"We still find formalin being used as preservative for chicken meat and soybean cake or tofu," head of the city health service Dyah Muryani said here on Monday.
Dyah said the city health service has taken samples from various market centers in the city for to be examined at laboratory.
The warning was also based on experience in previous years.
Dyah said the Food and Drug Control Agency (BPOM) had found dyestuff and preservatives that could cause health hazard in foods and beverages sold at market centers in the city.
She said the authorities found it difficult to punish the sellers, who claimed they were not aware of the dangerous materials as they were not the producers of the food products.
"We believe the products we received from suppliers or producers are safe," said a trader Rodiyah, at the Ramadan market in Sepinggan.
Dyah said formalin, which is used generally to preserve dead body of human before being buried, and certain other purpose, is dangerous to human health.
It is a source of fatal diseases like cancer and could poison the lungs , intestines, and nose.