Accumulation of Heavy Metals in Selected Medicinal Plants

Abstract

In this review, we evaluate the reports published between 1993 and 2011 that address the heavy metal accumulation in 88 medicinal plant species. We compare the safe limits for heavy metals set by governmental agencies vs. the levels at which such metals actually exist in selected medicinal plants. We also evaluate the uses and effectiveness of medicinal plants in health care, and assess the hazards of medicinal plant uses, in view of the growing worldwide use of medicinal plants. From our extensive review of the literature, we discovered that a maximum permissible level (MPL) of Pb is exceeded in 21 plant medicine species, Cd in 44 species, and Hg in 10 species. Vetiveria zizanioides a potential candidate species for the treatment of cardiovascular diseases absorb a wide range of heavy metals from metal-contaminated soils. We believe that this species is the single most impressive example of a potentially hazardous medicinal plant. Based on our review, we endorse the hypothesis that heavy metal accumulation by medicinal plants is mainly caused by extraction of soluble metals from contaminated soil, sediments and air. One continuing problem in protecting consumers of plant-based medicines is that permissible levels of all heavy metals in herbal medicine have not yet been standardized by regulating governmental entities. Moreover, there are few limit tests that exist for heavy metal content of medicinal plants, or permissible limits for essential dietary minerals, in most medicinal plants. The dearth of such limits hamstrings development of medicinal plant research and delays the release of either new or improved versions of medicinal plants or their components. In the present review, we emphasize that medicinal plants are often subjected to heavy metal contamination and that the levels at which these heavy metals sometimes occur exceeds permissible levels for some species. Therefore, collecting medicinal plants from areas that are, or may be, contaminated should be discouraged and banned if possible.

Accumulation of heavy metals from soil in medicinal plants

“…Medicinal plants are still widely harvested from wild habitats for individual use and production of plant-based products (35). It is therefore important to emphasize that the safety and benefits of plant-based products are directly related to the quality of the raw materials (36), including the presence of heavy metals within safe limits.…”

Monitoring heavy metals, residual agricultural chemicals and sulfites in traditional herbal decoctions

“…In addition, adulteration with undeclared other substances and contamination with undeclared toxic or hazardous substances are most likely to be found in herbal materials or herbal products [10]. The toxic heavy metals, residual pesticides or improper use of sulfites are regarded as potent risk factors in use of herbal medicines because they can be easily contaminated in herbal materials due to soil pollution and process of cultivation, harvesting and storage [11–13]. …”

Dietary Content and Potential Health Risks of Metals in Commercial Black Tea in Kampala (Uganda)

“…Therefore, all samples had Cd concentration within the internationally acceptable range of 0.3 to 0.5 mg/kg recommended for raw herbal material (Sarma et al, 2011). Cadmium content was also lower than that reported for black tea in Nigeria but higher than in other studies (Table 4).…”