Guide

What is AQI? The US AQI and PM2.5 explained

Last reviewed 4 Nov 2026 · DanixSoft

The Air Quality Index (AQI) turns pollution levels into one number from 0 to 500. Up to 50 is good, 51–100 moderate, 101–150 unhealthy for sensitive groups, 151–200 unhealthy for everyone, 201–300 very unhealthy and above 300 hazardous. In Pakistan in winter the number is almost always driven by PM2.5, fine particles small enough to reach deep into the lungs.

The six categories

The PM2.5 ranges are the ones the US EPA set in 2024 (in force from May 2024), when it lowered the top of "Good" from 12.0 to 9.0 µg/m³.

US AQI	Category	PM2.5, 24-hour average (µg/m³)	What it means
0–50	Good	0–9.0	Little or no risk.
51–100	Moderate	9.1–35.4	Acceptable; unusually sensitive people may notice it.
101–150	Unhealthy for sensitive groups	35.5–55.4	Children, older adults, pregnancy, asthma, heart or lung disease: cut long or hard activity outdoors.
151–200	Unhealthy	55.5–125.4	Everyone may feel effects; sensitive groups more seriously.
201–300	Very unhealthy	125.5–225.4	Health alert: risk is higher for everyone.
301+	Hazardous	225.5 and above	Health warning of emergency conditions.

What PM2.5 is

PM2.5 means particles 2.5 micrometres across or smaller, about thirty times thinner than a hair. They come from vehicle exhaust, burning crop residue and rubbish, brick kilns, industry, generators and road dust. Because they are so small they get past the nose and throat into the lungs and, from there, into the blood.

The World Health Organization’s 2021 guideline is 15 µg/m³ as a 24-hour average (and 5 µg/m³ as a yearly average). Saans shows how many times that guideline the current PM2.5 is.

Why two apps show different numbers

Averaging: the US AQI for particles is based on a 24-hour average. Some apps show a faster "NowCast" or a one-hour value, which jumps more.
Model or station: Saans’s main number is a model forecast (CAMS, on a ~45 km grid). A station measures one street corner. Both are useful; they rarely match exactly.
Scale: some sites use a different national index or a different year’s breakpoints. Saans always uses the US EPA scale and says so.
Time: check the update time. Air in Lahore can change a lot between 3 pm and midnight.

Questions people ask

What is a good AQI?

Up to 50 is good and up to 100 is acceptable for most people. Above 100, sensitive groups should cut long or hard activity outdoors; above 150, everyone should.

Is AQI 200 dangerous?

AQI 151–200 is "Unhealthy": everyone may feel effects such as coughing or a sore throat, and people with asthma, heart or lung disease, children and older adults are at more risk. Keep time outdoors short and exercise indoors.

What is the difference between AQI and PM2.5?

PM2.5 is a concentration in micrograms per cubic metre. The AQI turns that concentration (and other pollutants) into a 0–500 scale with health categories. In Pakistan’s winter the AQI is usually set by PM2.5.