SINGAPORE - Air quality in Singapore that was mostly in the high end of the moderate range on Saturday could deteriorate on Sunday.

The National Environment Agency (NEA) said in its haze update on Saturday evening that the situation could get worse if denser haze from the surrounding region is blown in. Visibility could worsen as well.

The 24-hour Pollutant Standards Index (PSI) for the next 24 hours is forecast to be in the high end of the moderate range and the low end of the unhealthy range, but could enter the mid-section of the unhealthy range if denser haze is blown in.

As of 11pm on Saturday, the 24-hour PSI reading was 85-98, in the moderate range. The three-hour reading, which is not tied to any specific health advisory, was 85.

The 24-hour reading was in a similar range early Saturday morning, when the reading was 98-115 at 2am.

Meanwhile, Indonesia has launched its biggest ever operation to fight the fires that have been shrouding the region in haze. Thirty-two planes and helicopters - of which six are from Singapore, Malaysia and Australia - were deployed on Friday to conduct water-bombing and weather modification efforts.

More than 22,000 troops on the ground have been battling the fires for weeks.

On the home front, more than 40,000 people have received haze subsidies for treatment from polyclinics and private GPs for medical conditions made worse by the polluted air.

That figure is already more than double the number who received such subsidies in 2013, when Singapore experienced its worst haze in recent years.