An air quality reading is only useful if you know what the number is telling you. The air quality index meaning is straightforward once you have the scale in front of you, and this page gives you that first: the bands, the colours, what each one means for your day, and the actual PM2.5 concentration behind each number.
It then covers something most explainers skip, and which causes a great deal of confusion in Asia in particular. Not every country uses the AQI. Singapore reports a PSI, Malaysia reports an API, India and China run their own national indices, and those numbers are not interchangeable with a US AQI. A PSI of 150 is not an AQI of 150, and reading one as the other leads people to the wrong conclusion about their own air.
What the AQI Actually Measures
The Air Quality Index converts pollutant concentrations, which are measured in micrograms or parts per billion and mean nothing to most people, into a single number on a 0 to 500 scale with a colour attached.
The US AQI, developed by the Environmental Protection Agency, is the version most global air quality apps and platforms report, which is why it has become the default reference even outside the United States.
One feature of the calculation is worth understanding because it surprises people. The AQI is not an average of the pollutants. Each pollutant is converted to its own sub-index, and the reported AQI is the highest of them. So an AQI of 160 does not mean everything is moderately bad; it means at least one pollutant is at an unhealthy level, and that pollutant is the one setting the number.
That is deliberate. The index is designed to report the risk you are actually exposed to, not to dilute a serious single pollutant by averaging it against five clean ones.
The AQI Scale: Numbers, Colours and What to Do
| AQI | Colour | Category | What it means | What to do |
|---|---|---|---|---|
| 0–50 | Green | Good | Little or no health risk | Normal outdoor activity |
| 51–100 | Yellow | Moderate | Acceptable, with some risk for unusually sensitive people | Sensitive individuals watch for symptoms |
| 101–150 | Orange | Unhealthy for Sensitive Groups | Children, older adults, and people with asthma, lung or heart conditions are affected | Sensitive groups reduce prolonged outdoor exertion |
| 151–200 | Red | Unhealthy | Everyone may feel effects; sensitive groups more seriously | Limit outdoor exertion; keep windows closed; run filtration indoors |
| 201–300 | Purple | Very Unhealthy | Significant risk of serious effects for the whole population | Stay indoors; avoid outdoor exercise; filtration and a fitted mask outdoors |
| 301+ | Maroon | Hazardous | Emergency conditions; everyone is at serious risk | Remain indoors with filtration running; follow official advisories |
A note on the orange band, because its name misleads people. “Unhealthy for Sensitive Groups” sounds reassuring if you are healthy. It is not a clean bill of health, it is a statement that the effects begin with the people most exposed to them: children, whose lungs are still developing, older adults, and anyone with asthma, COPD or a heart condition. Pregnant women are also in that group. In a household with any of those, the orange band is the point at which you act, not the point at which you relax.
What AQI Numbers Mean in PM2.5 Micrograms
The AQI is a derived number, and sometimes you need the concentration behind it, particularly if you are comparing against World Health Organization guidelines or reading a monitor that reports µg/m³ directly.
For PM2.5, which is the pollutant that sets the AQI on most polluted days, the EPA’s 24-hour breakpoints as revised in 2024 are:
| AQI range | Category | PM2.5, 24-hour average (µg/m³) |
|---|---|---|
| 0–50 | Good | 0.0–9.0 |
| 51–100 | Moderate | 9.1–35.4 |
| 101–150 | Unhealthy for Sensitive Groups | 35.5–55.4 |
| 151–200 | Unhealthy | 55.5–125.4 |
| 201–300 | Very Unhealthy | 125.5–225.4 |
| 301–500 | Hazardous | 225.5–325.4 |
One change worth knowing if you are comparing against older data. The 2024 revision lowered the Good to Moderate boundary from 12.0 to 9.0 µg/m³, following a strengthened annual PM2.5 standard. A reading that would have been reported as Good a few years ago can now read Moderate without the air having changed at all. If a chart seems to show air quality deteriorating around 2024, check whether it is the air or the scale.
For context on what those concentrations look like in practice, see our data on PM2.5 levels in major cities.
Why Singapore’s PSI and Malaysia’s API Are Not the Same as AQI
This is the part that causes the most confusion, especially during a regional haze episode, and it is not a minor technicality.
Singapore: the PSI
Singapore reports the Pollutant Standards Index. Like the AQI it takes the highest of six pollutant sub-indices, covering PM2.5, PM10, sulphur dioxide, ozone, carbon monoxide and nitrogen dioxide, and on a haze day PM2.5 is almost always the one setting it. The headline PSI is a 24-hour moving average.
The bands are where it differs:
| PSI | Singapore label | US AQI equivalent band |
|---|---|---|
| 0–50 | Good | Good |
| 51–100 | Moderate | Moderate |
| 101–200 | Unhealthy (one band) | Split into two: 101–150 and 151–200 |
| 201–300 | Very Unhealthy | Very Unhealthy |
| Above 300 | Hazardous | Hazardous |
So Singapore uses five bands where the US AQI uses six. The entire 101 to 200 range is a single “Unhealthy” band, which means a PSI of 120 and a PSI of 190 carry the same label despite being very different conditions. The US AQI separates those into “Unhealthy for Sensitive Groups” and “Unhealthy”.
