Singapore's air is likely to be "very unhealthy" today, according to the official forecast of the 24-hour PSI.
Elderly people, pregnant women, children and those with chronic diseases should avoid outdoor activity or wear an N95 mask,
the National Environment Agency (NEA) said at a briefing yesterday.
It did not provide one-hour PSI updates, it added, as no scientific study shows the health impact of short-term spikes.
Recent hourly updates have been the averages of readings taken in the previous three hours. These were introduced in 1997 to give more current information.
A spokesman said its health advisories are based on average PSI readings taken over 24 hours.
From today, the NEA will issue a rolling 24-hour PSI reading every hour on its website, in addition to the three-hourly updates, to give a better sense of the likely impact on people's health.
"One-hour updates would result in a lot of spikes over time," said the NEA's Mr Joseph Hui.
Mr Choi Shing Kwok, permanent secretary for the Ministry of the Environment and Water Resources, added: "We have no scientific studies done on very short exposure periods."
Dr Erik Velasco from the Singapore-MIT Alliance for Research and Technology said the most common practice by the international environmental authorities is to report both hourly and daily average concentrations of PSI readings.
This is alongside pollutant concentrations by regions and from each monitoring station. He said: "With hourly data, we would have seen a gradual increment in the PSI yesterday evening, and not a sudden jump from 190 to 290."
He said studies show health is affected by less than one hour's exposure to high concentrations of pollutants as well as hours of exposure to lower-level concentrations, but these studies were mainly in temperate and not tropical regions. The PSI is often higher at night because the sun's heat helps the air to rise and mix. Without this, pollutants could be trapped near the ground.
The NEA's 11 air-quality monitoring stations use US Environmental Protection Agency benchmark methods. Some particle monitors may show different readings as they use different methods, the NEA said.