Influence of Urban Land Surface Temperature on Heavy Metal Accumulation in Cabbage and Lettuce Across the Greater Accra Metropolitan Area

This study assessed the concentrations and health risks of heavy metals in cabbage (Brassica oleracea) and lettuce (Lactuca sativa) cultivated across three urban land surface temperatures in the Greater Accra Metropolitan Area (GAMA): Atomic (low land surface temperature, LST), Ashaiman (moderate LST), and Korle-Bu (high LST). The objective was to assess the influence of urban land surface temperature on heavy metal accumulation and associated human health risks. Results revealed that arsenic (As) and mercury (Hg) levels were consistently low (≤0.002 mg/kg) and remained below the maximum residue limits (MRLs) at all sites. However, cadmium (Cd), lead (Pb), and nickel (Ni) concentrations exceeded MRLs in both vegetables. Cd ranged from 1.40 ± 0.27 mg/kg (lettuce, Ashaiman) to 3.13 ± 0.99 mg/kg (cabbage, Atomic), while Pb varied between 0.90 ± 0.84 mg/kg (lettuce) and 2.62 ± 1.22 mg/kg (cabbage). Ni concentrations exceeded the permissible limit (0.2 mg/kg) across all LST zones, with the highest at Korle-Bu (0.65 ± 0.07 mg/kg). Cumulative heavy metal concentrations increased significantly (p < 0.005) with rising LST, particularly in cabbage. Noncarcinogenic risk assessment indicated that Cd and Ni were the dominant contributors to human health risk, with target hazard quotients (THQ) and hazard indices (HI) exceeding the safety threshold (HI > 1) for both adults and children, especially in Atomic and Korle-Bu. Children were more vulnerable, exhibiting higher exposure levels. Carcinogenic risk (CR) analysis further identified As, Cd, and Ni as the main carcinogens, with total cancer risk (TCR) values across all sites and age groups exceeding the USEPA acceptable range (1 × 10⁻⁶–1 × 10⁻⁴). The findings suggest that increasing urban temperatures exacerbate heavy metal accumulation in leafy vegetables, posing significant noncarcinogenic and carcinogenic health risks, particularly to children.