Open two apps during a haze episode and you get two answers. One says the air near you is 168 and colours it red. The other says 94, and calls it moderate. Neither is broken. They measure on different scales, over different time windows, and only one of them is what schools and agencies act on.
That distinction matters more than usual right now. Smoke from open burning in Sumatra, pushed across by the south-west monsoon and compounded by local burning, has kept readings elevated through August. On 22/08/2026 the Department of Environment reported no station in the country recording "good" air across its 68-station network, and on 26/08/2026 sixteen areas were still in the unhealthy band.
Two different scales, two different numbers
Malaysia reports the Air Pollutant Index, or API, through the Air Pollutant Index Management System — APIMS — run by the Department of Environment. Most international apps, including the ones that ship with phones, report the US Air Quality Index, or AQI, on the United States EPA scale.
Both compress pollutant concentrations into a single number from 0 upwards, and both use colour bands. That similarity is what makes them so easy to confuse.
But the breakpoints — the concentration thresholds that map onto each index value — are set differently. A Malaysian API of 150 and a US AQI of 150 do not describe the same air. Comparing them directly is like comparing a temperature in Celsius to one in Fahrenheit and concluding it got hotter.
What Malaysia's API actually measures
The API is a sub-index system. The Department of Environment calculates a separate index value for each of six pollutants, then reports the highest one as the API.
The six are PM2.5 and PM10 (fine and coarse particulate matter), carbon monoxide, ozone, sulphur dioxide and nitrogen dioxide.
During a haze episode, PM2.5 is almost always the pollutant driving the headline figure. The rest sit well below it. One consequence of the sub-index design is that the API tells you how bad the worst pollutant is, not how the mix adds up. The DOE publishes its full API calculation methodology if you want the breakpoint tables.
Why the government number lags what you can see
The API is published hourly, but the PM2.5 sub-index is derived from a 24-hour running average, not an instantaneous reading. Other sub-indices use shorter windows — the NO₂ sub-index, for example, is based on a one-hour average.
So when a fresh plume rolls in at 3pm, an app reporting live PM2.5 concentration reacts within the hour. The API, averaging that spike against the previous 23 hours, climbs slowly and keeps climbing after the smoke has thinned.
That is not a flaw. A 24-hour average is a better proxy for the exposure that actually affects health than a single spiky hour. But it means two things in practice: when haze arrives suddenly the API understates what you are breathing right now, and when it clears the API overstates it for the rest of the day.
If your eyes are stinging and the skyline has disappeared, believe your eyes over a moderate reading. If the API is high but you can see the hills again, the number is still catching up.
What each band is supposed to trigger
The API bands are fixed, and they are attached to real institutional consequences.
| API | Band | What it generally means |
|---|---|---|
| 0–50 | Good | No restrictions |
| 51–100 | Moderate | No restrictions for most people |
| 101–200 | Unhealthy | Outdoor activities in schools are to be stopped once readings exceed 100 |
| 201–300 | Very unhealthy | Schools, kindergartens and childcare centres are to close immediately once readings exceed 200 |
| Above 300 | Hazardous | Emergency-level response |
Two thresholds get quoted constantly. Above 100, activities outside classrooms are to be stopped. Above 200, schools, kindergartens and nurseries close immediately under the National Haze Action Plan — which is what happened in Serian, Sarawak, earlier this month.
There is a third, less-reported trigger. If an API reading stays above 150 for more than 24 hours, the National Disaster Management Agency, NADMA, activates its disaster management committee at district, state or central level.
Those numbers all refer to the Malaysian API. Screenshotting a US AQI of 210 and asking why school is still open is a category error.
Who should act earlier than the bands suggest
The bands are population-level guidance. Some people should not wait for them.
Children, older adults, pregnant women, and anyone living with asthma, chronic lung disease or a heart condition are consistently identified as the groups most affected during haze episodes, and are generally advised to reduce outdoor exertion well before the index crosses into the unhealthy band.
On masks: a loose surgical mask is designed to catch droplets, not fine particulate. Respirators rated N95 or KN95 are designed to filter fine particles, and their performance depends heavily on seal — a gap at the bridge of the nose defeats the rating. Advisories during this episode have also emphasised staying hydrated and keeping any child who becomes unwell out of school. If a breathing symptom is worsening rather than merely uncomfortable, that is a matter for a doctor, not an index reading.
For the seasonal picture of how these episodes build, our earlier look at how Malaysia's haze season develops covers the pattern — and because dry spells and haze often arrive together, it is worth knowing what the heat alerts actually mean too.
FAQ
Which number should I follow, API or AQI?
Follow the API for anything institutional — school closures, workplace policy, official advisories. Use a live PM2.5 app as an early-warning signal that conditions are changing before the API catches up. The official API comes from APIMS, run by the Department of Environment, and is updated hourly from its national monitoring network.
Why is the API still high when the air looks clear?
Because the PM2.5 sub-index is a 24-hour running average. It keeps carrying the previous day's pollution for hours after conditions improve.
At what API do schools close?
Outdoor activities are to stop once readings exceed 100. Schools, kindergartens and childcare centres are to close immediately once readings exceed 200.
Does a surgical mask help against haze?
Surgical masks are not designed to filter fine particulate matter. Respirators rated N95 or KN95 are, and their effectiveness depends on getting a proper seal against the face.
