Singapore's main air quality index has been tweaked to reflect more precisely the smallest, most harmful pollution particles.
The change, which took effect yesterday, means that the Pollutant Standards Index (PSI) reading will tend to be higher even if the air quality is the same as before.
More days will be classified as "moderate" under the new index, when they would have been "good" previously.
The tiniest particles are known as PM2.5 - less than 2.5 microns in diameter or a 30th of the diameter of a human hair.
The finer the particles, the more harmful their effects, as fine particles can enter the lungs; very fine ones can slip into the blood and even enter the brain.
These tiny particles have always been included in the PSI, but until now, they were part of a component called PM10 - less than 10 microns in diameter.
But the proportion of PM2.5 within the wider PM10 category may vary, so having PM2.5 as a separate component will better reflect its impact.
The PSI is calculated based on the highest reading of its components - PM10, sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide. PM2.5 has now been added as a sixth component.
The level of each component corresponds to a PSI level, based on a formula.
For example, at 11am yesterday, the 24-hour PSI reading for the central region was 59, based on the highest component, PM2.5. There were 19 micrograms per cubic metre of PM2.5 measured, which corresponds to a PSI reading of 59.
Under the previous system, the 24-hour PSI reading for the central region would have been 28, based on the 28 micrograms per cubic metre of PM10.
The change was made to simplify the readings, said Minister for the Environment and Water Resources Vivian Balakrishnan when announcing it in his ministry's budget debate last month.
Previously, PM2.5 levels were reported separately from the PSI reading.