الفهرس 
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Abstract
All living organisms have evolved in a hostile environment. Indeed, survival in challenging environments has been the driving force in the continuous remodeling of all multi-cellular organisms. Development of defense mechanisms and ability to withstand insults and subsequent disease development is crucial in the process of evolution. Increasing human activities have modified the global cycle of heavy metals and metalloids (Clarkson, 1995). In the second half of the nineteenth century, pollution of air, water, soil and food has become a threat to the plant and animal and the human race.
Environmental pollution causes instability, disorder, harm or discomfort to the living organisms. Environmental factors have important links with infectious as well as non-infectious diseases of both acute and chronic nature. Global burden of disease attributable to selected sources of environment like water sanitation and hygiene, urban outdoor and indoor pollution, occupational carcinogens, noise and airborne particulates has been assessed to be 8-9%, measured either in terms of mortality or disability adjusted life years (Ezzati et al., 2002). The increase in pollution is a major and global problem. This is due to the use of toxic chemicals or xenobiotic substances or by certain synthetic compounds such as heavy metallic compounds (Foulkes, 1990; Jagadeesan and Pillai, 2007). Of these heavy metallic compounds few reveal potential effects. They reach the environment after their liberation through industries (Migliore et al., 1999).
Exposure to toxic metals remains a wide spread occupational and environmental problems in world. Human exposure to heavy metals has risen dramatically in the last 50 years as a result of an exponential increase in their use in industry, agriculture and even as medicine (Malaev et al., 2004; Conesa et al., 2007).
Heavy metals are persistent environmental contaminants since they cannot be degraded or destroyed. Many heavy metals are urgently necessary for functioning of the body of humans and other living organisms in small amounts and belong to the range of nutrients. Others, when passed on to the living organisms cause poisoning or death (Danielyan, 2010). A heavy metal is a member of an ill-defined subset of elements that exhibit metallic properties, which would mainly include the transition metals, some metalloids, lanthanides, and actinides. Heavy metals have a high atomic weight and a density much greater (at least 5 times) than water. There are more than 20 heavy metals, but lead, cadmium, and inorganic arsenic (As) are of special concern. According to the U.S. Agency for Toxic Substances and Disease Registry (ATSDR) these three heavy metals are in the top six hazards present in toxic waste sites. They are highly toxic and can cause damaging effects even at very low concentrations. They tend to accumulate in the food chain and in the body and can be stored in soft (e.g., liver, kidney) and hard tissues (e.g., bone). Being metals, they often exist in a positively charged form and can bind on to negatively-charged organic molecules to form complexes. If heavy metals enter and accumulate in body tissue faster than the body’s detoxification pathways can dispose of them, a gradual buildup of these toxins will occur.
Acute heavy metal toxicity is rare; however, chronic low-grade toxicity may be more damaging, contributing to chronic illness. Heavy metals have specific neurotoxic, nephrotoxic, hepatotoxic, fetotoxic and teratogenic effects. They can directly influen.