Singapore's National Environment Agency has started providing hourly PSI updates on its website, in addition to the three-hourly updates it previously provided.
Around 300 schools in southern Malaysia have now been closed as a result of the smog. Schools in Singapore are currently closed for the holidays.
There are also reports of flight delays affecting both Singapore's Changi airport and Riau province in Indonesia.
The fires are caused by illegal slash-and-burn land clearance in Sumatra, to the west of Singapore.