The New York Times reports that exposure to polychlorinated biphenyls, also called PCBs, can make children's diphtheria and tetanus vaccinations less effective. The research leading to this conclusion was carried out in the Faroe Islands of the North Atlantic where the human diet includes pilot whale blubber. Pilot whale blubber leads to heightened PCB blood levels. Philippe Grandjean, of the Harvard School of Public Health and co-author of the study, said the data were useful in determining how much PCB children can be exposed to before vaccination levels were threatened. “We used the method of the Environmental Protection Administration to calculate an exposure limit that would be protective against this effect. Our data suggest an exposure limit that would be exceeded by most Americans.”