Heavy Metals Such as Iridium,

Heavy metals such as iridium, platinum, gold, tungsten, uranium, mercury, lead, and silver are essential ingredients in many modern industrial applications, but there are growing indications that such metals, especially in high concentrations, represent significant health threats for humans, animals and even plants. The studies to date have found some mixed results in this regard, and there is a growing consensus among many researchers that more studies are needed to sort out the fact from the fiction concerning heavy metals. To this end, the objective of this paper was to provide a critical review of the relevant peer-reviewed and scholarly literature concerning heavy metals, including the causes, effects, and solutions of heavy metals, and how these are known to adversely affect humans. An analysis of the political, societal and economic ramifications is followed by a summary of the research in the conclusion.

Review and Discussion

The encyclopedic entry for the term shows that heavy metals include the most dense metals, such as osmium, iridium, platinum, gold, tungsten, uranium, tantalum, mercury, hafnium, lead, and silver (Heavy metals, 2007). According to Farmer, there are a number of metals that are toxic to plants and animals, many of which occur naturally in varying concentrations, due to rock and soil weathering and volcanic activity; human activity, though, can cause significant amounts of additional emissions of such substances. This author advises, "Most metal processing operations will emit varying quantities of metals, as is to be expected. However, there are also other significant sources, such as fossil fuel burning (coal and oil contain traces of heavy metals)" (Farmer, 1997, p. 26). To date, the most heavily publicized heavy metals emissions involve those with lead in motor vehicle exhausts, where the lead component is specifically added to the gasoline product as an anti-knocking agent (Farmer). The heavy metals that represent the most concern for human health are arsenic, cadmium, cobalt, copper, chromium, iron, lead, manganese, mercury, nickel, silver, tin, titanium, vanadium and zinc (Farmer). Aerial emissions represent important sources and impacts of heavy metals; however, some authorities maintain that even more extensive impacts take place in freshwater and marine ecosystems (Farmer).

In most industrialized countries, the contamination of soils with heavy metals represents a longstanding and significant problem. For example, Dunn (2001) reports that that past quarter century has witnessed a growing number of industrial countries equipping the exhaust systems of gasoline-powered cars with catalytic converters in an effort to reduce emissions of nitrogen oxide, carbon monoxide, and hydrocarbons which are fundamental components of urban air pollution. This author adds that, "The converters have significantly reduced levels of these gases, as well as those of lead, since they require unleaded gasoline. Some scientists, however, raised concerns that the metals on which the converter technology is based -- platinum, palladium, and rhodium -- could become widely dispersed in the environment" (Dunn, p. 11). According to Harley, Foulkes, Hilborne, Hudson and Anthony (1998), "Heavy-metal compounds in solution are usually highly reactive and can exert broad cytotoxic effects. Heavy metals, including uranium, exhibit great affinity for biological molecules containing phosphate residues, such as glucose phosphate, phospholipids, and nucleic acids; or sulfhydryl groups, including cysteine, glutathione, and many proteins and oxyanions (oxygen-containing anions)" (p. 7). An increasing number of developing nations are also experiencing the same types of problems with heavy metals as they pursue aggressive regimens of industrialization (Carter & Turnock, 2002).

As a result of their high affinity for biological molecules, heavy metals do not exist in nature as free ions in biological systems except very transiently in some isolated instances (Harley et al.). Rather, such heavy metals are present in the environment as complexes with a great variety of molecules known as ligands (Harley et al.). In this regard, Govil, Reddy and Rao (1999) report that the formation of complexes that contain dissolved inorganic and organic ligands and high background concentrations of metals serve to increase the mobility of trace metals in soil; in addition, the mobility of such substance is also influenced by pH levels, soil cation exchange capacities, soil redox potentials, and organic complexation (Govil, Reddy & Rao, 1999). According to these authors, "Basically, metals are accommodated in three reservoirs: water, sediments, and biota. Nevertheless, heavy metals accumulate in the soil and present a serious long-term hazard. Not all heavy metals are harmful to humans, but very high concentrations are toxic in nature. Studies have shown that toxic metals in soil influence plants" (Govil et al., p. 23).

Climatologists and scientists have expressed concern over the widespread emission of heavy metals to far-flung regions of the earth, including the remotest parts of the arctic. According to Vidas (2000), throughout the arctic region, "Heavy metals, including mercury, lead, nickel, cadmium and copper, are mainly transported from sources in Eurasia and North America" (p. 179). Likewise, Dunn (2001) reports that, "Scientists from France and Italy have found remote regions of Greenland to be contaminated by heavy metals, many of which are likely to have originated from automobile catalytic converters" (p. 11). Furthermore, researchers have identified a link between exposure to heavy metals during childhood and the incidence of multiple sclerosis in humans (Henry, Williamson, Shiffe, Wagner, Shire & Garabedian, 2007). Further, a number of epidemiologic studies have indicated that there is a connection between Parkinson's disease and exposure to heavy metals (Coon, Stark, Peterson, Gloi, Pounds, Chettle & Gorrell, 2006). Comparable studies, though, have failed to establish such an association between exposure to heavy metals and thyroid problems in white storks (Baos, Blas, Bortolotti, Marchant & Hiraldo, 2006).

Nevertheless, some heavy metals may build up concentrations in treated soils and, under certain conditions, are capable of being transferred from the soil into crops, and subsequently into the humans or animals that consume them; not surprisingly, wildlife can also be adversely affected such as when insects in the soil absorb toxins, which then accumulate in birds (Bothwell, 2001). In fact, Girdner (2003) emphasizes that, "Any food that we buy today may have been grown on land fertilized by hazardous waste, and may have absorbed toxic heavy metals from the soil, including lead, cadmium, mercury, and arsenic. There is simply no way one can know" (p. 41). This author cautions, though, that there are a growing number of industries that are recycling their industrial waste as fertilizer in an effort to save the high cost of storing it in a hazardous waste landfill; therefore, contamination by heavy metal concentrations is becoming more and more likely (Girdner).

In fact, recycled human waste in the form of sludge is a growing practice in both industrialized and developing nations, but such compounds can contain large concentrations of heavy metals. According to Gardner (1998), "As industries hooked up to public sewers, their waste flows - often containing toxic chemicals and heavy metals - mixed with human waste. Sewage treatment processes, while somewhat effective at killing pathogens, did not eliminate these other contaminants; instead, the pollutants simply accumulated in the sludge" (p. 28). Of primary concern to many researchers is the fact that much remains unknown concerning the long-term behavior of metals in soils that are treated with such sludge. As Gardner emphasizes, though, scientists are worried that heavy metals could eventually be freed up and absorbed by crops over time: "This 'time bomb' theory postulates that as organic matter in sludge breaks down over time, the bonds that keep metals from travelling - either down into groundwater or up into crops - will be weakened. Metals that are largely immobile in the short run could well be found in our food and our drinking water in the long run" (p. 28).