The 3-hour Pollutant Standards Index (PSI) reading hit its highest level of the year last night.

It reached 91 at 8pm before dipping to 89 an hour later, then 74 by 11pm.

The 24-hour PSI reading was between 63 and 81 at 11pm, which is within the moderate range of 51 to 100.

Throughout the morning and early afternoon, the three-hour measure, which is not tied to a health advisory, was in the range of 60s, before it peaked at 91.

The one-hour reading for PM2.5 pollutants, which are fine particles a thirtieth the diameter of a human hair, ranged from 15 in the north to 41 in the south at 11pm. It was 21 in the east, 18 in the west and 38 in the central region.

PM2.5 pollutants 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 three days, and experts say that the burning smell detected is likely to be from Peninsular Malaysia, Borneo or even Singapore.