Singapore's Pollutant Standards Index (PSI) will be revised from May 1 to include very small, toxic particles called PM2.5. This means the PSI will be based on these particles if they make up the most pollution in the air.

The National Environment Agency will also start giving hourly, more current updates of PM2.5 levels that are averaged from the previous hour. Its hourly PM2.5 updates now are averaged from the past 24 hours, which means the figures may not accurately reflect the air quality at any given time.

Minister for the Environment and Water Resources Vivian Balakrishnan, announcing the changes in his ministry's budget debate, said the merging of PM2.5 into the PSI is to "simplify the system". "This revised single index will reflect whichever of the six pollutant parameters is the worst."

PM2.5 is expected to be the key pollutant that sets the index almost all of the time, he said.

"In summary, we will simplify the reporting system, but we will publish far more raw data that will be useful for academics and scientists studying this problem," he added.

Experts say the revised index would give people a better sense of how toxic the air is, as the PM2.5 particles can enter people's lungs and blood to cause harm.