What the API number means
Malaysia reports air quality as the Air Pollutant Index — the API. It is a single number that stands in for six pollutants at once, published every hour by the Department of Environment.
The five bands
Air quality poses little or no risk. No precautions needed.
Acceptable air quality. Unusually sensitive people should consider limiting prolonged outdoor exertion.
Sensitive groups — children, the elderly, and people with heart or lung conditions — should limit outdoor activity.
Everyone should avoid outdoor exertion. Sensitive groups should remain indoors with windows closed.
Emergency conditions. Everyone should remain indoors. Follow official DOE and health ministry directives.
How the number is worked out
The DOE calculates a sub-index for each of six pollutants — PM2.5, PM10, ground-level ozone, carbon monoxide, sulphur dioxide and nitrogen dioxide. Each is averaged over the window that matters for that pollutant: PM2.5 and PM10 over 24 hours, carbon monoxide over 8 hours, sulphur dioxide and nitrogen dioxide over 1 hour.
The API you see is simply the highest of those six sub-indices, and the “main pollutant” shown alongside it is whichever one produced that highest value. During a haze episode it is almost always PM2.5.
Why a reading can lag the air outside
Because PM2.5 is averaged over a rolling 24 hours, the API responds slowly. Air that turns sharply worse in the space of an hour will not be fully reflected in the index until that hour has worked through the average. If visibility and smell tell you the air is bad, trust them even when the published number has not caught up yet.