Some metals are of concern more because of their toxic effects on plants and some because of their potential toxic effects on animals and humans. Chromium, copper, nickel, and zinc are more of a toxicity concern with plants than with animals. This is not the case with cadmium, mercury, or lead. With these metals it is conceivable that a plant could accumulate enough of the metal to pose a health risk with little or no detrimental effect to the plant. These three are usually the metals of most concern regarding human health.
Lead is by far the most common contaminant of soils.
The most common source of lead contamination is lead-based paint which has been chipped or scraped off building exteriors over several decades or centuries. Lead in soil is virtually a permanent resident. Organic matter, especially, will bind and hold lead and other metals very effectively. In some circumstances this can be advantageous, as will be explained shortly. Moderate levels of lead contamination can be actively managed to minimize the risk of lead intake.