People here breathed a little easier yesterday, as the air quality turned out to be better than forecast.

The 24-hour Pollutant Standards Index (PSI) dipped into double digits last night for the first time since Sunday. As of 9pm, the index ranged from 99 to 133.

The National Environment Agency had said on Thursday that the haze yesterday was likely to be from the high end of the unhealthy range (101 to 200 on the 24-hour PSI) to the mid-section of the very unhealthy range (201 to 300).

But overnight rain "brought a reduction in the extent of the haze in our surrounding sea areas", the agency said yesterday morning.

For today, the air quality is expected to be between the mid- and high sections of the unhealthy range, and may enter the low end of the very unhealthy range if denser haze is blown in by prevailing winds from the south-east or south.

Experts said that forecasts are not foolproof.

"Any small change in the meteorological patterns may produce deviations between the forecast and observed conditions," said Singapore-MIT Alliance for Research and Technology research scientist Erik Velasco.

Assistant Professor Jason Cohen of the National University of Singapore's civil and environmental engineering department said air quality forecasts are "even more challenging" than weather ones.

"Chemistry and physics of the particles are required, in addition to exact knowledge of the source regions... These are all not very well understood," he said.

For example, there could be more fires burning than captured by satellite images, and wind direction can change and be affected by pollutants themselves, he explained.

Despite better air quality yesterday, at least two events today - Pedestrian Night in Orchard Road and NTUC U Family's event U Picnic - have been shelved or cancelled.

Several events this weekend still hang in the balance. For instance, sessions at the Singapore leg of the Fina Swimming World Cup will be cancelled if the three-hour PSI exceeds 200 at the start of a session.

The Light Up Your Life Run, planned for today at East Coast Park, may be called off too. A spokesman for Save Our Street Dogs, which is organising the run with Assisi Hospice, said organisers will use the same gauge to decide at 4pm whether or not to cancel it.

The event's carnival at Big Splash will continue as planned, she said.

• Additional reporting by Seow Bei Yi and Chua Siang Yee