While studies have established that prolonged exposure to the pollutant PM2.5 can be highly hazardous, some experts have warned that even short-term exposure can be damaging.
For the elderly, being exposed to high levels of the tiny particles for just an hour can lead to higher risks of heart attacks, a recent study suggested.
One in five children here has asthma and the symptoms may worsen if exposed to high PM2.5 levels for even a few minutes, said research scientist Erik Velasco of the Singapore-MIT Alliance for Research and Technology's Centre for Environmental Sensing and Modelling.
Experts said this is why the Government's decision to revise its air quality reporting system is timely. From May 1, the National Environment Agency (NEA) will add PM2.5 to its five other pollutants for calculating the Pollutant Standards Index (PSI). This means that if PM2.5 is the most significant pollutant in the air, the PSI figure will be based on it.
The NEA will also begin giving new, hourly PM2.5 updates based on the previous hour's readings. It gives hourly updates now but these figures are based on data in the previous 24 hours and may not reflect the air quality as well at any given time.
PM2.5 particles, which can enter people's lungs, make up the majority of haze particles - as much as 80 per cent during last year's record-busting pollution. They must be watched closely.
The existing PSI is usually based on PM10, which are particles 10 microns or smaller. The Government had said PM10 includes the smaller subset of PM2.5, so an increase in the tiny particles will be reflected in the PSI.
However, experts said PM10 is measured by the weight of the particles per cubic metre of air. This means two pockets of air could both have 100 micrograms of particles - and thus create the same PM10 and PSI values.
But one pocket of air could have fewer but larger particles, while the other could have more of the smaller, more toxic PM2.5 particles.
The NEA reckoned, based on existing PSI calculations, that each of the past five years had between 91 per cent and 96 per cent of "good" air quality days, and just 4 per cent to 9 per cent of "moderate" days.
Under the new reporting system, the figures would have been flipped, with just 1 per cent to 4 per cent of good days in each year, and 92 per cent to 98 per cent of moderate days.
"NEA will have to acknowledge that it was a bit conservative on quantifying the risk posed by the smoke," said senior research scientist Santo Salinas at the National University of Singapore's (NUS) Centre for Remote Imaging, Sensing and Processing.
The NEA's PSI health advisories for moderate and good air are the same. They state that all people, even the vulnerable, can carry on normal activities unless they feel unwell. However, scientists said the vulnerable should take extra care when air quality is moderate.
Dr Salinas welcomed the new, more current hourly PM2.5 updates, which will be provided alongside the 24-hour averages. "A 24-hour average smooths out pollution spikes," he said. "It is not a good indicator for rapid reaction, for example, for people who work outdoors."
Dr Christopher Frey, chair of the United States Environmental Protection Agency's clean air scientific advisory committee, explained that the first generation of PM2.5 monitors needed to accumulate almost a full day's worth of particles to be useful.
Instruments that can measure PM2.5 over shorter time frames are a "relatively recent introduction" so there have not been enough health studies to reach a firm conclusion on the short-term exposure impact, he said.
Dr Velasco stressed: "There is no evidence of a safe level of exposure or threshold, below which no adverse health effects occur."
Some complained during last year's haze that NEA figures - averaged over periods longer than an hour - did not seem to match what was outside their windows.
Dr Frey said the new one-hour index will "change more quickly and be more consistent with people's observations of visibility reduction".
Nanyang Technological University Professor Ang Peng Hwa, who created the Haze Elimination Action Team Facebook campaign in 2007, said the new changes "were made in response to science". He added that the new system may highlight pollution generated here, which could signal the Government to take action.
Assistant Professor Harvey Neo of the NUS Department of Geography said: "Some people will get a sense that this was a long time coming, or feel they have been cheated for the past few years.
"But most would welcome the changes... I would see this as (the Government) trying to match reality better."
Additional reporting by David Ee