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Medicinal plants are rich sources of natural products, the principal constituents in herbal medicines, utilized for the treatment and prevention of diseases. High consumer expectations for health care, in the face of soaring cost of conventional pharmaceuticals, have popularized herbal medicines in different regions of the world. The consumption of these botanicals and their products has recently gained much impetus with the assumption that since these active principles in them are natural, they are, therefore, safe. Assertive as this idea could be, scrutinizing the perspectives on which it is premised is critical in minimizing probable risk on human health. Most plant parts are repositories of natural toxins, phytosteroids, and bioaccumulated toxicants, all of which are driven by natural processes. Besides intrinsic toxicity possessed by phytotoxins, some innocuous chemicals in medicinal plants can be biotransformed to toxic components. Some chemicals in medicinal plants that are of safety concerns include the endocrine-disrupting chemicals such as phytoestrogens, bisphenol A, and phthalates, among others. The persistent, bioaccumulative, and toxic metals have also been identified in medicinal plants. Some of these chemicals have been found to cause metabolic derangement and carcinogenicity. It is, therefore, imperative that linking natural products to safety should rather be empirical.

The contamination of soil with toxic metals poses serious threats to the survival of living organisms including humans. We determined the contamination levels of cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb) and nickel (Ni) in soil samples from a typical agrarian soil in Nigeria, using various single and complex geochemical indices along with principal component analysis (PCA) for source determination. Ten soil samples (S1-S10) were collected from depths of 20 cm, with a clean shovel and brush from farmlands in Ihiala, South-East Nigeria. Three single pollution indices: geoaccumulation index (Igeo), pollution index (PI) and ecological risk index (Er), as well as four complex indices: pollution load index (PLI), Nemerow pollution index (PINemerow), average single pollution index (PIave) and Potential ecological risk (RI) were used for the geochemical analysis. The mean soil levels of Cd (1.94ppm) and Pb (60.83ppm) exceeded their corresponding world averages. The results of the single pollution indices of the soil samples revealed heavy Cd, moderate Pb and low Ni, Cr and Cu contaminations, while the PIavg, PINemerow and RI graded the soil samples as moderately to seriously polluted. The correlation analysis revealed that the general contamination was mostly contributed by Cd and partly by Cr. The findings showed that Cd and Pb were the main heavy metal soil contaminants in the area. The levels of toxic metals found in the soils could pose health and ecological risks. The probable sources of these metals include pesticides use and poor waste disposal systems.