentration distance: 42.0 – 35.5 = 6.5
- Multiply and add the lower AQI bound
The resulting AQI is approximately 117, which would be reported in the Unhealthy for Sensitive Groups category.
Key breakpoint table used in U.S. AQI communication
| AQI Category | AQI Range | PM2.5 24-hour | PM10 24-hour | Ozone 8-hour | CO 8-hour |
|---|---|---|---|---|---|
| Good | 0 to 50 | 0.0 to 12.0 mcg/m³ | 0 to 54 mcg/m³ | 0.000 to 0.054 ppm | 0.0 to 4.4 ppm |
| Moderate | 51 to 100 | 12.1 to 35.4 mcg/m³ | 55 to 154 mcg/m³ | 0.055 to 0.070 ppm | 4.5 to 9.4 ppm |
| Unhealthy for Sensitive Groups | 101 to 150 | 35.5 to 55.4 mcg/m³ | 155 to 254 mcg/m³ | 0.071 to 0.085 ppm | 9.5 to 12.4 ppm |
| Unhealthy | 151 to 200 | 55.5 to 150.4 mcg/m³ | 255 to 354 mcg/m³ | 0.086 to 0.105 ppm | 12.5 to 15.4 ppm |
| Very Unhealthy | 201 to 300 | 150.5 to 250.4 mcg/m³ | 355 to 424 mcg/m³ | 0.106 to 0.200 ppm | 15.5 to 30.4 ppm |
The ranges above are real EPA AQI breakpoint values commonly used in public reporting. Depending on pollutant and concentration, some high-end categories require additional pollutant-specific logic or alternate averaging periods. For most educational and planning uses, however, the listed breakpoints capture the core AQI calculation process accurately.
Why truncation and averaging period matter
A frequent mistake in AQI calculation is to use the wrong averaging period or to skip pollutant-specific truncation. AQI is not based on any random concentration snapshot. For example, PM2.5 generally uses a 24-hour average concentration for AQI conversion, while ozone may be based on an 8-hour average. Carbon monoxide AQI also uses an 8-hour concentration, and sulfur dioxide plus nitrogen dioxide can be based on 1-hour standards in AQI frameworks.
Truncation matters too. PM2.5 is typically truncated to one decimal place before calculating AQI. PM10 is handled as a whole-number concentration. Ozone is generally handled to three decimal places in ppm. These details may look minor, but they can shift a result near a category boundary. If a concentration sits close to an AQI threshold, the official rounding or truncation rule can determine whether the result is 100 or 101, a meaningful public health change.
Fine particles small enough to penetrate deep into the lungs. Common sources include smoke, combustion, traffic, and industrial emissions.
Coarser particles from dust, construction, roads, and mechanical processes. Still important, but usually less deeply penetrating than PM2.5.
Not emitted directly. It forms when sunlight drives reactions between nitrogen oxides and volatile organic compounds.
Combustion-related gases that can affect respiratory and cardiovascular health and often signal traffic or industrial emissions issues.
Real-world AQI and pollution statistics
To understand why AQI calculation matters, it helps to look at actual public health and emissions context. The AQI was created because direct pollutant concentration units are not intuitive for most people. A standardized index makes communication and action easier, especially during wildfire smoke events, summertime ozone episodes, and urban winter inversions.
| Statistic | Value | Why it matters |
|---|---|---|
| U.S. AQI scale maximum | 500 | Provides a common upper bound for extreme pollution episodes. |
| Good AQI range | 0 to 50 | Represents little or no expected risk for the general population. |
| Moderate AQI range | 51 to 100 | Usually acceptable, but some unusually sensitive individuals may notice symptoms. |
| AQI level where sensitive groups are first flagged | 101 | This threshold triggers more specific risk messaging for children, older adults, and people with heart or lung disease. |
| Fine particle diameter for PM2.5 | 2.5 micrometers or smaller | Small enough to enter deep portions of the respiratory system. |
| Coarse particle diameter for PM10 | 10 micrometers or smaller | Useful for tracking dust and mechanically generated particles. |
How official reporting chooses the daily AQI
Many people assume agencies average all pollutant AQIs together. That is not how the system works. Instead, each pollutant gets its own AQI score from its measured concentration. The highest of those scores becomes the headline AQI for that place and time. That pollutant is called the dominant pollutant. This method is intentionally conservative because public health messaging should reflect the most hazardous current pollutant, not an average that could dilute an urgent problem.
Imagine a city with these pollutant-specific AQI values on the same afternoon:
- PM2.5 = 137
- Ozone = 91
- CO = 22
- NO2 = 44
The reported AQI would be 137, and the dominant pollutant would be PM2.5. This approach is one reason wildfire events often dominate AQI reports even when ozone and traffic gases are present too.
Health interpretation by category
Knowing how to calculate AQI is valuable, but knowing how to interpret it is even more important. AQI categories are designed to trigger behavior changes:
- Good: Outdoor activity is generally safe for everyone.
- Moderate: Sensitive individuals should monitor symptoms if they are unusually reactive.
- Unhealthy for Sensitive Groups: Children, older adults, pregnant people, outdoor workers, and those with asthma, COPD, or heart disease should reduce longer or heavier exertion outdoors.
- Unhealthy: The general population may start to feel effects; sensitive groups face greater risk and should limit exposure.
- Very Unhealthy: Health alert conditions. Outdoor exertion should be minimized broadly.
- Hazardous: Emergency conditions with elevated risk for everyone.
Common mistakes when calculating AQI
- Using an instantaneous reading instead of the correct 1-hour, 8-hour, or 24-hour average
- Applying the wrong pollutant breakpoint table
- Skipping truncation rules for PM2.5, ozone, or carbon monoxide
- Averaging pollutant AQIs together instead of selecting the highest one
- Assuming all countries use the exact same AQI system and breakpoints
That last point is especially important. Different countries use different index methods, standards, and concentration breakpoints. If you are comparing U.S. AQI to another nation’s reported index, the values may not be directly equivalent even if the labels look similar.
Where to verify AQI methods and official guidance
For official U.S. reference materials, use authoritative sources such as the AirNow AQI Basics page, the EPA explanation of how AQI is calculated, and the EPA particulate matter basics resource. These sources explain pollutant categories, health messaging, and technical conversion methods used in public reporting.
Bottom line
If your goal is to understand “air quality index how to calculate,” remember the process in one sentence: measure the pollutant concentration over the correct averaging time, locate the matching breakpoint interval, interpolate to an AQI value, and then classify the result into the corresponding health category. The largest pollutant-specific AQI becomes the headline AQI. This is why AQI is both simple for the public and rigorous enough for environmental communication.
The calculator above gives you a practical way to estimate that value for common pollutants. It is especially helpful for students, environmental professionals, journalists, community groups, and homeowners trying to understand what a concentration number actually means for health and daily activities. AQI transforms raw monitoring data into a decision-making tool, and once you understand the formula, the daily air quality report becomes much easier to interpret.