API 180 Shuts Schools. But 180 of What? The Maths Behind Malaysia’s Haze Index

On Thursday, 8 October 2026, the Education Ministry set a simple rule for the haze: a school closes if the Air Pollutant Index where it stands goes above 180, or stays above 150 for three days in a row. By Friday the rule had shut schools across Kuala Lumpur, Putrajaya, Selangor, Johor, Melaka, Negeri Sembilan and Pahang, affecting roughly two million students, with Kuala Lumpur reading around 170 as smoke from fires in Indonesia settled over the peninsula. This morning, Monday 12 October, schools in Melaka, three districts of Perak and parts of Kuala Lumpur reopened as readings fell. Schools in Bangsar and Pudu stayed shut.

Everyone now knows the thresholds. Far fewer people know what the number itself means. 180 of what? It isn't a percentage, and it isn't a count of anything. The Department of Environment describes it as a non-unitary value: a number with no unit at all. Where it comes from is a short piece of arithmetic, and it explains a few things about haze days that otherwise seem strange.

Malaysia's monitoring stations measure six pollutants: sulphur dioxide, nitrogen dioxide, carbon monoxide, ground-level ozone, and two sizes of fine dust, PM10 and PM2.5. Each one is averaged over a period chosen for how long people can be exposed to it. Sulphur dioxide and nitrogen dioxide are averaged over one hour, carbon monoxide and ozone over eight hours, and both kinds of dust over 24 hours.

Each average is converted into a score of its own, called a sub-index, and the API is simply the highest of the six. Not the total, and not the average: the worst one. A reading of 180 means the single worst pollutant at that station is at level 180, whatever the other five are doing. That is deliberate. Air that is fine on five counts and dangerous on the sixth is still dangerous air, and an average would hide it.

During haze, the worst one is almost always the fine dust. The Department of Environment's own notes say so, and since 2017 the dust that counts has included PM2.5: particles no more than 2.5 micrometres across. The US Environmental Protection Agency puts that size in perspective. An average human hair is about 70 micrometres thick, 30 times wider than the largest of these particles, which is small enough to be breathed deep into the lungs.

So when the haze is in, the API is really a PM2.5 measurement in disguise: micrograms of dust per cubic metre of air, averaged over 24 hours and run through a conversion table. The table is where it gets interesting, because it is not one straight line. It is a series of straight lines joined end to end, and each band has its own slope.

Here is the Department of Environment's table for PM2.5. An API of 0 to 50, rated good, covers 0 to 12 micrograms per cubic metre. An API of 51 to 100, moderate, covers 12.1 to 75.5. An API of 101 to 200, unhealthy, covers 75.5 to 150.4. An API of 201 to 300, very unhealthy, covers 150.5 to 250.4. Above 300 is hazardous, and above 500 is an emergency.

Look at the widths. The whole good band is 12 micrograms wide. The moderate band is more than 63 micrograms wide, about five times as much dust spread across the same 50 points. So API 100 is not twice as much haze as API 50. It is about six times as much: 75.4 micrograms against 12. Further up, the scale evens out. API 200 works out at 150.4 micrograms, almost exactly twice API 100. The scale stretches hardest at the bottom, where it separates clean air from merely tolerable air, and becomes close to proportional once the air is genuinely bad.

That makes this morning's good news worth reading carefully. Melaka's schools reopened after readings of 91 at Alor Gajah, 94 at Bukit Rambai and 96 in Bandaraya Melaka, all in the moderate band. That is a real improvement on last week. But a reading of 96 corresponds to roughly 70 micrograms of PM2.5 per cubic metre, close to six times the top of the good band. Moderate covers a lot of ground.

The school thresholds convert the same way. API 150 is about 113 micrograms per cubic metre, and API 180, the immediate closure line, is about 135. Kuala Lumpur's reading of around 170 on Friday was roughly 128 micrograms. Cheras, which was still at 139 on Sunday afternoon, was at about 104.

You can do these conversions in your head, because the slopes are friendly. In the moderate band, each microgram adds about three quarters of an API point, so 4 micrograms is roughly 3 points. In the unhealthy band it flips: each API point is about three quarters of a microgram, so 4 points is roughly 3 micrograms. To turn API 180 into dust, take the 79 points above 101, find three quarters of them, and add the 75.5 the band starts at. Three quarters of 79 is a congak sum on its own: three quarters of 80 is 60, less three quarters of one, so 59.25. Add 75.5 and you get 134.75, a whisker under the exact answer of 135.3.

The averaging explains the other strange thing about haze days. The dust figure behind the API is a 24-hour average, so every reading carries the previous day with it. A morning that looks clearer can still show an unhealthy reading, because the smoky hours of the night before are still inside the average. It works the other way too: a sudden thick spell takes hours to push the number up. The API is built to describe what you have been breathing over a day, not in the last few minutes.

The Department of Environment pairs each band with advice. From 101 to 200 it advises limited outdoor activity for high-risk people: the elderly, pregnant women, children, and anyone with heart or lung conditions. From 201, it advises older and high-risk people to stay indoors and reduce physical activity. Above 300, it advises everyone to avoid outdoor activities.

None of this needs more than a times table and the habit of asking what a number is made of. A headline figure like 180 is the end of a calculation, not the start of one, and knowing the steps behind it — the worst of six, a 24-hour average, a scale that stretches at the bottom — tells you far more than the figure alone. Taking numbers apart like that is what congak builds, and the daily challenge at cerdikcerdik.com/math practises it in a few minutes a day.