Health risks Potential chemical and biological hazards in sushi include environmental contaminants, pathogens, and toxins. Large marine apex predators such as tuna (especially bluefin) can harbor high levels of methylmercury, one of many toxins of marine pollution. Frequent or significantly large consumption of methylmercury can lead to developmental defects when consumed by certain higher-risk groups, including women who are pregnant or may become pregnant, nursing mothers, and young children. A 2021 study in Catalonia, Spain reported that the estimated exposure to methylmercury in sushi consumption by adolescents exceeded the tolerable daily intake. A 2011 article reported approximately 18 million people infected with fish-borne flukes worldwide. Such an infection can be dangerous for expecting mothers due to the health risks that medical interventions or treatment measures may pose on the developing fetus. Parasitic infections can have a wide range of health impacts, including bowel obstruction, anemia, liver disease, and more. These illnesses' impact can pose health concerns for the expecting mother and baby. Sashimi or other types of sushi containing raw fish present a risk of infection by three main types of parasites: Clonorchis sinensis, a fluke which can cause clonorchiasis Anisakis, a roundworm which can cause anisakiasis Diphyllobothrium, a tapeworm which can cause diphyllobothriasis For these reasons, EU regulations forbid using raw fish that had not previously been frozen. It must be frozen at temperatures below in all product parts for no less than 24 hours. Fish for sushi may be flash frozen on fishing boats and by suppliers to temperatures as low as . Super-freezing destroys parasites, and also prevents oxidation of the blood in tuna flesh that causes discoloration at temperatures above . Calls for stricter analysis and regulation of seafood include improved product description. A 2021 DNA study in Italy found 30%-40% of fish species in sushi incorrectly described. Some forms of sushi, notably those containing the fugu pufferfish and some kinds of shellfish, can cause severe poisoning if not prepared properly. Fugu consumption, in particular, can be fatal. Fugu fish has a lethal dose of tetrodotoxin in its internal organs and, by law in many countries, must be prepared by a licensed fugu chef who has passed the prefectural examination in Japan. Licensing involves a written test, a fish-identification test, and a practical test that involves preparing the fugu and separating out the poisonous organs; only about 35 percent of applicants pass. Sustainable sushi Sustainable sushi