Heavy-Metal Toxicity—With Emphasis on Mercury

ticular concern are “heavy metals,” which occur naturally in the earth’s crust and are defined in physiochemical terms as metals with a density at least 5 times as great as water. This definition translates into an approximate heavy-metal minimum density of 5, and—in addition to cadmium, lead, and mercury—the metals zinc, copper, iron, cobalt, nickel, tin, manganese, and molybdenum also qualify. Scientifically, then, some heavy metals are essential nutrients. Cadmium (with a density of 8), lead (10), and mercury (14) are strikingly high in density compared with such common essential minerals as magnesium, calcium, or potassium, which all have densities below 2. Exposure to environmental contaminants comes through various routes, including natural sources (eg, groundwater, metal ores, and metal leaching from the soil), industrial processes, commercial products, and contaminated dietary supplements and food (eg, fish). The risk for chronic toxicity depends on the frequency, intensity, and duration of contact with the contaminant along with the exposure route. Toxicity risk also depends on the inherent toxic potential of the metal itself; thus, mercury, a nonessential metal, possesses more inherent toxic potential than copper, a metal essential for physiologic function. Metal toxicity affects all organ systems and can result in wide-ranging and nonspecific symptoms; however, the central nervous system (CNS) is especially susceptible to damage from metals (see Table 1 for examples of mercury toxicity). This article summarizes information on the most clinically important toxic metals—mercury, lead, cadmium, and arsenic—with a special emphasis on mercury. The ability for clinicians to assess risk for acute and chronic toxic metals exposure is essential to identifying the underlying cause for many different conditions.