Singapore's air quality readings have been updated from Tuesday to include fine particles, so the Pollutant Standards Index (PSI) levels now looks higher than before.

The PSI, a measure of air quality, now includes the reading of PM2.5 - fine particles smaller than 2.5 microns in diameter - as a component.

The change was implemented to simplify the readings, said Minister for the Environment and Water Resources Vivian Balakrishnan, when announcing the changes in his ministry's budget debate last month.

Previously, the PM2.5 level was reported separately from the PSI reading.

The finer the particles, the more harmful their effects, as fine particles can enter the lungs and very fine ones can slip into the blood to enter even the brain.

The PSI is calculated based on the highest level of those of six components: PM10, or particles smaller than 10 microns in size; sulphur dioxide; nitrogen dioxide; ozone; carbon monoxide and PM2.5.

The level of each component corresponds to a PSI level based on a formula.

So for example, at 11am on Tuesday, the 24-hour PSI reading for the central region in Singapore was 59, based on the component with the highest level, PM2.5.

That is because 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, because the measured 28 micrograms per cubic metre of PM10 corresponds to a PSI of 28.

The 24-hour PSI reading is available on the National Environment Agency's website www.nea.gov.sg, and is updated every hour.

Measured pollutant concentrations, and the PSI readings that they correspond to, are also available at www.haze.gov.sg.