The practical consequence: a PSI number and an AQI number are not interchangeable. Converting between them requires going back to the underlying concentration, not reading one scale as the other.
Malaysia: the API
Malaysia reports the Air Pollutant Index, also built from the highest pollutant sub-index, and it uses the same five-band structure as Singapore’s PSI: Good, Moderate, Unhealthy, Very Unhealthy, Hazardous, with Hazardous above 300. Malaysian authorities treat readings above 500 as the threshold for emergency measures.
So PSI and API are structurally similar to each other, and both differ from the US AQI. India and China each run their own national indices with their own breakpoints as well.
Why You See Two Different Singapore Numbers on the Same Day
This is the single most common question during a haze event, and it has a clean answer.
Singapore’s National Environment Agency publishes two separate figures:
- The 24-hour PSI, a moving average across the previous day, which is what the health advisories are based on.
- The 1-hour PM2.5 reading, by region, which is a snapshot of right now.
Because the PSI is a 24-hour average, it moves slowly. The 1-hour PM2.5 can climb sharply hours before the PSI reflects it, and fall again just as fast. During the October 2026 haze, 1-hour PM2.5 readings in the West region reached 284 while the 24-hour PSI sat well below that figure.
Neither number is wrong. They answer different questions. The 1-hour PM2.5 tells you what the air is doing at this moment, which is what matters if you are deciding whether to go for a run right now. The 24-hour PSI tells you what the day has been like overall, which is what matters for cumulative exposure and for official advisories.
If the two disagree sharply, the air is changing fast, and the 1-hour figure is the one to act on.
Which Pollutants the AQI Covers
Six pollutants feed the index, and it is worth knowing which is likely to be driving a given reading.
PM2.5, fine particulate under 2.5 micrometres. The most health-significant and usually the one setting the number during haze, wildfire smoke or winter inversions. Small enough to reach deep into the lungs and cross into the bloodstream. Our page on PM2.5 health effects covers the mechanism.
PM10, coarser particulate from road dust, construction and some industrial processes. Dominant during dust events.
Ground-level ozone (O₃), not emitted directly but formed when nitrogen oxides and volatile organic compounds react in sunlight. Peaks on hot, still, sunny afternoons, which is why ozone often drives summer AQI readings in cities with otherwise clean air.
Nitrogen dioxide (NO₂), largely from vehicle exhaust. Follows traffic patterns, with peaks at rush hour and along busy roads.
Sulphur dioxide (SO₂), from coal combustion, power generation and some industry.
Carbon monoxide (CO), from incomplete combustion. Rarely the driving pollutant outdoors in modern cities but significant near heavy traffic and in smoke.
Knowing which pollutant is dominant changes the right response. An ozone-driven reading on a hot afternoon calls for staying inside during the peak hours and ventilating in the evening. A PM2.5-driven haze reading calls for the opposite: keep the windows shut and filter the indoor air.
Health Effects of Poor Air Quality
Short term, within hours to days, exposure causes irritation of the eyes, nose and throat, coughing, chest tightness, shortness of breath and headaches. People with asthma experience more frequent and more severe attacks, and those with heart conditions report more symptoms.
Long term, the picture is more serious. Sustained PM2.5 exposure is associated with increased risk of cardiovascular disease, stroke, chronic obstructive pulmonary disease and reduced life expectancy. In children, continued exposure during the years when lung tissue is still developing is linked to lower lung function reached in adulthood, which does not recover later.
That is the reason the index exists. A single bad day is uncomfortable. A city that averages the orange band year after year produces a measurable population health burden, and the index is how both individuals and policy makers see it.
For vulnerable groups, the threshold for action is lower than the index labels suggest. Someone with moderate asthma can react within the yellow band, and the labels describe averages, not individuals. If you know you react, your own symptoms are a better guide than the colour.
Indoor sources matter too, and they are not in the outdoor index at all. See reducing PM2.5 from cooking, removing formaldehyde from indoor air and indoor asthma triggers.
What to Do Indoors When the AQI Is High
Most AQI advice stops at “stay indoors”, which is incomplete. Indoor air is not automatically clean air, and during a smoke event it tracks outdoor air within hours unless you do something about it.
Close the envelope. Windows and doors shut, and where you have mechanical ventilation, switch it to recirculate rather than drawing in outside air. Check obvious gaps: bathroom extracts left running pull smoke in through every other leak in the building.
Filter the particles. A HEPA filter removes the PM2.5 that is setting the AQI reading. H13 to EN 1822 captures at least 99.95% at the most penetrating particle size; the US HEPA definition is 99.97% at 0.3 microns. Size the unit to the room: an undersized purifier in a large living room cannot turn the air over fast enough to keep up.
For smoke, you also need carbon. This is the point that gets missed. HEPA media captures particles and does nothing whatsoever to gases. Smoke from forest and peat fires carries carbon monoxide, sulphur dioxide, nitrogen oxides and volatile organic compounds alongside the particulate, and those pass straight through a HEPA filter. If the room smells of smoke despite a clean PM2.5 reading, that smell is the gas fraction and it needs activated carbon in meaningful quantity, not a thin carbon-coated pre-filter. Our comparison of HEPA and activated carbon filters explains both mechanisms.
