Risky shellfish? Assessing hazards of clam consumption.

Although the north shore of the St. Lawrence River's lower estuary is regularly inspected for the presence of toxic algae and biological contamination, this coastal ecosystem is not regularly monitored for chemical contaminants. Bioaccumulation of such contaminants does not significantly harm shellfish, so they remain viable for harvesting. Yet chemical contaminants concentrated in their meat may pose a threat to human consumers. A study of Canadian recreational fishermen who harvest soft-shell clams in the St. Lawrence estuary now suggests that eating as few as 15 meals per year of these shellfish may represent a risk of cancer to consumers that exceeds cutoffs used in various U.S. Environmental Protection Agency (EPA) programs such as setting of fish consumption advisories [EHP 112:883-888]. The study is the first to document the consumption habits of recreational harvesters in this area while simultaneously characterizing the nature and degree of chemical contaminants present in the area.

Fabien Gagnon and colleagues from Quebec's Direction de Sante Publique de la Cote-Nord and Laval University interviewed 162 harvesters at 18 popular shellfish-collecting sites about the number of shellfish meals they had eaten in the last week and the last year. Harvesters were also asked to keep a food diary over the next 30 days recording the amount and type of shellfish they consumed and the location of harvest. This information and other published data formed the basis for four consumption scenarios.

The researchers sampled soft-shell clams at eight locations that were close to sources of point and nonpoint chemical pollution, for laboratory analysis. They focused their study on soft-shell clams because these were the primary shellfish that the fishermen ate. They tested for 56 potentially harmful chemicals that are known to bioaccumulate in clams and whose presence is relatively constant in the environment, including 10 metals, 22 polycyclic aromatic hydrocarbons, 14 polychlorinated biphenyls (PCBs), and 10 chlorinated pesticides.

Thirty-six of the chemicals were found in at least one shellfish sample; 25 were detected in at least 70% of the samples. To estimate the daily intake of contaminants, the researchers applied a mathematical formula factoring in the contaminant concentration, the four different consumption scenarios, and the average weight of a Canadian adult (70 kilograms) to determine a dose expressed as micrograms per kilogram per day. They evaluated both cancer and noncancer health risks.

PCBs and inorganic arsenic were the only chemicals found at concentrations that exceeded cancer risk thresholds. The researchers assessed cancer risks for the detected concentrations of PCBs and inorganic arsenic based on the dose-response relationship established by the EPA for these two contaminants. Even for the lowest clam consumption scenario (6.2 kilograms per year, or about 15 meals), risk assessments were greater than the level of 1 x [10.sup.-6] for lifetime exposure to PCBs and the level of 1 x [10.sup.-5] for lifetime exposure to inorganic arsenic; long-term exposure to such levels of these substances would lead to more than 1 case of cancer per 100,000 persons exposed.

Assessment of noncancer risks--which included skin reactions, diabetes mellitus, high blood pressure, and kidney disease--relied on exposure limit recommendations proposed by many agencies, including the EPA. The team found no significant risk of noncancer health effects to consumers based on their consumption scenarios of soft-shell clams from the study area. However, the authors recommend that other factors be considered before ruling out threats in that risk category. For example, not all harvesting areas were sampled, and the sample size was small for each study area. Also, the highest estimated exposures for contaminants such as inorganic arsenic, cadmium, and chromium were very close to the conservative exposure limits set by the EPA and other agencies. Furthermore, some scientists believe the acceptable thresholds set by regulatory and advisory agencies may be too liberal. Finally, risks to children may be greater because they ingest larger amounts of food in proportion to their body weight.

The authors recommend the implementation of a program for monitoring the chemical contamination present in areas of the St. Lawrence River estuary where shellfish are harvested. Such a program could provide data on which a shellfish consumption guide could be based.