For young children and women of childbearing age, excessive consumption of mercury-contaminated fish can severely impact a child's development. Older women and men may find it an acceptable tradeoff to exceed recommended seafood meal limits to increase their omega-3 intake.
People at high risk of cardiovascular disease must weigh the cancer risk of eating fish high in PCBs with the benefits of eating fish high in omega-3s, in which case the benefits of omega-3s may outweigh the cancer risk (1 in 100,000 - the level recommended by the EPA).
However, these chemicals are known to cause serious health problems besides cancer, so the trade offs are not simple. The good news is that there are several low-contaminant, high-omega-3 seafood options available, so there’s no need to risk eating contaminated fish.