SINGAPORE - The three-hour Pollutant Standards Index (PSI) reading surged to 91 at 8pm on Thursday (March 31), the highest this year.

It dipped to 89 at 9pm, and fell further to 84 at 10pm. The 24-hour PSI reading was between 63 and 81 at 10pm, which is within the moderate range of 51 to 100. The highest readings were logged in the central and south of Singapore.

The three-hour measure was in the range of 60s in the morning and early afternoon, before surging in the evening to peak at 91.

The one-hour PM2.5 reading ranged from 19 in the north to 55 in the south of Singapore at 10pm. It was 20 in the east, 25 in the west and 51 in the central region.

PM2.5 pollutants are fine particulates that are no larger than 2.5 microns, or a thirtieth the diameter of a human hair. These can become trapped deep in the lungs and are tiny enough to pass through linings into the bloodstream.

Air quality in Singapore has deteriorated in the last two days, and a burning smell could be detected in many areas.

The pollution is likely to have come from Peninsular Malaysia, Borneo or even Singapore's own backyard, experts say.