It's just common sense: if kids are exposed to toxic substances, bad things happen to them. PCBs are polychlorinated biphenyls: they were commonly used in electronics and other industrial machinery and products. It's very toxic and causes cancer. And now the Harvard School of Public Health finds that it makes children susceptible to diseases because vaccines don't work very well if the child has been exposed to PCBs.

The Harvard study looked at two groups of children in the Faroe Islands (that's in the North Atlantic Ocean between the United Kingdom and Iceland). The diet there sometimes includes whale fat that may be contaminated with PCBs (PCBs are also present in fatty fish that have been caught in contaminated waters).

By the way, PCBs are also common in meat and dairy products from contaminated areas. Old fluorescent light fixtures, TVs, refrigerators, and other appliance made about 30 or more years ago can also leak small amounts of PCBs into the air.

Finally, PCB can also mean "printed circuit board" - don't confuse them!