Pollution from forest fires in Indonesia pushed Singapore's air quality to unhealthy levels for the first time in three years on Saturday, the government said, a week ahead of the Formula One night race in the city.
The toxic smoke caused by burning to clear land for plantations is an annual problem for Indonesia's neighbours, but has been worsened this year by particularly dry weather.
"There has been a deterioration in the haze conditions in Singapore this afternoon," the National Environment Agency (NEA) said in a statement.
"This was due to a confluence of winds over the nearby region that led to more smoke haze from Sumatra being blown toward Singapore," it said, referring to one of the Indonesian islands where fires are raging.
The NEA said the pollutant standards index (PSI) worsened to 112 in parts of the island Saturday night.
An index reading between 101-200 is considered unhealthy, with residents advised against doing prolonged strenuous exercises outdoors.
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Singapore may continue to experience hazy conditions over the next few days, the agency warned.
The city-state of 5.6 million people was shrouded in a thin white haze, with a few residents seen wearing face masks, but there was no major disruption to daily activities.
The F1 race is scheduled from Friday to Sunday on a street circuit in the Marina Bay financial district.
Singapore GP, the Formula One organisers, said the possibility of haze is one of the potential issues covered in their contingency plan for this year's grand prix.
"The plan was formulated and refined with stake holders, government bodies and the Formula One community," Singapore GP said in an emailed statement.
"In the event that the haze causes visibility, public health or operational issues, Singapore GP would work closely with the relevant agencies before making any collective decisions regarding the event."