Droughts and unlawful burning of rain forests by farmers and illegal loggers are a regional environmental challenge.

In Malaysia's worst environmental crisis since 1997, when a similar haze enveloped large areas of the region, the smoky haze this year has raised serious health concerns and worries over its economic effects. A spokesman for the Malaysian prime minister's office said the state of emergency had been imposed on Kuala Selangor and Port Klang, with residents advised to stay indoors.

Schools have also been shut as the haze has spread to cover much of the populous central state of Selangor, he said, including the capital, Kuala Lumpur.

"Selangor has been quite badly affected," said the spokesman, who asked not to be named because he was not authorized to speak officially. "There's been an increase in respiratory complaints in a lot of areas."

Unlawful Burning of Rain Forests

Two Malaysian government ministers flew to the Indonesian city of Medan on Thursday morning to discuss the situation with their counterparts there. They were Adenan Satem, minister of natural resources and environment, and Peter Chin, minister of plantation industries and commodities.

The Indonesian president, Bambang Susilo Yudhoyono, on Thursday accepted an offer from Malaysia's prime minister, Abdullah Ahmad Badawi, to send firefighters and disaster-relief teams to Indonesia to fight the hundreds of fires that have been reported there. The two countries also agreed to carry out cloud-seeding efforts in the hope of bringing rain.

Haze from fires on the Indonesian island of Sumatra has become a periodic nuisance in recent years for residents of Singapore and Malaysia, which are separated from Sumatra by as little as 75 kilometers, or 46 miles.

Droughts and unlawful burning of rain forests by farmers and illegal loggers in Sumatra are a regional environmental challenge. But the latest episode of haze appeared to be the most severe since 1997, when a drought brought on by El Nino led to fires that left much of the region swathed in haze for several months, causing billions of dollars in lost tourism and lower agricultural production.

Some scientists now link that haze with the spread of the deadly virus Nipah that year, saying the haze forced fruit bats to fly closer to human habitations in search of food.

When haze began blanketing the region in June 2004, Malaysian officials demanded a regional conference on the issue. But Indonesia has refused to sign a regional protocol on "transboundary haze," saying that Singapore and Malaysia are partly responsible for the haze because of their failure to curb trade in smuggled Indonesian timber.

Fear of Raising Public Alarm

Since 1997, haze has become such a regular problem that, at one point, the government stopped publishing air quality statistics for fear of raising public alarm and driving away tourists.

This week, even as the capital was shrouded in haze, Malaysia's deputy prime minister and defense minister, Najib Razak, defended the policy of keeping the air pollution index secret and said it would continue. On Wednesday, however, the government unexpectedly reversed its policy and began publishing the index.

As of 5 p.m. on Thursday, Kuala Lumpur was rated 295; Putrajaya, the administrative capital, 354; Port Klang 486; and Kuala Selangor 527. The government's policy is to declare an emergency if the index exceeds 500, as it did in at least two locations on Thursday.

The latest fires reportedly broke out last week in the Sumatran provinces of Riau and in Aceh, the same area that was devastated by last year's tsunami.

Schools Closed, Students Sent to Hospitals

Singapore, though it is directly across the strait from Riau, has so far been unaffected. But southwesterly monsoon winds are carrying the haze from Sumatra into the Klang Valley, home to most of Kuala Lumpur's suburbs, pushing visibility in some places to less than 400 meters, or 1,300 feet.

More than 400 schools were closed on Wednesday, with reports of students' being sent to hospitals with breathing problems.

New reports said residents were rushing to buy particle masks to filter out the microscopic particles carried by the haze.

Port Klang shut down on Thursday. It was not clear what was happening to cargoes due to go in or out of the port, and officials at Port Klang could not be reached for comment.

But port officials in Singapore reported a nearly 25 percent surge in vessel arrivals.

Airport officials at Kuala Lumpur International Airport could not be reached, but according to Singapore Changi Airport, flights were operating normally from Kuala Lumpur after delays earlier Thursday.