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Haze glossary: PSI, PM2.5, API, ISPU, hotspots and more

Updated · Every figure links to its source below.

Haze reports mix two kinds of number: concentrations, such as PM2.5 in µg/m³, and index scores, such as Singapore's PSI, Malaysia's API, Indonesia's ISPU and the US AQI. Concentrations are measured; indices convert them into a 0–500 health scale, and each country uses its own scale.

Key facts

  • PM2.5 is a concentration (µg/m³); PSI, API, ISPU and AQI are unitless index scores.
  • Singapore, Malaysia and Indonesia all put the Unhealthy threshold at an index value above 100.
  • A satellite hotspot is a heat detection, not a confirmed fire count; smoke and cloud can hide fires.
Singapore now · loading the official reading…

How to use this glossary

Each term links to a stable anchor, so you can link straight to a definition, for example haze.day/en/learn/glossary#psi. For the numbers behind each scale, see the comparison of air-quality indices.

Terms

Haze
Air made hazy by fine, dry particles. In Southeast Asia the word usually means smoke haze from land and peat fires, mainly in Sumatra and Kalimantan, carried across borders by the wind.
Transboundary haze
Haze that starts in one country and is carried by the wind into another. It is the subject of the ASEAN Agreement on Transboundary Haze Pollution and of Singapore's Transboundary Haze Pollution Act.
PM2.5
Particles 2.5 micrometres across or smaller, roughly 30 times thinner than a human hair. They are the main pollutant in smoke haze because they reach deep into the lungs. Measured in µg/m³.
PM10
Particles 10 micrometres across or smaller. PM10 includes PM2.5 plus coarser dust and is one of the pollutants in Singapore's PSI and Malaysia's API.
µg/m³ (micrograms per cubic metre)
The mass of particles in one cubic metre of air. A PM2.5 of 100 µg/m³ is a measured concentration; it is not the same as a PSI, API or AQI of 100, which are index scores.
PSI (Pollutant Standards Index)
Singapore's air-quality index, published by the National Environment Agency (NEA). It combines six pollutants, including PM2.5 and PM10, into one score. The 24-hour PSI is Good up to 50, Moderate 51–100, Unhealthy 101–200, Very Unhealthy 201–300 and Hazardous above 300.
1-hour PM2.5 bands (NEA)
Singapore's guide for the next few hours: Band I Normal (0–55 µg/m³), Band II Elevated (56–150), Band III High (151–250) and Band IV Very High (251 and above). Sports organisers and schools usually act on these rather than the PSI.
API (Air Pollutant Index, Malaysia)
Malaysia's air-quality index (Indeks Pencemar Udara, IPU), published by the Department of Environment (DOE) on the APIMS website. It uses the same 0–500 band names as the PSI: Good, Moderate, Unhealthy (101–200), Very Unhealthy (201–300) and Hazardous (above 300).
ISPU (Indonesia)
Indonesia's air-quality index (Indeks Standar Pencemar Udara), set by the environment ministry. Bands: Good (Baik) 0–50, Moderate (Sedang) 51–100, Unhealthy (Tidak Sehat) 101–200, Very Unhealthy (Sangat Tidak Sehat) 201–300 and Hazardous (Berbahaya) above 300.
AQI (Air Quality Index)
A generic name for an air-quality index. Most global apps show the US EPA AQI, whose bands differ from the PSI (101–150 is Unhealthy for Sensitive Groups), so the same air gives a higher number in US AQI than in PSI. Thailand also publishes its own AQI.
Hotspot
A satellite pixel where a sensor such as VIIRS or MODIS detected unusually strong heat, usually an active fire. Hotspot counts show fire activity, but thick smoke and cloud can hide fires, and smouldering peat fires are often under-detected.
Peatland (peat fire)
Waterlogged soil built from partly decayed plant matter over thousands of years. Once drained, peat dries out and can smoulder underground for weeks, releasing far more smoke than a surface fire and resisting rain and firefighting.
Land clearing by fire
Using fire to clear vegetation for farms and plantations because it is cheap and fast. Fires lit for clearing can escape into drained peat, which is how most severe haze episodes start.
Southwest Monsoon
The season from about June to September when winds over the region blow mainly from the south-east or south-west. It is the drier season in Sumatra and Kalimantan, and the winds carry fire smoke towards Singapore and Peninsular Malaysia, so it is the main haze season.
El Niño
A warming of the central and eastern tropical Pacific that usually brings drier weather to Indonesia. Dry years make fires easier to start and harder to put out; the severe haze of 1997 and 2015 came in strong El Niño years.
Indian Ocean Dipole (IOD)
A see-saw in sea temperatures across the Indian Ocean. In a positive phase the water off Sumatra is cooler than usual and rainfall over western Indonesia drops; the strong positive IOD of 2019 helped fuel that year's fires.
ASMC
The ASEAN Specialised Meteorological Centre, hosted by the Meteorological Service Singapore. It monitors hotspots and smoke across ASEAN, publishes daily haze maps and seasonal outlooks, and issues haze alert levels for the southern and northern ASEAN regions.
ASEAN Agreement on Transboundary Haze Pollution
A regional treaty signed in 2002 that commits ASEAN members to prevent and monitor land and forest fires and to cooperate on haze. Indonesia was the last member to ratify it, in 2014.
Transboundary Haze Pollution Act (Singapore)
A 2014 Singapore law that makes it an offence to cause or contribute to haze pollution in Singapore, even when the burning happens outside the country, with civil and criminal liability for companies.
CAMS (Copernicus Atmosphere Monitoring Service)
The EU's atmospheric-composition service, run by ECMWF. It produces global forecasts of PM2.5 and smoke from satellite data and fire emissions. It is a model estimate, not a measurement, and can differ a lot from ground stations during haze.
Rolling average (24-hour)
An average over the most recent 24 hours, recalculated every hour. Because the 24-hour PSI averages a full day, it lags sudden changes: it can stay Unhealthy after the air has cleared, or look fine while a smoke plume has just arrived.
Vulnerable groups
In NEA's health advisories: the elderly, pregnant women and children, with separate advice for people with chronic lung or heart disease. They are advised to cut back on outdoor exertion at lower readings than healthy adults.
HEPA filter
A high-efficiency particulate air filter. Under the common US standard a true HEPA filter removes at least 99.97% of particles 0.3 micrometres across, the size that is hardest to catch, so it removes haze PM2.5 well in a closed room.
N95 / KN95 / FFP2 respirator
Masks certified to filter most fine particles: N95 (US) at least 95%, KN95 (China) at least 95% and FFP2 (EU) at least 94%. They only work as rated when they seal around the face; surgical and cloth masks do not filter smoke particles well.
Haze vs mist and fog
Haze is made of dry particles such as smoke and often smells of burning; mist and fog are tiny water droplets and usually clear as the day warms. Low visibility alone does not tell you the PM2.5: check the reading.
Upwind
The direction the wind is coming from. Fires upwind of a city can send smoke to it; on Haze Day, 'upwind' fires are a wind-direction hint that smoke may arrive, not a smoke-trajectory forecast.

