SINGAPORE - Air quality crept into the unhealthy range from as early as 3am on Sunday morning, with the 24-hour Pollutant Standards Index (PSI) registering readings of between 88 and 104 across Singapore.

The 24-hour PSI, a measure of air quality here, remained in this range throughout most of the day. As at 7pm on Sunday, the 24-hour PSI readings across Singapore ranged from 92 to 104.

The three-hour PSI hit 116 at 5pm, and fell to 107 at 6pm.

When 24-hour PSI readings are below 100, normal activities can be carried out. But when they start to inch above 100, authorities advise healthy people to reduce prolonged or strenuous outdoor activity. The elderly, pregnant women and children should do as little of such activity as as possible, while those suffering from chronic lung or heart disease should not engage in these activities.

The National Environment Agency (NEA) said in a haze situation update on Sept 5 that haze experienced in Singapore was due to " the spread of haze from Sumatra and the surrounding region under light wind conditions over Singapore".

Widespread smoke haze was also observed in central and southern Sumatra, the weather agency said.

For updates, the public may visit the haze microsite at www.haze.gov.sg, follow NEA's Facebook and Twitter page, or download the myENV app.