Some chemical metals are micronutrients at low concentrations and are essential for key biological processes. At higher concentrations, these essential metals, e.g., calcium (Ca), copper (Cu), zinc (Zn), manganese (Mn), etc., can have toxic effects. Likewise, nonessential metals, even in low doses can also be toxic. Toxicity can be attributed to the metal element binding to structural proteins, enzymes, and nucleic acids, inhibiting or altering the function of the bound object. Both essential and nonessential metals are found naturally in the environment as well as in concentrated sources stemming from human activity through mining and industrial wastes; vehicle emissions; lead-acid batteries; fertilizers, paints and treated woods. Metals, in particular heavy metals such as cadmium (Cd), arsenic (As), antimony (Sb), lead (Pb), mercury (Hg), and chromium (Cr), have been shown to be able to impede or alter basic cellular functions causing severe neurological damage. C. elegans, the most abundant soil invertebrate, provides a model organism for the study of metal toxicity. As a free-living soil nematode, C. elegans is exposed to many toxins naturally present in the soil. Heavy metals such as Cd result in larval lethality and arrested development. Among its repertoire of defenses, C. elegans has a highly tuned nervous system that allow the animals to recognize and avoid toxins, for example the ADL, ASE and ASH neurons direct behaviors in response to Cu or Cd. In cases of exposure to toxic concentrations of metals, a generalized cellular stress response is activated that includes an increase in expression of a number of genes, including heat shock stress proteins such as
daf-21/Hsp90, that direct cellular repair and defense. Studies of metal toxicity in C. elegans has also revealed a strong correlation between toxicity and temperature-dependent metabolic activity.