Malaysia has declared a state of emergency in two southern districts after smog triggered by forest fires in Indonesia reached hazardous levels.
The coastal towns of Muar and Ledang are in shutdown, and residents have been advised to stay indoors.
Air pollution has also worsened in the capital, Kuala Lumpur, shrouding its landmark Petronas Towers in hazy smoke.
Malaysia's environment minister is to meet his Indonesian counterpart on Wednesday to discuss the problem.
Smog has become an annual problem in Malaysia, but this is the first time in eight years that a state of emergency has been called, the BBC's Jennifer Pak, in Kuala Lumpur, reports.
People are angry that the authorities have not been able to address the health hazard, our correspondent says.
Officials on Sunday confirmed the Pollutant Standards Index (PSI) had exceeded 700 in two districts. A reading above 300 indicates that air pollution has reached dangerous levels.
Schools in the region have been ordered to remain closed. Local authorities have distributed face masks to residents.
Even in Kuala Lumpur, where smog levels have so far remained moderate, visibility is now strongly reduced and the smell of thick smoke hangs in the air, correspondents say.
Kuala Lumpur resident Raj Ahmed told the BBC: "You wake up in the morning and you can smell burnt wood - you look out the window and there is constant smog clouding the major landmarks that you would ordinarily see.
"If you go outside, it's like constantly standing close to a small barbecue."
The haze drifted across from Singapore, which has experienced record pollution levels as a result of the fumes originating in Indonesia's Riau province.
The smog is being blamed on illegal land-clearing fires burning near the provincial capital Pekanbaru.
Palm-oil firms - some of which are based in Malaysia and Singapore - have been accused of using slash-and-burn techniques to clear space for their plantations.
Environmental group Greenpeace International said its analysis of Nasa data between 11 and 21 June had "revealed hundreds of fire hotspots in palm oil concessions".
Firefighters in Sumatra are continuing to try to bring the blazes under control.
The issue is likely to feature prominently at a meeting of South-East Asian Nations (Asean) taking place in Brunei next week.
The South East Asia haze in 1997 and 1998 covered Indonesia, Malaysia, Singapore, Brunei and the southern Philippines, causing environmental damage and, by some estimates, around $9.3bn (£5.9bn) in economic losses.
Tourist numbers to the region fell sharply, and reports said the smog made around 20 million people ill.
The smoke also contributed to collisions on the roads and at sea, and a number of air disasters were blamed on poor visibility.