The haze prompted organisers of several races to turn their competitive runs yesterday into fun walks or call off their events altogether.
This comes as prevailing winds continued to blow in haze from Sumatra, Indonesia, with air quality expected to deteriorate further today.
This weekend's Formula 1 night race organisers are also monitoring the situation in case it worsens and affects driver visibility.
"The possibility of haze is just one of the many potential issues that are covered in the overall 2015 Formula 1 Singapore Airlines Singapore Grand Prix Contingency Plan," a Singapore GP spokesman told Reuters.
"In the event that the haze causes visibility, public health or operational issues, Singapore GP will work closely with the relevant agencies before making any collective decisions regarding the event."
Neither the National Environment Agency (NEA) nor race organisers would speculate on whether this Sunday's night race around the Marina Bay street circuit would be affected if the haze worsened.
In its 6pm update yesterday, NEA said the prevailing winds are forecast to blow from the south or south-west today, and air quality "could deteriorate as the... unfavourable winds may blow in denser haze from Sumatra".
Thundery showers forecasted here in the late morning and early afternoon today might bring only a short, temporary respite, NEA said.
The 24-hour Pollutant Standards Index (PSI) for the next 24 hours is expected to be in the mid to high sections of the unhealthy range (101 to 200). It may enter the low level of the very unhealthy range (201 to 300) if the unfavourable winds bring in denser haze from Sumatra, said NEA.
Yesterday, the 24-hour PSI crept into the low end of the unhealthy range islandwide at 4am, two hours before the start of most races. At 7am, the index stood at between 112 and 132. But by 10pm, it was between 130 and 152.
The Singapore Cancer Society decided just after 9pm on Saturday night to cancel its race yesterday.
POSB also cancelled the competitive races and family walk for its PAssion Run for Kids yesterday, but a carnival went on as planned at the Marina Bay Promontory.
Organisers of other events turned their competitive runs into fun walks. Among them was the Yellow Ribbon Prison Run, where its 10km competitive and 6km fun runs gave way to a 6km walk.
This article by The Straits Times and Reuters was published in MyPaper, a free, bilingual newspaper published by Singapore Press Holdings.
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