Extent of Heavy Metals Contamination in Leafy Vegetables among Peri-Urban Farmers

Heavy metals are metalloids of environmental concern, especially in urban and peri-urban small scale farming systems. Plants are contaminated with heavy metals through different pathways, including uptake from soil and from the atmosphere. These pathways need to be distinguished from each other in order to understand where massive contamination comes from and the one suitable for a particular metal. The uptake is affected by various factors, among them are environmental conditions (climatic conditions, soil characteristics, and root system), plant species/morphology and chelating agents. Most vegetables are known to be contaminated by heavy metal like cadmium (Cd), chromium (Cr), lead (Pb), arsenic (As), iron (Fe), zinc (Zn), copper (Cu), with leafy vegetables mostly affected. The contamination levels of these metals have largely exceeded the recommended consumption levels by the world health organization (WHO), thus likely to cause adverse health effects to human beings if regulation measures are not put in place. Research on heavy metal contamination mostly focuses on other vegetables and less has been done with respect to African Indigenous Vegetables (AIVs). On the other hand, plant species, especially AIVs, have different physical, morphological and physiological traits that may affect the movement and accumulation of various heavy metals and thus the extent of contamination and metal affiliation may vary from one vegetable to the other. Since contamination occurs along the value chain, this calls for research that guides farmers to choose the best farming practices, package and transportation measures for reduced heavy metals contamination of vegetables will be vital.