Accumulation of Heavy Metals in Selected Medicinal Plants - Reviews of Environmental Contamination and Toxicology - page 64

soil and surface waters has increased (Nriagu and Pacyna 1988 ; Larison et al. 2000 )
and now constitutes a potential threat to terrestrial and aquatic biota (Ives and
Cardinale 2004 ; Nasim and Dhir 2010 ) and to humans by entering the food
chain (Hsu et al. 2006 ; Meena et al. 2008 ). Because of the widespread presence of
heavy metals in the environment, their residues also reach and are assimilated into
medicinal plants.
Any nonbiologically degradable metal or metalloid that causes an environmental
problem should be considered to be a “heavy metal” (Herrera-Estrella and Guevara-
Garcia 2009 ). Fifty three elements now fall into the category that can be properly
be referred to as heavy metals; such elements are deined as the group of elements
whose densities are greater than 5 g cm −3 (Sarma 2011 ). The responses that plants
display to a HM-contaminated environment for purposes of adaptation are deter-
mined by many physiological, molecular, genetic, and ecological traits. But in gen-
eral, exposure to very high HM concentrations affects both the growth and
metabolism of plants (Dhir et al. 2009 ).
Traditional healers often prescribe mixtures of medicinal plants in raw form for
diseases ranging from common cold to malaria, arthritis, ulcers, hepatitis, and diabe-
tes among others (Obiajunwa et al. 2002 ; Sarma and Sarma 2008 ; Sarma et al. 2008 ).
The use of medicinal plants/herbal products as the irst choice in self-treatment con-
tinues to expand rapidly across the developing world, viz., India, China, and South
Africa, to prevent imminent development of certain diseases. In primary healthcare,
about 70-80% of the world's population rely on unconventional medicine, mainly of
herbal origin (WHO 2002 ), and in some parts of the world, herbal medicines are the
only options for primary health care for poor people. In Ethiopia, more than 85% of
the population depends on herbs for primary health care (Meena et al. 2010 ).
Medicinal plants are particularly important in developing countries because such
plants are also dietary components and are essential for health (Maiga et al. 2005 ;
Cantarelli et al. 2010 ); this, therefore, accounts for the increasing popularity of
phytotherapy (Gjorgieva et al. 2010 ). The average annual volume of medicinal and
aromatic plants that are utilized in EU countries has increased by 21% since 1992,
meaning that more than 100,000 ton of herbal drugs, worth US$330 million, are used
in traditional or processed forms (Bernath 2002 ). Most medicinal plants are rich in
minerals and metals and are usually consumed in low doses as nonprescription herbal
drugs for debility prevention ( Hay 1984 ; Obiajunwa et al. 2002 ). Despite widespread
use, however, the adverse effects of long-term ingestion of high doses of minerals
and metal supplements are still not well undocumented (Ivey and Elmen 1986 ).
In India, about 2,000 drugs of plant origin are used. The distribution of medicinal
plants is now under great pressure in India because excessive amounts of them are
collected from wild habitats and are exploited for use in medicine. Such overcollec-
tion has endangered 20-25% of existing plant species in India (Laloo et al. 2006 ).
Medicinal plants are normally procured indiscriminately from noncultivated wild
habitats by untrained and uneducated people, and pushed to the market or raw drug
suppliers of the pharmaceutical industry without any analysis of the plants for metal
content. Such premarket analysis is pertinent to ensure that levels of different toxic
heavy metals do not exist in polyherbal medical formulations (Meena et al. 2010 ).