The haze situation should improve today, as the winds carrying smoke from Sumatra to Singapore are expected to weaken.

The 24-hour Pollutant Standards Index (PSI) is forecast to drop from the middle of the 101 to 200 unhealthy range to the low end of the unhealthy range and the high end of the moderate range.

And while widespread smoke haze was observed in Sumatra and the surrounding region, the number of hot spots detected in Sumatra decreased from 256 on Friday to 125 yesterday.

Still, the National Environment Agency warned yesterday that conditions may get worse, if denser haze from the surrounding region is blown in by unfavourable winds.

Hazy conditions had returned on Friday night, and the three-hour PSI peaked at 173 at 5am yesterday because of haze being blown in by weak prevailing winds.

However, air quality improved in the afternoon because of a shift in wind directions, and continued to get better in the evening.

At 8pm yesterday, the 24-hour PSI was 119 to 145, in the middle of the unhealthy zone. The three-hour PSI at the time was 89.

Yesterday morning's spike came at the end of the week when air quality hovered in the moderate range.

But the hazy conditions did not disrupt some outdoor activities.

At the Singapore Sports Hub, more than 10,000 participants carried lanterns yesterday evening on a 1.4km walkathon organised by the Association for Persons with Special Needs. The three-day Singapore Grand Prix, which started on Friday, was also unaffected.

Other organisers, however, took more precautions. Pink Apple, an events company that organised the Garfield Run in Sentosa yesterday, shortened its races.

Its 1km race for children and 6km run for adults were cut to 700m and 4km respectively. Its 4km fun run was also truncated to 2.5km.

Elderly folk, children and pregnant women are advised to minimise prolonged or strenuous outdoor activities when the 24-hour PSI exceeds 100.