Air quality in Singapore deteriorated yesterday, with the 24-hour Pollutant Standards Index (PSI) crossing into the unhealthy range in the afternoon.

At 5pm, the 24-hour reading stood at 85-103, in the moderate to unhealthy ranges.

The three-hour PSI, which is not tied to a health advisory, hit 161 at 5pm yesterday.

The haze conditions worsened after two days of respite - the 24-hour PSI had hovered in the moderate range since 11am on Sunday.

Air quality is considered moderate when the 24-hour reading is in the range of 51-100, and unhealthy when it is from 101-200.

In an update last night, the National Environment Agency (NEA) said the deterioration in air quality here was due to haze from Sumatra, Indonesia, being blown in by the prevailing south-southeasterly winds.

As of 9pm yesterday, the 24-hour PSI was 85-113.

A total of 18 hot spots were detected in Sumatra yesterday, the NEA said. The low hot spot count was due to cloud cover over parts of Sumatra. The agency added that there has been an increase in the extent of moderate to dense haze over southern and central Sumatra.

Today, hazy conditions can still be expected as the prevailing winds are forecast to blow from the south- southeast or south-southwest. "The 24-hour PSI for the next 24 hours is expected to be in the low to mid sections of the unhealthy range, and may enter the high end of the unhealthy range if denser haze from Sumatra is blown in," the NEA said.

Given the air quality forecast, healthy people are advised to reduce prolonged or strenuous outdoor physical exertion, while the elderly, pregnant women and children should keep such activities to a minimum. Those with chronic lung or heart disease should avoid these activities altogether.

Updates on the haze conditions can be found at the NEA website (www.nea.gov.sg) or the haze microsite (www.haze.gov.sg). People can also follow the NEA on Facebook and Twitter, or download the myENV app.