The ministry says the practice poses serious harm to both consumers and vendors due to severe side effects associated with the chemical.
Formalin is a solution containing 40% formaldehyde.
Narong Sahametapat, permanent secretary for public health, said ministry officials found the use of the corpse-preserving substance at five fresh and weekly markets in Nakhon Sawan late last month.
Staff at the Department of Health collected 275 food samples from two fresh markets and three weekly markets in Muang Nakhon Sawan, Tha Tako and Chumsaeng districts. Of the samples, 102, or 37%, contained formalin. At big, urban markets, formalin was found in as many as 59% of collected samples.
Contaminated foods included shrimp, squid, chopped ginger, chopped finger root, straw mushrooms, grey oyster mushrooms, black mushrooms, long beans and oak-leaf ferns.
Dr Narong said that formalin was prohibited from use on food. Its use posed dangers to both consumers and vendors because the substance was carcinogenic and a lethal substance.
He ordered health officials to locate the sources of formalin use and take action against them.
Porntep Siriwanarangsun, director-general of the Department of Health, said the discoveries of formalin use by food vendors could indicate they are turning from sodium hydrosulfite to formalin to preserve produce.
Dr Porntep said formalin is a clear liquid that vapourises easily and can attack almost all systems of the human body. Its acidic properties destroy cells. Vendors exposed to low levels of formalin vapour will suffer eye and respiratory irritation. Those heavily exposed to formalin risk serious breathing difficulties, lung inflammation, pulmonary congestion and even death. Consumers of food with high formalin content will experience severe headache, chest pains and stomachache, vomiting and diarrhoea.
“Consumers can test vegetables for formalin by smelling their leaves or breaking their stems and smelling them. If they have a strong smell and cause irritation, consumers should not buy them. Meat with a strong smell should not be bought either,” said Dr Porntep.
He said consumers can remove formalin from vegetables by rinsing them for 5-10 minutes under running water or placing them for one hour in still water. Health officials are testing samples of foods nationwide to check for hazardous contamination. Cancer continues to be the most common cause of death among Thai people and consumption of hazardous foods is one cause.
The tests focus on six hazardous substances known for use on fresh food, namely borax, sodium hydrosulfite, pesticides, salicylic acid, formalin and salbutamol.