The haze is back.
After creeping up over the last few days, the 24-hour Pollutant Standards Index (PSI), an indication of air quality, reached unhealthy levels (101-200) on Tuesday morning.
The National Environment Agency (NEA) has warned that the conditions might worsen, although rain over parts of Singapore yesterday provided some respite.
Forest fires are raging in Indonesia due to the widespread clearing of forests for oil palm plantations, and smoke has been blown over here by prevailing winds from the south and south-west.
In Sumatra, air quality hit hazardous levels on Tuesday in Pekanbaru and Dumai, both in Riau, with flights grounded at the Pinang Kampai airport by poor visibility. An operations manager for Pelita Air has even likened the conditions to the haze in 2013.
It is bad news if the haze turns out to be as bad as that in 2013. In June 2013, the three-hour PSI, an indicator of the amount of air pollution in the previous three hours, hit a historic high of 401, and the 24-hour PSI was at a high of 246.
Continuous exposure to unhealthy PSI levels over a few days can cause irritation of the eyes, nose and throat in healthy people.
The effects are worse for the elderly, pregnant women and children, and people with chronic lung or heart disease.
The haze is unhealthy for the economy too.
In 2013, the Singapore Retail Association estimated an 8 per cent to 12 per cent dip in business during the normally busy school-holiday period.
The 1997 haze here, which lasted three months, is estimated to have led to economic losses of almost US$300 million (S$426 million) by Professor Euston Quah, head of Nanyang Technological University's Department of Economics.
Some economists have also estimated the damage to Singapore from Indonesia's forest fires at $5 million a day.
When the haze hits, it is not just the health of Singaporeans that is at stake but that of the economy as well.