Watch the carbon dioxide. A well-sealed room with people in it accumulates CO2, and no filter of any kind removes it. The drowsiness and poor concentration people often blame on the purifier is usually CO2 building up. This is why fresh air units with heat recovery and filtration exist: they ventilate while filtering, rather than forcing a choice between smoke and stuffiness. Our campus fresh air ventilator is built for that duty in schools and offices.
If you must go out, a properly fitted N95 or KN95 respirator filters fine particulate. A surgical or cloth mask does very little against PM2.5, because the fit and the media are both wrong for it.
Our air purifier range covers portable household units, higher-capacity commercial models and ducted fresh air equipment. Distributors and retailers sourcing stock for a haze season will find the commercial side covered in our guide to HEPA air purifier wholesale sourcing.
“Once the AQI reaches the orange band I close the windows and run a purifier, and I would tell anyone with children or older relatives at home to do the same rather than waiting for red. The part people get wrong is assuming the filter handles everything. HEPA takes out the particles the index is measuring, but during haze the smell in the room is the gas fraction, and that needs carbon. And if you seal a room up tight with people in it, carbon dioxide climbs and no filter touches that. Clean air indoors is filtration and ventilation together, not one instead of the other.”
— Maggie Shen, Director of Legom
How to Check the AQI
Most phone weather apps now show an air quality figure, and dedicated air quality apps report by station with forecasts. Government and environmental agency sites are the authoritative source for your own country, and they are the ones to use for official advisories.
Three things to check before trusting a number:
- Which index is it? US AQI, local AQI, PSI, API or a raw concentration. An app reporting “AQI” in Singapore may be converting to the US scale, which will not match the NEA figure.
- How far away is the station? Air quality varies substantially across a city. A reading from a station 15 km upwind may not describe your street at all.
- What averaging period? A 1-hour figure and a 24-hour figure on the same day can differ widely, and both can be correct.
For an indoor picture, a monitor reporting PM2.5 in µg/m³ alongside CO2 in ppm tells you more than an outdoor index does, because it measures the air you are actually breathing and shows whether your filtration is working.
Frequently Asked Questions
What does the air quality index mean?
It converts pollutant concentrations into a single 0 to 500 number with a colour band, so air quality can be read at a glance. The reported value is the highest of the individual pollutant sub-indices, not an average, so the number reflects whichever pollutant is worst that day.
What AQI level is unhealthy?
151 and above is “Unhealthy” for everyone. The 101 to 150 band is “Unhealthy for Sensitive Groups”, meaning children, older adults, pregnant women and people with asthma, lung or heart conditions are already affected there even though the general population may not notice.
Is a PSI of 150 the same as an AQI of 150?
No. Singapore’s PSI uses five bands with 101 to 200 as a single “Unhealthy” band, while the US AQI uses six and splits that range in two. The two scales are built differently and the numbers are not interchangeable. The same applies to Malaysia’s API.
Why does Singapore show a PSI and a separate PM2.5 number?
The PSI is a 24-hour moving average used for health advisories, so it moves slowly. The 1-hour PM2.5 reading is a snapshot of current conditions by region, and it can rise and fall hours before the PSI follows. If they disagree sharply the air is changing quickly, and the 1-hour figure is the one to act on.
What PM2.5 concentration is an AQI of 100?
35.4 µg/m³ as a 24-hour average, under the EPA breakpoints revised in 2024. An AQI of 50 corresponds to 9.0 µg/m³, down from 12.0 before the revision, so some older readings described as Good would now be reported as Moderate.
Does an air purifier help when the AQI is high?
For the particulate component, yes, which is what sets the AQI on most polluted days. A HEPA filter removes PM2.5 effectively provided the unit is sized for the room. For smoke, you also need activated carbon, because HEPA media does nothing to the gases in smoke, which is what you are smelling.
Does any filter remove carbon dioxide?
No. Neither HEPA nor activated carbon removes CO2. In a sealed room CO2 rises from the occupants and causes drowsiness and poor concentration that is often blamed on the purifier. The answer is ventilation with filtration, such as a fresh air unit with heat recovery.
Which pollutant usually sets the AQI?
PM2.5 on most polluted days, and particularly during haze, wildfire smoke and winter inversions. Ground-level ozone often drives readings on hot still afternoons in cities with otherwise clean air, and PM10 dominates during dust events. The dominant pollutant determines the right response.
Reviewed by Maggie Shen, Director at Legom, on October 15, 2026. This article was reviewed for technical accuracy, including the US EPA AQI band structure and the 2024-revised PM2.5 breakpoints, the fact that the reported index is the highest pollutant sub-index rather than an average, the five-band structure of Singapore’s PSI and Malaysia’s API and their non-equivalence to the US AQI, the distinction between the 24-hour PSI and the 1-hour PM2.5 reading, and the inability of HEPA media to remove gases or of any filter to remove carbon dioxide.