The air pollution index (API) in Kuala Lumpur reached 97, while in the nearby district of Port Klang it reached 103, the government's Environment Department said.
An API of 100 to 200 is considered unhealthy, while 200 to 300 is very unhealthy and more than 300 is hazardous.
Also on Tuesday, Singapore warned of an imminent pollution spike, saying satellite data showed “a marked increase in the number of hotspots in Sumatra”, referring to the heat signatures of apparent forest fires.
Ronnie Tay, head of the city-state's National Environment Agency, had contacted Indonesian authorities and “sought an urgent update of Indonesia's efforts to tackle the fires”, the agency said.
He offered Singapore's help, including data gathering the fires and the use of aircraft for cloud-seeding operations.
Jakarta assured Singapore that it was monitoring the situation and taking action to suppress the fires, the statement added.
Malaysia's Meteorological Department said there were 252 hotspots or forest fires in Sumatra on Tuesday up from 159 on Sunday.
Haze from forest fires on Sumatra island mostly affects Indonesia's Riau province, Malaysia and Singapore during the dry months of June to September.
The fires are started illegally to clear land quickly and cheaply for agricultural use, from land-clearing by plantation owners and accidental peat fires.
Last month, a large portion of Malaysia including Kuala Lumpur, and the island-state of Singapore were choked in dangerous levels of haze from smoke off forest fires in Sumatra