It may have started with pet food, but now the safety of a wide variety of food products from China is coming under close scrutiny.

A new study published in Environmental Toxicology and Chemistry found samples from Chinese markets that contained concentrations of contaminants high enough to pose threats to human health.

China has become the worlds largest producer and exporter of fish and fish products.

Organochlorine pesticides such as DDT can accumulate in top predators, including humans. Though these pesticides were officially banned in 1983, China had been using them for decades prior to the ban. Twenty-five years later, there is evidence that new sources, particularly of DDT, may be present and contaminating seafood, researchers say.

The current study focused on seafood from markets in 11 coastal cities in Guangdong Province.

The last two decades have witnessed explosive economic growth in that province. Rapid industrialization, urbanization, and conversion of agricultural lands to commercial use have accelerated the environmental deterioration in this region.

Samples of shrimps, crabs, and mollusks were analyzed for 21 organochlorine pesticides. Of those, DDT and HCH (hexachlorocyclohexane) were detected most frequently and measured at the highest concentrations.

These highest concentrations were observed in mollusks, specifically oysters, mussels, and squid. Concentrations of DDT in some of this seafood were high enough to pose human health threats. Other organochlorine pesticides present were at concentrations high enough to pose human cancer risks.

The studys researchers said further research was urgently required to identify the new sources of organochlorine pesticide contamination, so the food safety issues could be dealt with. Human health risk assessments are required to determine potential risks from local and overseas consumption and potential limits that should be imposed on such consumption.

China exports 3.2 million metric tons of seafood products, which is 10 percent of the global export volume. Exports primarily go to Japan, Korea, Canada, the United States, and the European Union.