Heavy Metal Contamination of Herbal Drugs: Implication for Human Health-A Review

Interest in the use of medicinal plants in treatment of diseases in Africa has increased tremendously over the past decade. Ingestion of contaminated medicinal plants and herbal medicinal products is regarded as potential source of heavy metal toxicity to both man and animals. Heavy metals are often not well defined in medicine, but include all toxic metals. They are released into the environment by both natural and a variety of anthropogenic sources. The presence of heavy metals in plant tissues is primarily dependent upon their availability and concentration in the soil. They can also be deposited directly on plant surfaces from the atmosphere. Heavy metals are persistent in nature due to their long biological half-life. The major heavy metals of health concern are arsenic, cadmium, lead and mercury. They are the redox inactive metals and show their toxic effects via bonding to sulphydryl groups of proteins and depletion of glutathione - an antioxidant. In order to ensure quality and safety of herbal drugs, cultivation and collection of medicinal plants in the immediate vicinity of industrial sites which utilize these metals and their compounds, and sites where these metals have been improperly disposed is highly discouraged; because plants from these areas are prone to high concentration of heavy metals, hence, increases human risk of contamination when taken. In addition, screening of plant extracts, herbal medicinal products and medicinal plants at large, for heavy metal contamination is of highly medical importance and must be given maximum attention in phytotherapy.