What is the difference between AQI and NAQI?

AQI is a generic term for any air quality index; in practice it usually refers to the US EPA Air Quality Index. NAQI is the National Air Quality Index used by India's Central Pollution Control Board. Both use the same piecewise-linear formula, but they differ in pollutant breakpoints, category names, and thresholds — the EPA categories shift at 50, 100, 150, 200, and 300, while the CPCB categories shift at 50, 100, 200, 300, and 400.

Why is the AQI the maximum of the pollutant sub-indices and not the average?

The maximum ensures the index reflects the worst pollutant, which is the one most likely to drive adverse health effects on a given day. If the index averaged the sub-indices, a high PM2.5 reading could be diluted by low readings for other pollutants, understating the true risk.

Can I calculate the AQI myself from raw monitor data?

Yes. You need the measured concentration for at least one pollutant, its correct averaging time, and the breakpoint table for the standard you want. Apply the piecewise-linear formula to get each sub-index and take the maximum. The easiest way is to use the Air Quality Index Calculator, which handles both the EPA and CPCB tables automatically.

Why do some countries use different AQI scales?

Each country or agency sets breakpoints based on its own health studies, regulatory standards, and policy priorities. The result is that the same measured concentration can produce different index values and categories in different countries. The World Health Organization publishes global air quality guidelines that are more protective than any national index, precisely to highlight these differences.

What is the difference between PM2.5 and PM10?

PM2.5 refers to particles smaller than 2.5 micrometres, which penetrate deep into the lungs and bloodstream; PM10 refers to particles up to 10 micrometres, which are coarser and mostly affect the upper airways. PM2.5 is generally the more dangerous pollutant and is often the main driver of a city's AQI.

Which pollutant is measured as an 8-hour average and why?

Ozone and carbon monoxide are measured as 8-hour averages because their health effects depend on prolonged exposure over several hours, not on short peaks. Ozone is produced by sunlight-driven chemistry during the day, so an 8-hour window captures the daylight accumulation; carbon monoxide's effects on blood oxygen build up over hours of exposure.

Is an AQI of 100 safe?

An AQI of 100 is exactly the boundary between the Moderate and the next band (Unhealthy for Sensitive Groups under EPA, or Moderately Polluted under CPCB). It is considered acceptable for the general population, but unusually sensitive individuals may begin to feel effects at this level. It is a benchmark for health-action triggers, not a guarantee of zero risk.

Why does the CPCB report 'Good' air when the WHO guidelines say the level is unhealthy?

National indices set breakpoints that balance health protection with the reality of prevailing pollution levels in that country. The WHO's guidelines are health-only targets and are considerably stricter. A 'Good' or 'Satisfactory' label under a national index does not mean the air meets WHO recommendations — it means the level is acceptable under that country's chosen standards.

What does 'Unhealthy for Sensitive Groups' mean?

It is the EPA category for AQI values of 101–150. It means the air is probably safe for the general public, but children, older adults, and people with heart or lung disease — including asthma — may experience health effects and should reduce prolonged outdoor exertion. The CPCB equivalent, 'Moderately Polluted', warns of breathing discomfort for people with existing lung, asthma, or heart conditions.

How often is the AQI updated?

Real-time monitoring networks update the index throughout the day, typically every hour, using the most recent available measurements and the appropriate averaging times. Agencies also publish daily summaries. Because the underlying concentrations fluctuate, the index can move between categories over the course of a single day.

How the Air Quality Index (AQI) Is Calculated

Understand the piecewise-linear formula behind the Air Quality Index, how the US EPA and India CPCB standards differ, and how to interpret the health categories.

Introduction

When a government agency reports "today's air quality is 145, unhealthy for sensitive groups," the number they quote is not a raw measurement. It is a converted, standardized score known as the Air Quality Index (AQI), designed so that a single number can communicate a complex mixture of pollutants to the general public. The AQI transforms raw concentrations of fine particles, ozone, carbon monoxide, and other gases — each measured in different units over different averaging times — into a common 0–500 scale where higher values mean worse air.

