TAIPEI (Taiwan News) — The Food and Drug Administration (FDA) said on Tuesday (Jan. 3) that 6,400 kilograms of Chinese mitten crabs imported from a northeastern Chinese province have been intercepted because of excessive levels of dioxins.

Samples of the seized crabs were detected to contain 10.4 pg/g wet weight of dioxins, and 10.8 pg/g wet weight of dioxins and dioxin-like PCBs. The figures exceeded the maximum permitted levels of 3.5 pg/g wet wt. and 6.5 pg/g wet wt., respectively.

This marks the first time the Chinese mitten crabs have been found tainted by the toxins in two years. Over 25 batches of the crustaceans from China have been vetted since October, wrote CNA.

The contaminated crabs were imported from an aquafarm in Panshan County of Liaoning Province, and all were returned or destroyed, said FDA. Products from the farm will be banned from import starting Tuesday (Jan. 3) due to tightened inspection, wrote Liberty Times.

Dioxins are persistent environmental pollutants and accumulate in the food chain. Exposure to the substances can cause damage to the muscle, nervous, immune, and reproductive systems. The chemicals have been classified as a carcinogen by the International Agency for Research and Cancer.