What is a "body burden"?

What is a "body burden"?
http://www.chemicalbodyburden.org/whatisbb.htm

"Toxic chemicals, both naturally occurring and man-made, often get into the human body. We may inhale them, swallow them in contaminated food or water, or in some cases, absorb them through skin. A woman who is pregnant may pass them to her developing fetus through the placenta. The term " body burden " refers to the total amount of these chemicals that are present in the human body at a given point in time.  Sometimes it is also useful to consider the body burden of a specific, single chemical, like, for example, lead, mercury, or dioxin." <snip>