The purpose of this guide is to explain, step by step, how that conversion actually works. We cover the two most widely used national standards: the US EPA AQI (used by the United States and adopted in modified form by many countries) and the India CPCB National Air Quality Index (NAQI). Although the two standards differ in their pollutant breakpoints and category names, they share the same underlying mathematics: a piecewise-linear interpolation between published breakpoint pairs. Once you understand that single formula, you can compute an AQI for any pollutant under either standard by hand, in a spreadsheet, or with the Air Quality Index Calculator, which implements both standards.

Air quality reporting matters because air pollution is one of the largest environmental health risks worldwide. The World Health Organization estimates that ambient (outdoor) air pollution contributes to millions of premature deaths each year, driven mainly by fine particulate matter, ozone, and nitrogen dioxide. An index that the public can read and act on — checking whether it is safe to let children play outside or whether to cancel outdoor exercise — is therefore a genuine public-health tool, not just a scientific statistic. Understanding how it is built helps you use it critically rather than treating it as an oracle.

Core Concepts: Pollutants and Averaging Times

Before any index value can be calculated, measurements must be collected for one or more of the six to eight pollutants that each standard monitors. Each pollutant is measured in its own unit, and each has a specified averaging time that matches how that pollutant affects health.

Criteria pollutants and their units

Pollutant	Abbreviation	Typical unit	Averaging time (EPA)	Averaging time (CPCB)
Fine particulate matter (≤2.5 µm)	PM2.5	µg/m³	24 hours	24 hours
Coarse particulate matter (≤10 µm)	PM10	µg/m³	24 hours	24 hours
Ground-level ozone	O3	ppm (EPA) / µg/m³ (CPCB)	8 hours	8 hours
Carbon monoxide	CO	ppm (EPA) / mg/m³ (CPCB)	8 hours	8 hours
Sulfur dioxide	SO2	ppb (EPA) / µg/m³ (CPCB)	1 hour (EPA)	24 hours (CPCB)
Nitrogen dioxide	NO2	ppb (EPA) / µg/m³ (CPCB)	1 hour (EPA)	24 hours (CPCB)
Ammonia	NH3	µg/m³	—	24 hours
Lead	Pb	µg/m³	—	24 hours

The averaging time is part of the definition of the index. The EPA computes its PM2.5 sub-index from a 24-hour average concentration, while SO2 and NO2 use a 1-hour maximum because those gases irritate the lungs within a short exposure. The CPCB, following the Ministry of Environment, Forest and Climate Change, uses 24-hour averages for most pollutants and 8-hour averages for ozone and carbon monoxide. Using the wrong averaging time against the wrong breakpoint table produces a meaningless AQI — a common source of error when people copy values from a real-time monitor that reports hourly data.

Why concentrations are standardized

Raw concentrations are incommensurable: 12.1 µg/m³ of PM2.5, 0.060 ppm of ozone, and 36 ppb of sulfur dioxide cannot be compared directly. The AQI solves this by mapping each concentration through its own pollutant-specific breakpoint table into a shared sub-index on the 0–500 scale. A sub-index of 100 for PM2.5 means the same severity of exposure as a sub-index of 100 for ozone, even though the underlying concentrations are completely different numbers. This is what makes the overall index interpretable across pollutants and across cities.

How the AQI Is Calculated: The Core Formula

Both the EPA and the CPCB calculate each pollutant's sub-index using the same piecewise-linear interpolation formula. For a measured (or truncated) concentration Cp that falls between a lower breakpoint BPLo and an upper breakpoint BPHi, the sub-index Ip is:

I_p = \\frac{I_{hi} - I_{lo}}{BP_{hi} - BP_{lo}} \\times (C_p - BP_{lo}) + I_{lo}

where Ilo and Ihi are the index values corresponding to BPLo and BPHi. The overall AQI is then simply the maximum of all computed sub-indices:

AQI = \\max(I_{PM2.5}, I_{PM10}, I_{O3}, I_{CO}, I_{SO2}, I_{NO2}, \\dots)

