Assessment of metal contaminants and human health risks in food crops at the uncontrolled Mbale dumpsite, Uganda

  • Florence Awino

Student thesis: Doctoral Thesis


Waste chemicals arising from poor municipal solid waste management across countries result in negative environmental impacts and health risks. Globally, over 200 infectious and non-communicable diseases, 600 million patient illnesses and 2 million deaths result from microbial and chemical contaminations of food and water. In some developing countries, chemical-related diseases (e.g., “itai-itai”, “chloracne” and “yusho”) have been reported. In 2012, 23% of the global deaths and 22% of the disabilities were attributed to environmental contaminant related risks. In Uganda, ten preventable diseases related to environmental hygiene and food safety contributed towards 75% of infant mortality (1997 – 2001). Limited studies on ecological and food chain risk assessments of chemical disease burdens, coupled with weak enforcement of environmental regulations and lack of effective food safety systems have negatively impacted on land and food governance. This has resulted in the co-occurrence of open dumping and urban agriculture, including dumpsite farming. In the Mbale municipality, in Uganda, several qualitative studies have reported more than 50% prevalence of sanitation-related diseases. Only one quantitative study on five soil and two surface water samples have been carried out at the Mbale dumpsite (2007), and no ground water or food crop risk analysis exists. In this thesis, the literature review (chapter 2) describes the problems associated with poor waste management across countries and identified the critical gaps that present risks to the human population. Also, described is an assessment of food chain contamination due to toxic metals from the Mbale mixed waste dumpsite (Uganda). Food crops on the Mbale dumpsite were used as bio-monitors to assess the presence of aluminium (Al), chromium (Cr), iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), selenium (Se), cadmium (Cd), mercury (Hg) and lead (Pb), and their potential health risks. The first objective of the study was to use 108 Zea mays and Amaranthus cruentus crop samples to assess if crop locations influenced metal uptake across the Mbale dumpsite. The second objective assessed if metal uptake in crops and crop parts were influenced by growth period (maturity age) using eighty-one short-term (2 – 6 months) and long-term (7 - 15, 18 – 72 months) crops from the Mbale dump’s centre and hill slope. The third objective assessed if the single and total dietary exposure risks in individual crop types, combined individual crop types and mixed diets (meal types) were within acceptable levels, using seventy-five short-term crops at the Mbale dump’s centre. The results were compared against World Health Organization food safety guidelines, toxicity limits and United States Environmental Protection Agency cancer risk categories.
It was observed that some metal (Pb, Cr, Al, Zn, Hg, Ni, Mn, Fe, Cu) concentrations in the food crops were above WHO/FAO safe consumption levels and could pose both non-carcinogenic and carcinogenic risks. The mean concentrations of some essential elements (Fe, Al, Zn, Mn and Cu) were higher than for most toxic elements (Cr, Pb, Ni, Co, Se, As, Cd, and Hg), that were only present in trace amounts, except for Al and Se. Metal accumulation in crops depended on crop type and part, with leaves accumulating higher metal concentrations compared to any other part of the plant, for most metals. Furthermore, Al, Zn, Fe, Cr and Co concentrations in crops varied significantly across the dumpsite; and the highest metal accumulation was at the dump centre and in crop leaves, with Zn and Cr concentrations higher at the slope. Metal concentrations in short-term (2 – 6 months) crops were higher than those in long-term (7 – 15 and 18 – 72 months) for most metals, with the highest metal accumulation in short-term crops at the dump centre and in crop leaves at the slope. The estimated daily metal intake, non-carcinogenic and carcinogenic risks were higher in children than adults. The total dietary exposure risks in consuming individual crop type or individual combined crop types was higher than that in meal types (common mixed diets). The carcinogenic risks from total metal dietary exposure in meal types and crop types were high (10-3 - 10-2) to very high (10-3 - 100) respectively. Also, the highest risks were via consumption of leafy crops compared to legumes, cereals and fruits. The study concludes with recommendations on the need to create consumer awareness through relevant stakeholder collaborations, inclusion and participations; identifies what crops to grow and where to grow them on the site and outside the urban setting. It also highlights the importance of crop type selection for meal type combinations. Other identified areas are the need to use scientific evidence to develop potential policies required in enforcing land and food governance, and future research on exposure assessments to further characterize the risks associated with exposure to metal contaminants found in food crops (washed and unwashed) grown on the Mbale dumpsite.
Date of Award2020
Original languageEnglish
SupervisorJasmyn Lynch (Supervisor) & Bill Maher (Supervisor)

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