Toxicological considerations pollutants

Mercury is directly below cadmium in the periodic table, but has a considerably more varied and interesting chemistry than cadmium or zinc. Elemental mercury is the only metal that is a liquid at room temperature, and its relatively high vapor pressure contributes to its toxicological hazard. Mercury metal is used in electric discharge tubes (mercury lamps), gauges, pressure-sensing devices, vacuum pumps, valves, and seals. It was formerly widely used as a cathode in the chlor-alkali process for the manufacture of NaOH and Cl2, a process that has been largely discontinued, in part because of the mercury pollution that resulted from it. 

Figure 13.3 An example of a substitution reaction of an aromatic hydrocarbon compound (biphenyl) to produce an organochlorine product (2,3,5,2, 3 -pentachlorobiphenyl, a PCB compound). The product is 1 of 210 possible congeners of PCBs, widespread and persistent pollutants found in the fat tissue of most humans and of considerable environmental and toxicological concern.

In nonindustrialized countries it is feared that, without legislative measures, considerably enhanced production and consumption will lead to similar or even worse Cd pollution as has occurred in the past in industrialized countries. There are no known beneficial effects of Cd, and the available literature on the exposure, pollution, and toxicology of Cd is enormous and still growing. Many conferences, symposia, colloquia, and workshops have been - and will continue to be - dedicated to Cd and its associated problems. It is impossible to deal with all problematic aspects of Cd within the scope and limits of this chapter hence a selection has been made of the most important issues of the exposure and toxicology of Cd. 

Cadmium ranks close to lead and mercury as a metal of current toxicological concern. Cadmium is used in electroplating and galvanization, and in plastics, paint pigments (cadmium yellow), and nickel-cadmium batteries. Because <5% of the metal is recycled, environmental pollution is an important consideration. Coal and other fossil fuels contain cadmium, and their combustion releases the element into the environment. Extraction and processing of zinc and lead also lead to environmental contamination with cadmium. Workers in smelters and other metal-processing plants may be exposed to high concentrations of cadmium in the air however, for most of the population, food is the major source of cadmium. 

Sediments can be sources of toxicants and are an important consideration in toxicological chemistry. Although heavy metal sulfides such as PbS and CdS are removed from water into sediments, when the sediments are stirred up the sulfides can be oxidized to toxic soluble forms. The dense, toxic pollutants polychlorinated biphenyls (PCBs) have accumulated in Hudson River sediments as discussed in Chapter 4, Section 4.12. As noted in Section 3.11, anoxic bacterial processes in sediments may convert insoluble inorganic mercury to mobile methylmercury compounds that contaminate fish tissue. Bottom-feeding organisms may bioaccumulate metal and organic pollutants that have accumulated in sediments. 

If a conversation regarding chemical pollution begins with metals, fuel combustion cannot be far behind. In fact, the two are inextricably linked, as the extraction of metals from ore, also known as smelting, requires considerable heat. Combustion, whether employed to coax metals from ore, to cook dinner, or simply to heat a home, is a messy business in its own right. Whether the fuel combusted is wood, coal, or oil, the consequences of combustion are soot, ash, and smoke. As we shall see, exposure to these end products comes with a significant toxicological price tag. 

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