Air Quality Index Calculator

Introduction

The Air Quality Index (AQI) is a number that turns complicated air-quality measurements into a single figure you can act on. Instead of parsing raw concentrations of six or eight different pollutants, you get one index value with a color and a plain-language health advisory — "Good", "Moderate", "Unhealthy", and so on. Government agencies around the world publish these indices every day so that the public can decide whether to keep children indoors, cancel a morning run, or wear a mask outdoors.

The two most widely used systems are the United States EPA AQI and India's National Air Quality Index (NAQI), published by the Central Pollution Control Board (CPCB). Both use the same underlying mathematics — a piecewise-linear interpolation between health breakpoints — but they differ in which pollutants they track and where the breakpoints sit, because each reflects a different country's air-quality standards and its population's exposure profile. India's NAQI tracks eight pollutants, including ammonia and lead, which the US index does not. [who-aq]

This calculator implements both standards. In the US EPA AQI mode you enter the concentrations you have measured (PM2.5, PM10, ozone, carbon monoxide, sulfur dioxide, or nitrogen dioxide), and the tool returns each pollutant's sub-index plus the overall AQI — which is always the worst (highest) sub-index. Switch to India CPCB NAQI mode to use that country's eight-pollutant table with its six categories, from Good (0–50) to Severe (401–500).

The AQI matters because it turns air pollution from an abstract chemistry problem into something with direct health consequences. Outdoor air pollution is responsible for millions of premature deaths each year, and fine particles (PM2.5) are consistently the pollutant most strongly linked to cardiovascular and respiratory harm. [who-aq] Knowing your local index — and knowing how to compute it from raw monitor data — is the first step toward acting on it.

How to Use

Select the standard that matches your data. The US EPA AQI mode uses the 2024 EPA breakpoints; the India CPCB NAQI mode uses the breakpoints published by CPCB. In both modes, every concentration input is optional — you only fill in the pollutants for which you have measurements. The calculator computes a sub-index for each pollutant you enter and reports the maximum as the overall AQI.

Example 1 — US EPA, PM2.5 only. Suppose a local monitor reads a 24-hour PM2.5 average of 35.5 µg/m³. In EPA mode, enter 35.5 in the PM2.5 field and leave the others blank. The PM2.5 sub-index is 101 (the boundary of "Unhealthy for Sensitive Groups"), so the overall AQI is 101 and the category is Unhealthy for Sensitive Groups. This matches the EPA table, where 35.5–55.4 µg/m³ maps to AQI 101–150. [aqs-breakpoints]

Example 2 — US EPA, multiple pollutants. Now suppose the same day also had an 8-hour ozone reading of 0.060 ppm and a 1-hour nitrogen dioxide reading of 54 ppb. Enter all three. Ozone at 0.060 ppm falls between 0.055 and 0.070, interpolating to an ozone sub-index of about 75. Nitrogen dioxide at 54 ppb interpolates to about 54. The highest sub-index is still PM2.5 at 101, so the AQI stays 101 and PM2.5 is identified as the primary pollutant. This is exactly how the EPA reports it: the AQI is the maximum sub-index, and the pollutant that drives it gets named in the forecast. [epa-basics]

Example 3 — India CPCB. A monitoring station in Delhi reports a 24-hour PM2.5 average of 65 µg/m³ and an 8-hour ozone value of 105 µg/m³. Switch to India CPCB mode and enter both. PM2.5 at 65 µg/m³ falls in the 61–90 band, which interpolates to an NAQI sub-index around 111 — squarely in the "Moderately Polluted" category (101–200). Ozone at 105 µg/m³ sits in the 101–168 band, interpolating to a sub-index near 102. The overall NAQI is the maximum, 111, with PM2.5 as the primary pollutant. [who-aq]

A few practical notes on data. Official AQI values are computed from concentrations truncated to specific precision — PM2.5 to one decimal, PM10 to an integer, ozone to three decimals. This calculator uses the values you enter directly. If your monitor reports a value that lands exactly on a breakpoint boundary, the sub-index equals the boundary's AQI value; the interpolation formula and the table agree in that case. [cfr-appendix-g]

The Formula

Both the EPA AQI and the CPCB NAQI use the same piecewise-linear interpolation. For a pollutant concentration $C_p$ that falls between two breakpoints, the sub-index is:

Where:

• $C_p$ — the truncated concentration of pollutant p
• $BP_{lo}$ and $BP_{hi}$ — the breakpoint concentrations that bracket the reading
• $I_{lo}$ and $I_{hi}$ — the AQI values corresponding to those two breakpoints

The overall AQI is the maximum sub-index across all pollutants measured:

(EPA mode; the CPCB mode adds ammonia and lead to the set.)

