The burning garbage dump covers more than 100 rai in tambon Praeksa in Muang district of Samut Prakan.
Provincial governor Kanit Iamrahong said he called a meeting of officials in tambon Praeksa yesterday and declared the site a disaster zone to keep people out of the area and make it easier for emergency services to battle the fire.
He said the extent of the fire had been contained, but it continued to send billowing smoke over the area. Firefighters were continuing to spray water to keep the fire from spreading.
At least 30 families living around the garbage dump site have been evacuated to a safe area. Health officials have examined them and provided them with necessary medical care, he said.
Protective masks have been distributed to people in the area. The Pollution Control Department has been called in to check the air pollution levels, Mr Kanit said.
Pol Col Wichit Boonchinwuthikul, the Bang Pu police chief, said he expected that agencies such as the tambon administration organisation and the provincial authority would jointly file a complaint against those responsible.