The thing they use for flotation is a tiny droplet of oil. But when any molecule of, say, a PCB touches the droplet of oil in a diatom, it will immediately go into solution in that oil-that is, in the diatom. Because the diatom has no way to break down the PCB molecule, it just stores it. Diatoms have fierce microscopic predators, and as a general rule these predators must consume at least ten diatoms before they can divide their cells to reproduce themselves. But the predators can't digest PCBs either, and the result is that each predator ends up with about ten times as many PCBs as there were in each diatom it preyed upon.
Whales feed on prey that live between the second and seventh levels of the food chain.
This means that the concentration of PCBs they take into their bodies with each meal is between 102 and 107. In fact, scientists have measured levels of PCBs in fish more than twenty-five million times higher than the levels of PCBs in the sea around them. When you take something that is measured in a single part per trillion and concentrate it ten million times, its concentration becomes ten parts per million. With substances as toxic as some PCBs, ten parts per million can be a very dangerous level. For example, in the United States the federal government forbids the sale of any food containing more than two parts per million of PCBs.