If you were in Singapore on Friday, you'd be gasping for air.
Singapore's Prime Minister Lee Hsien Loong urged people to remain indoors.
The toxic haze is the result of smoke that's drifting over from smouldering fires in the peat swamp forests on Indonesia's Sumatra island.
The BBC's Ashleigh Nghiem has been out talking with commuters in Singapore about the hazardous smog conditions.
She says many residents are now wearing face masks, or trying to leave the city.
Others who are more vulnerable to respiratory problems including children and the elderly are seeking emergency medical care.