Impurities in Herbal Substances, Herbal Preparations and Herbal Medicinal Products, IV. Heavy (Toxic) Metals

Abstract

The main source of available forms of heavy metals (toxic metals) for the plant kingdom is anthropogenic, resulting from diverse activities such as metallurgic processing of ore, cement plants, uncontrolled discharge of sewage sludge, burning of fossil fuels and waste incineration plants, and leaded petrol. Agricultural chemicals (e.g. phosphate fertilizers containing cadmium) may also contribute to the contamination of cultivated plants. The main threats to human health from toxic metals are associated with exposure to lead, cadmium, mercury (organic forms, especially methylmercury) and arsenic (mineral form only), which have no known vital or beneficial effect on living organisms. As their toxicity often takes years to manifest and may go unsuspected, their toxicological risk is defined on the basis of the so-called Provisional Tolerable Weekly Intake (PTWI) values. Beside anthropogenic causes, the main factors that may lead to high levels of toxic metals in medicinal plants are their availability in the soil with soil pH as the most important parameter for uptake by the plant. Indeed, genetic features of certain plant species show a tendency to accumulate certain trace elements, especially cadmium (“cadmium collector”). A very recent revision draft of the monograph “Herbal drugs” (Ph. Eur., 1433) includes acceptance criteria for lead, cadmium and mercury. This proposal is discussed in detail, based on literature data dealing with terrestrial plants and seaweed. Additionally, the need for inclusion of tests for inorganic impurities in quality control specification is examined, based on a risk assessment. As the daily intake of food supplements is very similar to the one of herbal remedies, it would be advisable to take into account the same acceptance criteria. The specific situation has also been considered of exotic herbal remedies, particularly those of Asian origin, which have been repeatedly reported to contain toxic levels of heavy metals and/or arsenic resulting in heavy metal poisoning.