Working through an example by hand

Suppose a monitor reports a 24-hour PM2.5 concentration of 35.5 µg/m³ under the EPA standard. The EPA breakpoint table for PM2.5 24-hour averages defines the pair (BPLo = 35.5, BPHi = 55.4) with corresponding index values (Ilo = 101, Ihi = 150). Plugging these into the formula:

I_p = \\frac{150 - 101}{55.4 - 35.5} \\times (35.5 - 35.5) + 101 = 101

The sub-index is exactly 101 because 35.5 µg/m³ sits exactly on the lower boundary of that band. Now repeat with a PM2.5 reading of 45 µg/m³:

I_p = \\frac{150 - 101}{55.4 - 35.5} \\times (45 - 35.5) + 101 = \\frac{49}{19.9} \\times 9.5 + 101 \\approx 124

So 45 µg/m³ maps to a PM2.5 sub-index of about 124. If no other pollutants were measured, the overall AQI would be 124, placing the air in the Unhealthy for Sensitive Groups band. If a concurrent ozone reading produced a higher sub-index, the ozone value would win, because the overall AQI is the maximum of all pollutant sub-indices — the "worst pollutant rules" rule.

Why the formula is linear within each band

The interpolation is piecewise-linear, not a single straight line over the whole 0–500 range. Between any two consecutive breakpoints the mapping is a straight line, but the slope changes from band to band. This design means that the index is always exactly 50, 100, 150, and so on at the published boundaries, keeping category thresholds crisp, while smoothly interpolating between them. It also means the same concentration can map to different sub-indices under different standards, because the breakpoint tables differ — a fact we examine in the next section.

US EPA vs India CPCB: Comparing the Two Standards

The two standards organize their categories differently. The EPA defines six categories with thresholds at AQI values of 50, 100, 150, 200, and 300. The CPCB also defines six categories, but its thresholds sit at 50, 100, 200, 300, and 400 — meaning "Moderately Polluted" spans the 101–200 range that the EPA splits into two separate categories.

EPA categories

AQI range	Category	Color
0–50	Good	Green
51–100	Moderate	Yellow
101–150	Unhealthy for Sensitive Groups	Orange
151–200	Unhealthy	Red
201–300	Very Unhealthy	Purple
301–500	Hazardous	Maroon

CPCB categories

AQI range	Category	Color
0–50	Good	Green
51–100	Satisfactory	Light green
101–200	Moderately Polluted	Yellow
201–300	Poor	Orange
301–400	Very Poor	Red
401–500	Severe	Dark red

Side-by-side: how the same concentration scores

The breakpoint tables are not merely rescaled versions of each other. The CPCB's bands for particulate matter are wider in the higher ranges (for example, the "Severe" PM2.5 band starts at 250 µg/m³ under CPCB, while the EPA's "Hazardous" PM2.5 band starts at 250.5 µg/m³), but the intermediate bands differ enough that the same measured concentration can land in a different category. Consider PM2.5 at several concentrations:

PM2.5 concentration	EPA sub-index	EPA category	CPCB sub-index	CPCB category
20 µg/m³	78	Moderate	61	Satisfactory
40 µg/m³	115	USG	125	Moderately Polluted
65 µg/m³	165	Unhealthy	212	Poor
100 µg/m³	208	Very Unhealthy	258	Poor
180 µg/m³	260	Very Unhealthy	361	Very Poor

The EPA PM2.5 sub-index climbs faster than the CPCB sub-index across the moderate-to-very-high range, because the EPA's breakpoint bands above 35.5 µg/m³ are narrower.

This table shows that the EPA's bands compress more index range into the mid concentrations, so the EPA sub-index runs higher than the CPCB sub-index for the same PM2.5 reading above roughly 30 µg/m³. That is not a defect in either standard — it reflects different choices about how strictly to flag health risk at intermediate pollution levels. When comparing air quality between an Indian city and a US city, you must use a single standard for both, never mix the two.

Practical Applications: Reading and Acting on the Index

The point of an index is action. Public-health agencies publish guidance that tells people what to do at each level, and the guidance differs for sensitive groups (children, the elderly, pregnant people, and those with asthma or heart disease) versus the general population.

