Current U.S. Food and Drug Administration guidelines aim to reduce mothers' intake of methylmercury from fish, not POPs. Mercury can harm an unborn baby's nervous system. Mercury builds up in the food chain, so expectant and nursing mothers should especially avoid eating predatory fish, such as shark, swordfish and king mackerel. However, mercury naturally clears out of a woman's system after approximately one year in most cases.
POPs, including the banned pesticide DDT, also build up in the food chain. Unlike mercury, POPs don't easily flush out of humans or animals. The chemicals build up in an animal's fat, then are passed on to whatever predator dines on that fat. Predators at the top of the food web - including humans - end up with the heaviest loads of pollutants.
BLOG: Sharks Contain More Pollutants Than Polar Bears
"We have to be careful in saying fish advisories don't work at all because they can work very well for reducing exposure to quickly eliminated contaminants, such as mercury," said lead author Matthew J. Binnington, doctoral candidate at the University of Toronto. "But for POPs we found that they are not very effective."