The haze continued to choke Singapore ahead of this weekend's Formula 1 grand prix as the Pollutant Standards Index (PSI) got even higher.
The PSI rose to a year-high 222 on Monday, going into the "very unhealthy" bandwith.
The 24-hr PSI was 130-161, according to Singapore's National Environment Agency (NEA).
"The haze situation could further deteriorate should the current unfavourable winds blow in denser haze from Sumatra," the NEA said.
"Thundery showers are forecast for Singapore in the pre-dawn and early morning hours, but this may only bring a brief temporary respite to the hazy conditions.
"The 24-hour PSI for the next 24 hours is expected to be in the mid to high sections of the unhealthy range, and may enter the low section of the Very Unhealthy range if denser haze from Sumatra is blown in."
Several outdoor events, including a charity race against cancer scheduled for Sunday, have been cancelled, local media reported on Tuesday.
The low visibility has also affected air traffic in the area, with severals flights having been diverted.