At Good (0–50), outdoor activity is safe for everyone. At Moderate (51–100), air quality is acceptable, but unusually sensitive individuals may consider reducing prolonged or heavy outdoor exertion. At Unhealthy for Sensitive Groups (101–150), children, older adults, and people with respiratory or heart conditions should limit prolonged outdoor exertion; the general public is unlikely to be affected. At Unhealthy (151–200), everyone may begin to experience health effects, and sensitive groups should avoid prolonged outdoor exertion. At Very Unhealthy (201–300), health alerts are issued and everyone should avoid prolonged outdoor exertion. At Hazardous (301+), emergency conditions are declared and everyone should avoid outdoor activity altogether.

The same ladder exists under the CPCB with its own wording: Moderately Polluted means breathing discomfort for people with lung, asthma, or heart conditions; Poor means breathing discomfort for most people on prolonged exposure; Very Poor means respiratory illness on prolonged exposure; and Severe means health impacts even for healthy people. The Air Quality Index Calculator reports the matching health advisory text automatically for whichever standard you select.

A real-world workflow

A practical use case: a runner in Delhi checks the morning report and sees a CPCB NAQI of 178 (Moderately Polluted). She can enter the reported pollutant concentrations — say PM2.5 at 75 µg/m³, PM10 at 190 µg/m³, and ozone at 92 µg/m³ — into the calculator, which will compute each sub-index and confirm that PM10 dominates the score. Seeing the per-pollutant breakdown tells her whether the day's primary irritant is dust (PM10) or combustion particles (PM2.5), which changes her choice of activity: both warrant a lighter workout, but PM10-dominant days are typically windy, dry days where a mask may help more than on ozone-dominant days.

Air quality is closely connected to other environmental quantities you can explore with our tools. The Carbon Footprint Calculator estimates the emissions your household and travel produce, which are a major upstream driver of the PM2.5 and NO2 measured at urban monitors. The Flight Carbon Emissions Calculator quantifies the aviation share. For other weather-environment quantities that pair naturally with air-quality readings, the Dew Point Calculator and the Wind Chill Calculator help you interpret the conditions that accompany pollution episodes — stagnant, humid air traps pollutants near the ground, while strong wind disperses them.

Common Mistakes and Limitations

Mistakes people make when calculating AQI

The most common error is using the wrong breakpoint table. Copying a CPCB breakpoint table into an EPA calculation (or vice versa) silently shifts every result. A second frequent mistake is using an hourly reading against a 24-hour breakpoint table without first computing the appropriate average — the index is defined on averaged concentrations, not instantaneous peaks. A third error is forgetting that the overall AQI is the maximum of the sub-indices, not the average; averaging six pollutant scores can hide a dangerous PM2.5 episode behind a mild ozone reading. Finally, people sometimes round the concentration before interpolating, which changes results at band boundaries; the formula uses the truncated, not rounded, concentration.

Inherent limitations of any AQI

Every national index is an imperfect simplification, and the CPCB documents its limitations explicitly. The index is reported for regulatory monitoring stations, which may not represent the air people breathe inside buildings, near roads, or in neighborhoods far from the monitor. Measurements are typically hourly and daily snapshots; the true exposure over a person's day can differ substantially from the index number. The index also says nothing about pollutant mixtures — two cities with the same AQI may have completely different pollutant compositions, and therefore different health profiles. And because the sub-index uses a single "worst pollutant," a day with five moderate pollutants and one high one looks identical to a day with six high pollutants, even though the latter is far worse for health.

Crucially, the AQI bands are health-effect benchmarks, not safety thresholds. The WHO's own air quality guidelines are far stricter than either national standard, because they are set to protect health, whereas national breakpoints also balance economic and practical considerations. A "Good" or "Satisfactory" label under EPA or CPCB does not mean zero health risk — it means the level is within the range the nation has chosen to describe as acceptable.

