All metals are persistent. However, not all metals are PBTs; that is, not all are also defined as bioaccumulative and toxic. Many factors affect whether exposure to a metal poses problems. A higher dose is, of course, more likely to cause adverse effects than a low dose. Another factor, illustrated by the metal, chromium is the chemistry. Chromium(III) has a valence of three. It is a nutrient and, because it is not very soluble, doesn't build up to high concentrations in water. But chromium(VI) has a valence of six, is toxic and is a carcinogen.

• Five nutrient metals are PBTs: copper, chromium, nickel, selenium, and zinc.

• Six non-nutrient metals are also PBTs: antimony, arsenic, beryllium, cadmium, lead, and mercury. The toxicity of these metals too varies with chemical form as illustrated by lead. Elemental lead is toxic, but tetraethyl lead is much more toxic.

Section I of this chapter is a metal primer, providing information on metals regardless of whether they are PBTs. Section II details four especially troublesome metal PBTs: lead, mercury, cadmium, and arsenic.

SECTION I

A metal primer

Metals are elemental and cannot be destroyed. But they do bond to other elements (thus becoming molecules) and then show properties different from the parent metal. Consider three cases.