The haze could get worse in Singapore today as winds from the south and south-west continue to blow in smoke from Sumatra. This has prompted organisers of several races today to consider whether they should go on.

The Singapore Cancer Society decided to cancel this morning's Race Against Cancer just after 9pm last night. Fees for the annual fund-raising run were between $29 and $59, and a spokesman confirmed to The Sunday Times that no refunds will be given due to set-up costs that were already incurred.

The society posted on its website that it was calling off the run "for the safety and health of runners, volunteers, staff and beneficiaries".

Participants for today's Yellow Ribbon Prison Run were told to wait till 5am - an hour before it is set to start - before a decision would be taken. It is organised by the Yellow Ribbon Project, which aims to help former prisoners rejoin society. On its website, POSB also said it would make a decision on its Passion Run for Kids depending on the three-hour PSI reading at 5am.

The National Environment Agency (NEA) said last night that the 24-hour Pollutant Standards Index (PSI) is expected to be at the low end of the 101 to 200 unhealthy range. This may climb into the mid to high sections if denser haze is blown in by unfavourable winds.

The 24-hour PSI was within the moderate range of 51 to 100 for most of yesterday before creeping into unhealthy levels in the evening. By 11pm, the readings were between 90 and 107.

The three-hour PSI reading climbed to 147 at 4pm.

Yesterday saw a drop in the number of hot spots detected in Sumatra from 304 to 53. But NEA noted the low count was due to cloud cover over some areas.

The elderly, pregnant women and children are advised to minimise prolonged or strenuous outdoor activity when the 24-hour PSI exceeds 100. Those suffering from chronic lung or heart disease should also avoid such activities.