Frequently Asked Questions

❓ What is the difference between AQI and NAQI?: ✅ AQI is a generic term for any air quality index; in practice it usually refers to the US EPA Air Quality Index. NAQI is the National Air Quality Index used by India's Central Pollution Control Board. Both use the same piecewise-linear formula, but they differ in pollutant breakpoints, category names, and thresholds — the EPA categories shift at 50, 100, 150, 200, and 300, while the CPCB categories shift at 50, 100, 200, 300, and 400.
❓ Why is the AQI the maximum of the pollutant sub-indices and not the average?: ✅ The maximum ensures the index reflects the worst pollutant, which is the one most likely to drive adverse health effects on a given day. If the index averaged the sub-indices, a high PM2.5 reading could be diluted by low readings for other pollutants, understating the true risk.
❓ Can I calculate the AQI myself from raw monitor data?: ✅ Yes. You need the measured concentration for at least one pollutant, its correct averaging time, and the breakpoint table for the standard you want. Apply the piecewise-linear formula to get each sub-index and take the maximum. The easiest way is to use the Air Quality Index Calculator, which handles both the EPA and CPCB tables automatically.
❓ Why do some countries use different AQI scales?: ✅ Each country or agency sets breakpoints based on its own health studies, regulatory standards, and policy priorities. The result is that the same measured concentration can produce different index values and categories in different countries. The World Health Organization publishes global air quality guidelines that are more protective than any national index, precisely to highlight these differences.
❓ What is the difference between PM2.5 and PM10?: ✅ PM2.5 refers to particles smaller than 2.5 micrometres, which penetrate deep into the lungs and bloodstream; PM10 refers to particles up to 10 micrometres, which are coarser and mostly affect the upper airways. PM2.5 is generally the more dangerous pollutant and is often the main driver of a city's AQI.
❓ Which pollutant is measured as an 8-hour average and why?: ✅ Ozone and carbon monoxide are measured as 8-hour averages because their health effects depend on prolonged exposure over several hours, not on short peaks. Ozone is produced by sunlight-driven chemistry during the day, so an 8-hour window captures the daylight accumulation; carbon monoxide's effects on blood oxygen build up over hours of exposure.
❓ Is an AQI of 100 safe?: ✅ An AQI of 100 is exactly the boundary between the Moderate and the next band (Unhealthy for Sensitive Groups under EPA, or Moderately Polluted under CPCB). It is considered acceptable for the general population, but unusually sensitive individuals may begin to feel effects at this level. It is a benchmark for health-action triggers, not a guarantee of zero risk.
❓ Why does the CPCB report 'Good' air when the WHO guidelines say the level is unhealthy?: ✅ National indices set breakpoints that balance health protection with the reality of prevailing pollution levels in that country. The WHO's guidelines are health-only targets and are considerably stricter. A 'Good' or 'Satisfactory' label under a national index does not mean the air meets WHO recommendations — it means the level is acceptable under that country's chosen standards.
❓ What does 'Unhealthy for Sensitive Groups' mean?: ✅ It is the EPA category for AQI values of 101–150. It means the air is probably safe for the general public, but children, older adults, and people with heart or lung disease — including asthma — may experience health effects and should reduce prolonged outdoor exertion. The CPCB equivalent, 'Moderately Polluted', warns of breathing discomfort for people with existing lung, asthma, or heart conditions.
❓ How often is the AQI updated?: ✅ Real-time monitoring networks update the index throughout the day, typically every hour, using the most recent available measurements and the appropriate averaging times. Agencies also publish daily summaries. Because the underlying concentrations fluctuate, the index can move between categories over the course of a single day.

Summary

The Air Quality Index turns incommensurable pollutant concentrations into a single, actionable 0–500 score through a simple and transparent mechanism: a piecewise-linear interpolation between published breakpoints, with the overall index equal to the maximum of the per-pollutant sub-indices. The US EPA and India CPCB standards apply the identical formula to different breakpoint tables and category thresholds, so the same air can score differently depending on the standard in use. Whether you are a runner deciding on an outdoor workout, a parent checking whether school playgrounds are safe, or a student learning how environmental metrics work, understanding the formula — and its limitations — lets you interpret the daily AQI number as the measured, standardized, and deliberately simplified public-health signal that it is. For the calculation itself, the Air Quality Index Calculator applies both standards automatically.

References

Give us your feedback! Was this useful?

UnByte — Independent Software Engineering

All reference data cites its sources — Editorial policy