It was slightly hazy in some parts of Singapore yesterday, with particles probably drifting in from Sumatra, which has seen an increase in hot spot activity, the National Environment Agency (NEA) said.

At 6pm, the 24-hour Pollutant Standards Index (PSI) was 75 to 85, in the moderate range.

The unhealthy PSI range, for which NEA advises the public to cut down on time spent outdoors, is 101 to 200.

Meanwhile, the one-hour reading of PM2.5 at 6pm yesterday ranged from 41 to 59, in the normal band and the lower end of the elevated band.

NEA said PM2.5 concentration readings are a good indicator of current air quality, and those concerned should use them to decide whether to go for immediate outdoor activities, such as a jog.

PM2.5 is a measure of the concentration of tiny particles less than 2.5 micrometres in diameter - or about one-thirtieth the diameter of a human hair - in the air. When the PM2.5 reading is in the elevated range, haze particles can affect the heart and lungs, especially in people who have chronic heart or lung conditions like asthma. Those with such chronic conditions should ensure that their medication is on hand and readily available, according to NEA's website.

The hazy conditions yesterday afternoon came amid an increase in the number of hot spots in central and southern Sumatra.

A total of 380 hot spots were detected in Sumatra, up from 333 on Sunday, said NEA. This is also more than triple the 113 hot spots detected last Saturday.

"There were persistent hot spots with moderate to dense smoke haze in Riau and Jambi provinces, and some of the haze has spread across the Strait of Malacca to the western parts of Peninsular Malaysia," NEA said in its statement. Hot spots with smoke haze were also detected in South Sumatra and Lampung.

Dry weather is expected to continue in Sumatra for the next few days, while the prevailing winds are forecast to keep blowing from the south-east or south.

For the next 24 hours, the one-hour PM2.5 readings are expected to remain in the range between the normal and elevated bands. The 24-hour PSI reading is forecast to be in the moderate range.

"Given the air quality forecast for the next 24 hours, everyone can continue with normal activities. Persons who are not feeling well, especially the elderly and children, and those with chronic heart or lung conditions, should seek medical attention," said NEA.

Regular updates on the haze situation can be found on NEA's website at www.nea.gov.sg