Levels of fine particles in the air - known as "PM2.5" - spiked to the unhealthy range on wednesday, triggering warnings for some people to limit outdoor exercise.
This is even as the main Pollutant Standards Index (PSI) stayed in the moderate range. At 9pm last night, the 24-hour PSI in various parts of the island ranged from 48 to 70.
But PM2.5 levels hit 60 micrograms per cubic m at several points in the afternoon, the highest reading this year. A reading above 55 mcg would be considered unhealthy under a new way of calculating the PSI from May.
At these levels, the elderly, children and those with heart or lung disease should reduce prolonged or heavy outdoor exertion, while everyone else should limit it, said an advisory on the National Environment Agency (NEA) website.
This development comes a day after the Government announced it would incorporate PM2.5 levels into the calculation of the overall PSI starting in May.
PM2.5 are fine particles in the air smaller than 2.5 microns in diameter. Currently, the PSI value is based on the worst of five pollutants, including larger PM10 particles up to 10 microns in diameter.
From May, after PM2.5 becomes a PSI component, on Wednesday's levels of PM2.5 would translate to a PSI value of 105, in the "unhealthy" range, the NEA said.
Meanwhile, the short-term PSI tends to rise in the evenings and some think the smell gets worse. Without the sun's heat, experts explained, air in the boundary layer of the sky contracts and pollutants within it become more concentrated.
The atmospheric boundary layer is the stratum of air closest to the earth's surface, and it expands and contracts as it gains or loses heat.
Some fluctuations in the PSI at night might be seen if the wind changes direction, speed or strength, said Assistant Professor Jason Blake Cohen of the National University of Singapore's (NUS) civil and environmental engineering department.
But while the haze might smell bad, its odour cannot be used to judge how harmful it might be, said Dr Erik Velasco, a research scientist who studies air pollution at the Singapore-MIT Alliance for Research and Technology. "The acrid smell is due to the chemical composition of the pollutants, which depends on the soil, biomass and fire characteristics."
He added: "The intensity of smell cannot be used to quantify the concentration of pollutants in the air. Both visibility and smell are just qualitative indicators."
On Tuesday, the NEA had said the current hazy conditions were due to southern Johor hot spots. The NUS Centre for Remote Imaging, Sensing and Processing also captured images of smoke plumes at plantations near Kota Tinggi in Johor, and its senior research scientist Santo Salinas said: "Because it's extremely dry nowadays, this is something that could have happened accidentally."
There are also hot spots in other parts of peninsular Malaysia and northern ASEAN, and smoke haze over central Sumatra.
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