Malaysia Haze: How to Read the API — and Why AQI Numbers Differ

The API is a maximum, not an average

The Department of Environment measures six pollutants at its continuous air quality monitoring stations: PM10, PM2.5, sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide. Each is converted onto a common 0–500 scale to produce a sub-index, and the API published for a station is simply the highest of those six sub-indices — not a blend of them.

This is why DOE names a predominant pollutant alongside the reading. During haze it is almost always PM2.5 or PM10; on a still, sunny afternoon in a traffic corridor it can be ozone instead. An API of 95 driven by ozone describes very different air from an API of 95 driven by smoke particles, even though the headline number is identical.

The scale is anchored to Malaysia's own air quality standard: API 100 is set at the level the standard treats as the limit for safe exposure, and API 50 at roughly half of it. For PM2.5 that limit is 35 µg/m³ averaged over 24 hours. So API 100 is not a neutral midpoint — it is the line the standard says should not be crossed.

Why the API lags the haze you can see

Each sub-index uses the averaging period that the health standard for that pollutant is written against. Particulates — PM10 and PM2.5 — and sulphur dioxide are reported on 24-hour running averages. Carbon monoxide uses an 8-hour running average. Ozone and nitrogen dioxide use 1-hour averages.

The practical consequence catches people out every haze season. When smoke arrives suddenly in the afternoon, the PM2.5 sub-index is still averaging in the clean air of the previous night, so the API climbs slowly over the following hours rather than spiking. The reverse is also true: after a heavy downpour clears the air, the API stays elevated for the rest of the day because yesterday's smoke is still inside the 24-hour window.

So if the sky looks far worse than the API suggests, you are probably right and the index is simply behind. Trust the slope of the number over several hours, not a single reading.

What each API band means — and what it triggers

The bands are not just advisory labels. Several official responses are keyed directly to them, most visibly the school closure threshold that state education departments apply during an episode.

Why this site's number is not the DOE API

The Haze Meter on this site reports the US EPA Air Quality Index derived from modelled PM2.5 and PM10. It is useful because it covers the whole country on one screen and updates hourly, but it is a different statistic from the official API and the two should never be compared number-to-number.

Two things differ. First, the scales are cut differently above 100: Malaysia has a single wide "Unhealthy" band running from 101 to 200, where the US index splits that range into "Unhealthy for sensitive groups" (101–150) and "Unhealthy" (151–200). The same air can therefore sit in bands with quite different names on the two systems.

Second, the inputs differ. The DOE API comes from physical instruments at fixed monitoring stations, with quality control applied before publication. A modelled AQI is a calculation over a grid cell roughly eleven kilometres across — which is why two towns in the same corner of the Klang Valley can show identical readings, and why a localised plume from a single fire may not appear at all.

For anything that matters — a school decision, a medical precaution, an insurance or employment question — use the official figure at apims.doe.gov.my. Use the modelled view for the national picture and the trend.

What actually protects you

• A surgical or cloth mask does little against PM2.5. The particles are far smaller than the weave, and the loose fit lets air bypass the material entirely. An N95, KN95 or FFP2 respirator filters at least 94–95% of fine particles — but only if it seals against your face, which rules out most beards and all loose-strap wearing.
• Indoors is only better if you keep it that way. Close windows and doors on the windward side, run air conditioning on recirculate rather than fresh-air intake, and avoid anything that adds particles: smoking, frying, incense, candles.
• If you buy an air purifier, size it by CADR rather than by room-size marketing claims. As a rule of thumb, aim for a clean-air delivery rate of at least 12 m³/h per square metre of floor area — about 240 m³/h for a 20 m² bedroom. One correctly sized unit in the room where you sleep beats three underpowered ones spread around the house.
• In the car, switch the ventilation to recirculate. Drawing outside air in during an episode turns the cabin into the same air you are trying to avoid.
• Watch for symptoms rather than numbers if you have asthma, COPD or heart disease. Keep reliever medication accessible, and treat a cough that worsens over a day as a reason to stay in, whatever the index says.

When haze season happens, and why

Malaysia has two distinct haze patterns. The major one runs roughly from June to September, during the southwest monsoon, when winds carry smoke from land-clearing and peat fires in Sumatra and Kalimantan across the Straits. Peat fires are the reason these episodes last: peat smoulders below the surface and can burn for weeks, surviving ordinary rain.

The second pattern is shorter and more local — a dry spell around February and March that produces bush, scrub and peat fires within Peninsular Malaysia itself, sometimes compounded by smoke drifting down from mainland Southeast Asia. These episodes are usually less severe but can hit specific districts hard.

Transboundary haze is governed regionally by the ASEAN Agreement on Transboundary Haze Pollution, which Malaysia ratified in 2002. In practice it provides for monitoring, notification and mutual assistance rather than enforceable penalties against a neighbouring state, which is why domestic measures remain the main lever.

Open burning is a criminal offence

Within Malaysia, open burning is prohibited under section 29A of the Environmental Quality Act 1974. Conviction carries a fine of up to RM500,000, imprisonment for up to five years, or both; the Department of Environment can also issue compounds of up to RM2,000 per offence without going to court.

The prohibition applies to householders burning garden waste as much as to plantations, and enforcement is stepped up during dry spells and active haze episodes. Open burning can be reported to the DOE hotline or through its complaint channels.

Caveats

API band descriptors, the pollutants included in the index and the ambient standards behind it are set by the Department of Environment and have changed before — PM2.5 was only added to the Malaysian API in recent years. School closure decisions are made by state education departments and can depart from the general threshold described here.

Nothing on this page is medical advice. If you have a respiratory or cardiac condition, your own doctor's guidance about exposure thresholds overrides any index band.