Because of the limited number of samples, FDA data are clearly inadequate and present absolutely no meaningful overall information on contamination of many species. Some aquatic organisms are sampled sporadically.
Many aquatic species are never analyzed for mercury concentrations.
Many finfish are never analyzed for inorganic contaminants, such as arsenic, cadmium, lead, and selenium, in edible portions.
Very few shellfish are analyzed for organic chemical contamination or for other contaminants of concern such as organotins and polycyclic aromatic hydrocarbons (PAHs) even though it is known that bivalves may well be contaminated with such chemicals.
It is uncertain for which pollutants FDA routinely tests, and at what laboratory detection limits. For example, the FDA Total Diet Study was able to detect pesticides such as chlorpropham, dacthal, diazinon and malathion on 45 occasions when analyzing 48 composite samples of seafood dishes such as cod and handdock fillets, canned tuna, and shrimp–pesticides that the FDA industrial chemical and pesticide monitoring program rarely, if ever, detected among these seafood items in some five years of sampling (Gunderson, 1988). This may occur because these pesticides are applied to the food product after harvest and during processing, because the analytical methods in use are not designed to detect these particular chemicals, or because the laboratory detection limits in the Total Diet Study have quantitation levels that are 5-10 times lower than the limits used in FDA enforcement of regulatory limits (Gunderson, 1988). In any event, FDA regulatory data appear to underestimate consumers' exposure to a variety of organic and inorganic contaminants including pesticides, metals, and PAHs.
The disparity in sampling detection limits between the Total Diet Study and regular FDA seafood monitoring programs raises serious doubts as to whether the FDA sampling program is accurately estimating the actual concentrations of industrial chemicals and pesticides in the seafood supply. Indeed, the FDA laboratories' relatively high detection levels–particularly for PCBs and dieldrin, which can occur in seafood at levels below the current detection limits–lead to so-called nondetected zero values when the PCB concentration might actually be several parts per billion. Because of the relatively high carcinogenic potency of dieldrin and PCBs, and their frequency in the seafood supply, findings of several parts per billion in widely consumed aquatic species might add appreciably to overall exposure and risk.
Nevertheless, FDA data can be used to provide a rough estimate of daily dietary exposure and at least a highly tentative indication of potential risk. Table 6-31 presents the committee's estimates of U.S. daily dietary exposure to selected organic and inorganic chemicals in terms of milligrams per kilogram of body weight per day, the form that is best suited to calculation of upper-confidence-limit cancer risks. These calculations assume a standard body weight of 70 kg. Table 6-31 also provides EPA upper-confidence-limit cancer potency estimates (where they exist) and the indicated upper-confidence-limit estimates of national aggregate lifetime cancer risk based on the committee's estimation of daily exposure to carcinogenic organic chemicals.17