What the Air Quality Index Actually Measures — and the Number Where It Starts to Hurt

Direct answer: the Air Quality Index is not a measurement. It is a country-specific translation of measured pollutant concentrations onto a single scale, and different countries use different translations. The widely quoted 0 to 500 scale is the US EPA one, which is why an app can report "US AQI" for a city whose own government reports a different number for the same air.

How the index is built

Monitors measure concentrations of several pollutants — most importantly fine particulate matter (PM2.5), but also PM10, ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide. Each concentration is converted to a sub-index through a piecewise-linear formula, and the reported AQI is the highest of those sub-indices, not an average.

Two consequences follow, and both matter when you are reading a number about somewhere you are not:

• The headline AQI usually reflects one pollutant. In South Asian winter smog that is nearly always PM2.5; in a Californian summer it is often ozone. The same number means a different exposure.
• The relationship to concentration is not linear. Going from AQI 50 to AQI 100 is a much smaller change in actual PM2.5 than going from AQI 300 to AQI 350.

The US AQI bands, and what they actually mean for a person

• 0 to 50 — Good. No action needed.
• 51 to 100 — Moderate. Unusually sensitive people may notice symptoms.
• 101 to 150 — Unhealthy for sensitive groups. Children, older adults, pregnant women and anyone with asthma or heart disease should limit prolonged outdoor exertion.
• 151 to 200 — Unhealthy. Everyone may begin to feel effects. This is the level at which healthy adults should cut outdoor exercise.
• 201 to 300 — Very unhealthy. Health warnings of emergency conditions; the entire population is more likely to be affected.
• 301 and above — Hazardous. Serious risk to everyone. Stay indoors with windows closed.

For orientation, the World Health Organization's 2021 air quality guideline for PM2.5 is 5 micrograms per cubic metre as an annual average and 15 over 24 hours. Large parts of South Asia sit many times above that for months at a time, which is why a winter in Lahore or Delhi is a chronic exposure question and not a series of separate bad days.

Why the same air gets different numbers

India's National AQI, China's standard, the European Air Quality Index and the US EPA index all use different breakpoints, and some use different averaging periods. The result is that a city can be "severe" on its own national scale and a different value on the US scale at the same moment, with neither being wrong.

If you are comparing two places, or watching a place from far away, check which scale the number is on. This site and the app both use the US AQI, so that a reading for Lahore and a reading for London can be compared with each other.

What PM2.5 does

Particles below 2.5 micrometres across are small enough to travel deep into the lungs, and the finest fraction crosses into the bloodstream. The established associations are with respiratory illness, heart attack and stroke, low birth weight, and worsening of asthma and COPD. The effects are strongest for the people least able to avoid exposure — outdoor workers, and households that cannot seal or filter their air.

What actually helps

• An N95, KN95 or FFP2 respirator filters PM2.5. A surgical mask or a cloth mask does not — the particles are far smaller than the gaps.
• Close windows during peaks and, where possible, run a HEPA filter in one room rather than trying to treat the whole house. A single clean room to sleep in is worth more than a small improvement everywhere.
• Move exertion indoors. Exercise multiplies the dose, because you breathe several times more air.
• Watch the daily cycle. In smog season, concentrations are usually worst in the early morning and late evening when the air is still and cold, and lower in the afternoon.
• Do not add to it indoors — smoking, unvented cooking and burning rubbish put the same particles straight into the room.

Why an alerting app should mostly stay quiet about air

Worth stating plainly, because it explains what you will and will not be told. Lahore sits above US AQI 150 for much of the winter. An app that alerted whenever the air was unhealthy would send a notification every day for three months, telling people something they can see out of the window — and they would turn it off before the day it mattered.

So the rule used here is "has it got worse than the last thing we told you", not "is it bad": nothing below 151 is ever raised, a place alerts only when it crosses into a worse band than the one it last alerted at, and air never breaks through quiet hours, because smog at three in the morning cannot be acted on.

Related: wet-bulb temperature, the heatwave safety guide, and how wildfires spread — wildfire smoke is the other common source of hazardous PM2.5.

Frequently asked questions

Is AQI the same everywhere?

No. AQI is a national translation of pollutant concentrations, and India, China, the EU and the US all use different breakpoints. The 0 to 500 scale most apps quote is the US EPA one. The same air can carry two different index numbers with neither being wrong, so always check which scale you are reading.

What AQI is dangerous?

On the US scale, 101 to 150 is unhealthy for sensitive groups, 151 and above begins to affect everyone, 201 and above is very unhealthy, and 301 and above is hazardous. The level at which a healthy adult should stop outdoor exercise is around 151.

Does a surgical or cloth mask help against smog?

No. PM2.5 particles are far smaller than the gaps in surgical and cloth masks. A fitted N95, KN95 or FFP2 respirator does filter them, and fit matters as much as the rating.

Why is the air worst in the morning?

In smog season, cold still air at night traps pollution near the ground under a temperature inversion, so concentrations build overnight and peak in the early morning. They usually fall through the afternoon as the ground warms and the air mixes.

Why does the app not alert every time the air is unhealthy?

Because Lahore and similar cities sit above AQI 150 for most of the winter, so a daily notification would tell people what they can already see and they would switch it off before the day it mattered. The rule is to alert only when the air gets worse than the last level you were told about.

Sources

• US EPA — AQI Basics
• US EPA — Technical Assistance Document for the Reporting of Daily Air Quality
• WHO — Global Air Quality Guidelines 2021
• WHO — Ambient air pollution fact sheet
• Central Pollution Control Board, India — National AQI
• European Environment Agency — European Air Quality Index
• Open-Meteo — Air Quality API

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