What PM2.5 is and why it matters
Published 19 October 2026
PM2.5 is the fine dust in smoke and traffic fumes, and the number Kabut's air quality reading is built on. Here is what it is and how to read it.
A particle you cannot see
PM stands for particulate matter: solid and liquid particles floating in the air. The number after it is a size limit. PM2.5 means particles 2.5 micrometres across or smaller, about thirty times thinner than a human hair. PM10, which some stations also report, includes everything up to 10 micrometres, so PM2.5 is the finer part of PM10.
Size is the whole point. Larger dust is mostly caught in the nose and throat. Particles as small as PM2.5 get past those defences and reach deep into the lungs, and the smallest of them pass into the blood. That is why health agencies treat PM2.5 as the most important single measure of polluted air, and why it is the pollutant Kabut's headline number is calculated from.
PM2.5 is measured as a concentration: micrograms of particles in each cubic metre of air, written µg/m³.
Where it comes from
Anything that burns makes PM2.5. In Indonesia the main sources are:
- Smoke from land and peat fires. In the dry season this is by far the largest source across Sumatra and Kalimantan, and it can push PM2.5 to levels that are rare anywhere else in the world. See the haze season guide.
- Traffic. Exhaust from cars, buses and especially motorbikes, all day, in every large city.
- Coal and industry. Coal-fired power plants, factories and smelters, which affect the cities near them and downwind of them.
- Burning at home and in the neighbourhood. Rubbish burning and cooking over wood or charcoal.
Particles also form in the air itself, from gases such as sulphur dioxide and nitrogen oxides. So PM2.5 can be high in a city with no obvious source nearby.
Why it matters for health
Breathing PM2.5 irritates the airways and the lungs. In the short term it can cause coughing, a sore throat, stinging eyes and shortness of breath, and it can set off an asthma attack. People with heart or lung conditions can get worse during a bad day. Over years, breathing polluted air is linked to heart disease, stroke, lung disease and lung cancer.
Some people are more at risk than others: children, whose lungs are still growing; older adults; pregnant women; and people with asthma, other lung disease or heart disease. That is why Kabut's advice asks these sensitive groups to ease up earlier, from the "Sensitive groups" class, before everyone else.
The practical advice is on every city page, and it is the same for every city in the same class. From Unhealthy upward: limit time outdoors, avoid hard exercise, keep windows shut during the worst hours, and wear an N95 outside. A cloth mask does not stop PM2.5; the masks guide explains why. If you have breathing difficulty, chest pain or dizziness, seek care.
How much is too much
The World Health Organization's 2021 air quality guidelines set two values for PM2.5:
- 5 µg/m³ as an average over a year.
- 15 µg/m³ as an average over 24 hours, not to be exceeded on more than a few days a year.
These are guideline levels, set where the evidence shows harm starts, not legal limits. Most cities in the world are above the annual value. Kabut's home screen shows the PM2.5 reading as a multiple of the annual guideline, so a city at 50 µg/m³ is shown as ten times the guideline. It is a way to read the number, not a class of its own.
From PM2.5 to a US AQI number
A concentration is hard to read on its own: is 40 µg/m³ bad? So Kabut turns it into the US AQI, the index of the US Environmental Protection Agency, using the breakpoints of its 2024 revision. The index runs from 0 upward and falls into six classes, from Good to Hazardous, each with a colour and its own advice.
Some examples of where PM2.5 lands on that scale:
- Up to 9.0 µg/m³ is Good.
- 35.5 µg/m³ is the start of Sensitive groups.
- 55.5 µg/m³ is the start of Unhealthy.
- 225.5 µg/m³ and above is Hazardous.
The full table and the advice for each class are on the methodology page. The index is calculated once on our server and stored with the reading; the site and the app only colour it.
Indonesian cities also show ISPU, Indonesia's own index, beside the US AQI. It uses different breakpoints, so the same PM2.5 can score differently; the ISPU guide explains the difference.
Station or model
Each city's reading comes from one of two kinds of source, and the city page says which:
- Station. A monitoring station reporting to the World Air Quality Index project measures the air at one place. It is the most direct evidence there is, but it describes the air around that station, which may not be the air across the whole city.
- Model. Where no station is close enough, the reading comes from the Copernicus Atmosphere Monitoring Service (CAMS) model, served by Open-Meteo. A model estimates the air for any point from satellite data, emission estimates and the weather. It covers every city, but it works on a coarse grid and smooths over local peaks.
So a station and a model can disagree, and both can be right about what they describe. In dense haze the model usually reads lower than a station inside the smoke. When a reading says model, treat it as an estimate, and if your eyes and your throat say the air is worse, believe them.