Malaysia has announced a state of emergency in two towns after air pollution reached dangerous levels.
Kuala Lumpur's landmarks have disappeared
The pollution is blamed on fires lit to clear land in neighbouring Indonesia, seriously affecting air quality and visibility across the Malacca Strait.
Air quality readings taken in the two towns showed pollution markers to be above the emergency level of 500.
The haze has prompted hundreds of schools to shut, as well as disrupting airports and busy shipping lanes.
Malaysian and Indonesian officials met to discuss the fires, which are an annual problem as poor farmers on Sumatra use fire to clear land for planting.
This year's haze is the worst since 1998, when Malaysia was affected for weeks, causing severe economic losses.
The readings over 500 were detected in the western towns of Port Klang and Kuala Selangor.
MALAYSIA AIR QUALITY
201-300: very unhealthy
Under the state of emergency, all workplaces must shut except for those providing essential services, and selling food.
Air quality levels throughout the Klang Valley have reached levels considered hazardous to health. Emergency measures could yet be introduced in the region's major cities, including the capital Kuala Lumpur, Petaling Jaya, Putrajaya and Shah Alam.
Schools in Kuala Lumpur and surrounding districts have been ordered to close, and people have been advised to stay indoors and wear masks if they venture out.
The BBC's Malaysia correspondent, Jonathan Kent, says the acrid smog that has enveloped Kuala Lumpur is like sandpaper on the back of one's throat.
Some bars and restaurants have even offered special haze discounts on drinks to tempt customers in.
However, the country's tourist destinations including Penang, Langkawi, the East Coast and Malaysian Borneo are almost entirely unaffected.
Nevertheless there will be concern that pictures of the haze from the Kuala Lumpur area will scare away visitors.
'900 fires burning'
With stock markets falling and hospitals inundated with people complaining of eye, throat and chest problems, Malaysia is offering Indonesia help to put out the fires that are causing the problem.
The two countries' leaders spoke on Wednesday, and on Thursday, Indonesian Forestry Minister Malam Sambat Kaban met Malaysian Environment Minister Adenan Satem in Sumatra, where more than 900 fires are reported to be burning.
Malaysian officials would say only that there is a definite willingness to co-operate.
Schools in Kuala Lumpur and parts of the state of Selangor will be closed until Monday.
Students in schools that remain open are being told to stay in their classrooms, drink warm water and wear masks if they have to venture outdoors.
Our correspondent says there is no silver lining to this smog for those students who have been told to stay at home.
They will have to make up for missed lessons on Saturdays once the pollution has eased.