The intuition is simple. Each pollutant's breakpoint table defines a ladder of (concentration → AQI) pairs. If your reading sits between two rungs, the formula slides linearly between them. Because the ladder segments have different widths, the same concentration change does not always move the index the same amount. For example, in the EPA PM2.5 table, the Good band spans 0 to 9.0 µg/m³ for AQI 0–50, but the Very Unhealthy band spans 125.5 to 225.4 µg/m³ for AQI 201–300 — a much wider concentration range per index point. [cfr-appendix-g]

Why the maximum rather than an average? The AQI is a public-health warning system. If one pollutant is dangerously high while all others are clean, the health threat is real and urgent — averaging would hide it. The maximum sub-index tells the most vulnerable person what they actually face. This is a deliberate design decision shared by the EPA, CPCB, and most national indices worldwide. [epa-basics]

Reference Table

The two tables below give the official breakpoints. In each row, a concentration range maps to an AQI range and a category name. The US EPA table (2024 update) is excerpted for the pollutants handled here; the full table in 40 CFR Appendix G adds ozone 1-hour values and additional rows. [cfr-appendix-g]

Practical Tips

• Use the right averaging period. The EPA AQI is built on 24-hour averages for particles, 8-hour for ozone and carbon monoxide, and 1-hour maxima for sulfur and nitrogen dioxide. CPCB uses 24-hour averages for most pollutants and 8-hour for ozone and CO. Feeding the wrong averaging period into any index produces a meaningless number.
• Cross-check the truncation. Official agencies truncate concentrations before interpolating — PM2.5 to one decimal, PM10 to a whole number, ozone to three decimals. If you are reproducing an official reading, apply the same truncation to your input.
• The AQI is the worst pollutant. Do not average pollutant sub-indices. The overall AQI is always the maximum sub-index, and the pollutant behind it is what drives public-health warnings that day.
• Sensitive groups should act before everyone else. An AQI of 101–150 affects people with asthma, heart conditions, children, and older adults. At 151–200 the general public starts to feel it. Plan outdoor activity around these thresholds if you or a family member is vulnerable.
• Pair with a carbon footprint check. Air quality and emissions are two sides of the same coin. Use the Carbon Footprint Calculator to estimate your contribution, and the Flight Carbon Emissions Calculator for the aviation slice of it.
• Watch the weather context. The Dew Point Calculator and Wind Chill Calculator describe the same atmosphere from a comfort perspective. Air stagnation — often tied to weather patterns — is what lets pollution accumulate.

Limitations

• Breakpoints differ by country. This calculator implements exactly two standards. Other countries (Canada, China, the UK, Australia) publish their own indices with their own breakpoints, and the numbers are not interchangeable. Always identify the standard behind an AQI value before comparing across borders. [who-aq]
• Concentrations, not forecasts. The tool computes an index from the concentrations you enter. It does not forecast air quality, account for indoor vs. outdoor settings, or incorporate real-time monitoring feeds.
• Truncation precision. The calculator interpolates from the values exactly as typed. For a published official figure, you may need to round or truncate your inputs to match the agency's convention.
• Simplified pollutant set. The US EPA AQI also supports 1-hour ozone sub-indices in certain areas and a 24-hour SO2 basis in others. This calculator uses the primary 8-hour ozone and 1-hour SO2/NO2 basis, which is the standard reporting convention. [cfr-appendix-g]
• Health advice is general. The advisory text is a condensed paraphrase of the official cautionary statements. It is educational and not a substitute for medical advice, especially for people with chronic respiratory or cardiovascular conditions.

Frequently Asked Questions

❓ What is a good AQI number?

✅ An AQI of 0 to 50 is considered Good in both the US EPA and India CPCB systems. 51–100 is Moderate (EPA) or Satisfactory (CPCB), where only unusually sensitive people might notice effects.

❓ How is the AQI calculated from pollutant concentrations?

✅ Each pollutant concentration is converted to a sub-index using piecewise-linear interpolation between the health breakpoints in the standard's table. The overall AQI is the maximum of all pollutant sub-indices.

❓ What is the difference between EPA AQI and CPCB NAQI?

✅ Both use the same interpolation formula, but they track different pollutants and use different breakpoints. The US EPA index uses 6 pollutants; India's NAQI tracks 8 including ammonia and lead, with breakpoints reflecting India's national ambient air quality standards.

❓ Why does PM2.5 get its own AQI?

✅ Fine particulate matter under 2.5 micrometers penetrates deep into the lungs and enters the bloodstream, making it the pollutant most consistently linked to cardiovascular and respiratory harm. It is usually the primary pollutant driving high AQI values.

❓ Can I calculate AQI for one pollutant only?

✅ Yes. If you enter a single pollutant's concentration, the sub-index for that pollutant is the overall AQI. In practice, the AQI is the maximum across all measured pollutants, so a single-pollutant calculation is valid when that is all the data you have.

❓ What AQI level is unhealthy?

✅ Under the EPA system, AQI 101–150 (Unhealthy for Sensitive Groups) affects vulnerable people, 151–200 (Unhealthy) begins to affect everyone, and above 200 the warnings become more serious. Under CPCB, Moderately Polluted is 101–200, Poor is 201–300, Very Poor is 301–400, and Severe is 401–500.

❓ Why is ozone worse in the afternoon?

✅ Ground-level ozone forms when nitrogen oxides and volatile organic compounds react in sunlight. Concentrations typically peak in the afternoon, which is why the 8-hour average can be much higher than the morning reading.

❓ How can I check my local air quality?

✅ The US AirNow service publishes EPA AQI for US locations, and India's CPCB publishes the live NAQI for Indian cities. Many weather apps also include an AQI readout sourced from these official agencies.

❓ Do indoor air quality numbers use the same AQI?

✅ No. The EPA AQI and CPCB NAQI are outdoor indices based on ambient monitoring. Indoor air quality is assessed with different instruments and standards, though the same pollutants (PM2.5, CO2, VOCs) are often measured.

❓ What is the most important number for my health?

✅ For most people, the PM2.5 sub-index is the number that matters most, because fine particles drive the majority of pollution-related health effects. If PM2.5 is low, the overall AQI is usually low as well.