br br br Heavy metals are natural constituents of the earth's crust, but indiscriminate human activities have drastically altered their geochemical cycles and biochemical balance. This results in accumulation of metals in plant parts having secondary metabolites, which is responsible for a particular pharmacological activity. Prolonged exposure to heavy metals such as cadmium, copper, lead, nickel, and zinc can cause deleterious health effects in humans. Molecular understanding of plant metal accumulation has numerous biotechnological implications also, the long term effects of which might not be yet known. br br Any toxic metal may be called heavy metal, irrespective of their atomic mass or density. Heavy metals are a member of an ill-defined subset of elements that exhibit metallic properties. These include the transition metals, some metalloids, lanthanides, and actinides. One source defines heavy metal as one of the common transition metals, such as copper, lead, and zinc. These metals are a cause of environmental pollution from sources such as leaded petrol, industrial effluents, and leaching of metal ions from the soil into lakes and rivers by acid rain.Three principal systems of medicine are practiced in India: Ayurveda, Siddha and Unani-Tibb. These systems utilize drugs of natural origin constituting plants, animals, and mineral preparations. br br History br Ayurvedic medicines originated in India more than 2000 years ago and rely heavily on herbal medicinal products (HMPs).Approximately 80 of India's population use ayurveda through more than one-half million ayurvedic practitioners working in 860 ayurvedic hospitals and 22100 clinics.As early as the 19th century, there were plants identified, which were capable of accumulating uncommonly high Zn levels and hyper accumulating up to 1 Ni in shoots. Following the identification of these and other hyper accumulating species, a great deal of research has been conducted to elucidate the physiology and biochemistry of metal hyper accumulation in plants.In the United States, ayurvedic remedies are now available from South Asian markets, ayurvedic practitioners, health food stores, and the Internet. Because ayurvedic HMPs are marketed as dietary supplements, they are regulated under the Dietary Supplement Health and Education Act (DSHEA), which does not require proof of safety or efficacy.Since 1978 more than 80 cases of lead poisoning associated with ayurvedic medicine use have been reported worldwide.Metal contamination of garden soils may be widespread in urban areas due to past industrial activity and the use of fossil fuels.