People in Lat Krabang district of Bangkok were warned on Tuesday there are carcinogens in the air within 700 metres of a burnt-out furniture warehouse still giving off smoke.
People were advised to avoid the area. Nearby communities were earlier evacuated.
The warning was issued by Industry Minister Akanat Promphan.
He said officials detected carcinogens in the air within a radius of 700 metres, probably coming from burnt plastic pellets, which required foam, not water, to put out.
He said investigators would find out if the company that owned the warehouse had illegally produced furniture there. The property was licensed only as a warehouse. Violation carried a prison term of up to two years, he said.
The destroyed two-storey building is owned by Siam House and Home Co. It is on Chalong Krung 55 Road in Lat Krabang.
The blaze started on Sunday evening and forced the evacuation of nearby communities. The fire-razed site continued to give off smoke on Tuesday.
Deputy Interior Minister Theerarat Samrejvanich also said the smoke came from burnt plastic pellets in the basement of the warehouse. People should stay well clear and avoid inhaling the air near the factory for the next three to five days, he said.
Environment and health lecturer Sonthi Kotchawat wrote in Facebook on Monday that black smoke and gases from the burnt warehouse could contain several carcinogens, including dioxin, furan, benzene and polycyclic aromatic hydrocarbons (PAHs).
Lat Krabang district chief Tharapong Phetkong said the flames had been put out and firefighters expected to finish fully extinguishing the fire later on Tuesday.
Water used to fight the fire was being contained to ensure it did not contaminate public water sources, he said.