Quick answers

Is a PSI of 100 the same as a PM2.5 of 100 µg/m³?

No. PM2.5 in µg/m³ is a measured concentration; the PSI is an index that converts concentrations into a health scale. NEA's 1-hour PM2.5 of 100 µg/m³ sits in Band II (Elevated), while a 24-hour PSI of 100 is the top of the Moderate band.

What is the difference between a hotspot and a fire?

A hotspot is a satellite pixel that looked unusually hot during an overpass. Most are fires, but one large fire can produce many hotspots, small or smouldering fires may be missed, and cloud or smoke can hide them.

Related questions

More reference pages

References

  1. PSI and 1-hr PM2.5 readings; health advisory. National Environment Agency (Singapore).
  2. Air Pollutant Index Management System (APIMS). Department of Environment Malaysia.
  3. ISPU: Indeks Standar Pencemar Udara. Ministry of Environment, Indonesia.
  4. AirNow: AQI basics. US Environmental Protection Agency.
  5. Regional haze situation, hotspot maps and alert levels. ASEAN Specialised Meteorological Centre.
  6. Fire Information for Resource Management System (FIRMS). NASA.
  7. ASEAN Agreement on Transboundary Haze Pollution. ASEAN Secretariat.
  8. Transboundary Haze Pollution Act 2014. Singapore Statutes Online.
  9. Copernicus Atmosphere Monitoring Service. ECMWF.

Sources

Cite this page

Haze Day. “Haze glossary: PSI, PM2.5, API, ISPU, hotspots and more.” haze.day, updated . https://haze.day/en/learn/glossary

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General information, not medical advice. Follow your doctor and your school, league or organiser's final call. Data come from official sources; see data sources.