Billowing smoke from Indonesian forest fires has worsened Singapore's air pollution, raising concern among organizers of the widely-anticipated Formula One night race this weekend.

The National Environment Agency said Singapore's main air pollution index rose to 222 early Monday, the highest level in a year and above the official "very unhealthy" bandwidth of 200.

This is particularly taxing on young children, the elderly and those with heart and lung diseases. Also Monday, schools reopened after a weeklong term break.

The organizers of the Singapore Grand Prix were also keeping a close watch on the situation. The race will be held Friday to Sunday.

In a statement, the organizers said Singapore GP would work closely with relevant agencies before making any collective decisions regarding the event if the haze worsens.