A total ban on open burning comes into effect in the Klang Valley tomorrow, as seven areas have reported unhealthy air quality.
This may be extended to other parts of the country over the next few days, if the Air Pollution Index (API) worsens.
The only exemption will be cremation, burning of articles as part of religious rites and outdoor grills and barbecues.
The worst-affected areas were Port Klang, Gombak, Petaling Jaya, Putrajaya, Shah Alam, Kuala Selangor and Tanjung Malim, Perak.
Department of Environment (DOE) director-general Datuk Rosnani Ibarahim, describing the situation as "bad", attributed the worsening air quality to hundreds of hotspots, mostly in parts of Indonesia.
There were 333 fires in Riau and Sumatra, 20 in Sarawak, 15 in Kalimantan and three in Sabah.
The DOE is also investigating reports of fires in Pahang, Selangor, Perak and Johor.
Rosnani said even the burning of articles in religious rites would be regulated.
"We don’t want to worsen the unhealthy air pollution index," she said, adding that people in the Klang Valley were discouraged from cooking outside their homes.
Thirty-two stations nationwide recorded "moderate" API levels, and 11 "good".
Those engaged in open burning are liable to a maximum RM500,000 fine or five years’ jail or both under Section 29A(1) of the Environmental Quality Act 1974.
Rosnani has directed State DOE offices to step up enforcement, especially between noon and evening when the haze worsened.
She said they should come down hard on those whose premises emitted industrial smoke and black exhaust smoke from vehicles.
"We are also monitoring all fire-prone areas nationwide for peat and forest fires," she added.
She said the department was concerned about the situation, which had not improved since last Tuesday.
The Meteorological Services Department continues to report poor visibility in several areas.
Its weather division senior assistant director Muhammad Helmi Abdullah said the the worst-affected areas were the Kuala Lumpur International Airport, with only 1.2km visibility, and the Straits of Malacca with visibility of one kilometre.
At 4pm, there were an undisclosed number of hotspots in Selangor and two in Perak and Pahang.
In Seremban, the air quality in the district, especially in the outskirts of town and Nilai, continued to worsen, with visibility reduced to about 1.2km.
State Department of Environment director Rahani Hussin said the API, especially in Nilai, had been at an unhealthy level since late yesterday.
"The air quality in Seremban is still ‘moderate’, while it is ‘unhealthy’ in Nilai. Visibility in both areas is about 1.2km," she said.
She urged the public to refrain from open burning, reduce outdoor activities and drink a lot of water.
Checks around town found a number of people, especially motorcyclists, using face masks.
Checks with clinics in the district revealed that more people had come in for treatment for eye irritation.
Said a doctor at a clinic in Kemayan here: "We have received a slight increase in the number of patients complaining of sore throat and nasal congestion."