Air Quality Index (AQI) Calculator

What an Air Quality Index (AQI) Calculator Actually Measures

The Air Quality Index (AQI) is designed to turn complex air pollution measurements into a single number you can use quickly. Instead of interpreting multiple pollutant concentrations with different units, timescales, and health thresholds, AQI converts each pollutant into a standardized scale and then reports the highest value as the day’s overall air quality level.

An AQI calculator does not measure air directly. It converts pollutant concentration values into AQI sub-index values using a published set of “breakpoints.” Breakpoints are concentration ranges that correspond to AQI ranges such as 0–50 (Good), 51–100 (Moderate), 101–150 (Unhealthy for Sensitive Groups), 151–200 (Unhealthy), 201–300 (Very Unhealthy), and 301–500 (Hazardous). The result is an index that supports clear, actionable communication about health risk and exposure management.

Why AQI Uses Multiple Pollutants Instead of One “Air Quality” Number

Air pollution is not a single thing. In many locations, particle pollution (PM2.5 or PM10) is the main driver of poor air quality during smoke, dust storms, or winter inversions. In other areas, ozone becomes the dominant pollutant during hot, sunny days when sunlight drives chemical reactions between nitrogen oxides and volatile organic compounds. Carbon monoxide can become important near traffic and combustion sources, while sulfur dioxide and nitrogen dioxide can spike near certain industrial or urban areas.

Because each pollutant has its own health profile and concentration distribution, the AQI approach computes a sub-index for each pollutant and then reports the maximum sub-index as the overall AQI. This “max-of-sub-indexes” approach is why your reported AQI might be “Unhealthy” even if only one pollutant is high. The AQI is communicating that at least one pollutant has reached a level that should guide behavior.

How the AQI Conversion Formula Works

AQI conversion is based on a linear interpolation between two breakpoints that surround your concentration. The formula is straightforward: find the lower and upper concentration breakpoints that contain your pollutant concentration, then map that interval onto the corresponding AQI interval.

In this formula, C is the pollutant concentration (often truncated using AQI rules), C_Lo and C_Hi are the breakpoints surrounding C, and I_Lo and I_Hi are the corresponding AQI breakpoints (for example, 51 and 100). The resulting AQI value is then rounded to the nearest whole number. This is why AQI outputs are reported as integers.

Why Truncation Matters in AQI Calculations

AQI calculations use specific concentration “handling” rules so that reporting is consistent. In many AQI systems, certain pollutants are truncated (not rounded) before converting to AQI. For example, ozone values are often truncated to three decimal places in ppm, PM2.5 values to one decimal place, PM10 values to whole numbers, and some gases to the precision used in reporting.

The difference between truncation and rounding can be small, but near breakpoints it can change category classification. That matters because AQI categories are tied to public guidance and behavior. This calculator includes a “Truncate per AQI rules” option so you can align the results with standardized reporting conventions, while still allowing a “Use as entered” option when you want a quick estimate.

Understanding AQI Categories and What They Mean

AQI categories are not just labels. Each category corresponds to an increasing level of health concern. The intent is to help you decide whether to change outdoor activity, reduce exposure, or take protective steps such as moving exercise indoors or using filtered indoor air.

Good (0–50)

Air quality is considered satisfactory and poses little or no risk. Most people can carry out normal outdoor activities without concern. If you still notice symptoms, it may be due to allergies, humidity, smoke odor, or other non-AQI factors rather than a high AQI pollutant level.

Moderate (51–100)

Air quality is acceptable, but there may be a moderate health concern for a small number of people who are unusually sensitive to air pollution. Sensitive individuals may consider reducing prolonged or heavy exertion outdoors if they experience symptoms such as coughing or shortness of breath.

Unhealthy for Sensitive Groups (101–150)

This category is especially important because it marks the point where sensitive groups should actively reduce exposure. Sensitive groups often include children, older adults, pregnant individuals, and people with heart or lung disease such as asthma or COPD. The general public is less likely to be affected at this level, but strenuous outdoor activity can still cause symptoms for some.

Unhealthy (151–200)

Everyone may begin to experience health effects, and sensitive groups may experience more serious effects. For many people, the main practical advice is to reduce outdoor exertion, take more breaks, and consider indoor alternatives for workouts, sports, or outdoor labor. If you must be outside for long periods, reducing intensity can meaningfully reduce inhaled dose.

Very Unhealthy (201–300)

This level triggers a stronger public health alert. The risk of health effects increases for everyone. At these levels, the best approach is to reduce exposure: avoid vigorous outdoor activity, consider staying indoors in filtered air, and follow local public health guidance. People with existing respiratory or cardiovascular conditions should be especially careful.

Hazardous (301–500)

This category is an emergency-level warning. Health effects are likely for everyone, especially with prolonged exposure or heavy exertion. If your area reaches Hazardous AQI due to smoke or dust storms, indoor filtered air becomes a priority. Keep windows closed, limit indoor sources of combustion, and consider high-efficiency filtration where possible.

