Heavy metal accumulation in Amaranthus hybridus and Ocimum gratissimum cultivated on automobile workshop soils in Ekiti Nigeria and associated human health risks

Abstract In rapidly urbanizing Nigerian cities such as Ado-Ekiti, Ekiti State, automobile workshops release heavy metals into surrounding soils, posing risks to food safety when such areas are used for urban agriculture. This study investigated growth responses, metal bioaccumulation, and health risks associated with Amaranthus hybridus and Ocimum gratissimum on soils from two automobile workshops (Workshop A: 20 years operational; Workshop B: 10 years operational) in Ado-Ekiti, Nigeria. Controlled pot experiments were conducted under greenhouse conditions, followed by heavy metal quantification using atomic absorption spectrophotometry and standardized health risk assessment. Workshop A soil exhibited lower pH (6.5 vs. 6.8), higher organic matter content (4.5% vs. 3.8%), and elevated concentrations of Cu (0.935 mg kg⁻¹), Cd (0.251 mg kg⁻¹), and Mn (29.55 mg kg⁻¹) compared to Workshop B. A. hybridus accumulated higher metal concentrations than O . gratissimum with stem tissues showing 1.2–2.1 times greater accumulation than leaves. Bioconcentration factors remained below 1.0 for most metals, except anomalously high Ni values (6.5–9.5) in Soil B due to extremely low baseline concentrations A. hybridus demonstrated superior growth resilience, achieving 54.44 cm height in Soil A versus O. gratissimum ’ s 45.64 cm. Health risk assessment revealed Target Hazard Quotients (THQ) and Hazard Index (HI) values below 1.0, indicating acceptable non-carcinogenic risk. Total Cancer Risk (TCR) ranged from 1.65 × 10⁻⁵ to 3.26 × 10⁻⁵, within USEPA acceptable limits (< 1 × 10⁻⁴), though children showed higher vulnerability. Findings indicate that while immediate health risks remain low, continuous monitoring and soil remediation strategies are essential for sustainable urban agriculture in automobile workshop-impacted areas.