What we don’t know about the toxic chemicals in our bodies: Scientists call the accumulation of chemical contaminants (such as PCBs, mercury, and pesticides) within a person’s body the “body burden.” Body burden is just a number, a concentration in parts per billion or micrograms per liter. But the term calls forth an image too, of a body bent over and struggling beneath a heavy load. When scientists start taking about body burden, I think about real bodies — my own and my children’s. Grist