Overall AQI: Why the Highest Sub-Index Wins

When multiple pollutant concentrations are available, the AQI is the maximum of the sub-index values. That means an “Overall AQI” calculation is not an average. It is a “worst-case pollutant” communication tool. This is why the “Overall AQI (Multiple Pollutants)” tab in this calculator lists each pollutant sub-index and highlights the main pollutant.

The main pollutant matters for two reasons. First, it helps you understand the likely source or condition: wildfire smoke tends to drive PM2.5, dust storms often drive PM10, and hot sunny days can drive ozone. Second, it helps with mitigation. Some protective steps apply broadly (reduce exposure, filtered indoor air), while others are pollutant-specific (for example, avoiding high-ozone afternoon hours in summer).

Special AQI Rules for Ozone and Sulfur Dioxide

AQI reporting includes special handling in very high ranges for some pollutants. Ozone often uses an 8-hour average for most reporting, but at very high values a 1-hour ozone value may be used to compute higher AQI levels. Similarly, sulfur dioxide uses a 1-hour value for lower AQI ranges but relies on a longer averaging window for higher AQI values. These conventions are designed to ensure that the AQI reflects meaningful health risk and does not understate severe episodes.

This tool supports both ozone 8-hour and ozone 1-hour modes, and both SO₂ 1-hour and SO₂ 24-hour modes. If you are entering values from a sensor or dashboard, choose the averaging window that matches your data. If you are unsure, ozone is commonly displayed as 8-hour in AQI reporting, while SO₂ displays can vary by system and reporting context.

PM2.5 vs PM10: Why Particle Size Changes Risk

PM stands for particulate matter. PM2.5 refers to particles smaller than 2.5 micrometers, while PM10 refers to particles smaller than 10 micrometers. The reason size matters is that smaller particles penetrate deeper into the lungs and can contribute to systemic inflammation, which is why PM2.5 is closely linked to health outcomes and why AQI systems place special attention on it.

PM10 can still be harmful, especially during dust events or when coarse particles carry irritants. Both PM2.5 and PM10 can aggravate asthma and respiratory conditions. When the AQI is driven by particle pollution, exposure reduction and indoor filtration tend to be the most effective protective strategies.

Using AQI for Exercise, Outdoor Work, and Daily Planning

AQI is particularly useful for planning activity intensity and duration. Even if you must go outside, you can reduce dose by reducing exertion, choosing lower-pollution times, and taking breaks indoors. For many areas, ozone peaks in the afternoon and early evening on hot days, while smoke-driven PM2.5 can remain high throughout the day depending on wind and fire behavior.

Outdoor workers, runners, cyclists, and people who spend long hours outside should pay attention not only to category but also to how they feel. AQI is a population-level indicator. Individual response varies, and symptoms are meaningful signals. If you notice chest tightness, wheezing, persistent coughing, dizziness, or unusual shortness of breath, it is appropriate to reduce activity and seek medical advice if symptoms are severe.

What This AQI Calculator Assumes and What It Does Not

This calculator converts concentrations to AQI using breakpoint interpolation and standard category ranges. It does not model local sensor bias, indoor exposure, microclimates, altitude effects, or personal medical risk. It also does not compute specialized real-time techniques such as NowCast, which some systems use to relate short-term particle measurements to 24-hour AQI reporting.

If you need decision-grade guidance for a specific medical condition, consult a clinician and follow local public health guidance during severe events. For general planning, however, AQI remains one of the clearest ways to translate complex air data into an actionable daily signal.

Practical Steps to Reduce Exposure When AQI Is High

When AQI moves into Unhealthy ranges, the most effective strategies focus on reducing inhaled dose. A few high-impact steps include:

• Reduce time spent outdoors, especially vigorous activity.
• Keep windows closed during smoke, dust, or high-ozone periods.
• Use filtered indoor air where possible and minimize indoor combustion sources.
• Plan workouts for lower-AQI hours or move them indoors.
• For sensitive individuals, treat “Unhealthy for Sensitive Groups” as a meaningful trigger for exposure reduction.

AQI is most helpful when used consistently. Even small reductions in exposure during repeated poor-air days can meaningfully reduce irritation and respiratory stress, especially for people who already have asthma, allergies, or cardiovascular concerns.

Why Your AQI Can Change Quickly

AQI can change quickly because pollutant concentrations can change quickly. Wind shifts, inversion layers, traffic patterns, industrial emissions, wildfire smoke, and sunlight-driven chemistry can all alter local pollution within hours. That is why AQI forecasts are useful, but real-time updates matter when conditions are unstable.

If you see sudden AQI jumps, check the main pollutant. A rapid rise in PM2.5 often suggests smoke or localized particle sources, while ozone patterns often reflect time-of-day and temperature. Understanding “what is driving the AQI” helps you choose protective actions that match the real cause.

Frequently Asked Questions