HEALTH RISKS FROM LETTUCE (LACTUCA SATIVA) FROM CONTAMINATED URBAN SOILS: A CRITICAL REVIEW OF METAL UPTAKE AND HUMAN EXPOSURE

Abstract

Urban agriculture is emerging as an effective approach toward food security, social equity and green space utilization. Urban soils are, however, often polluted with old and current pollutants, including heavy metals (lead, cadmium, arsenic, mercury) and organic pollutants (polycyclic aromatic hydrocarbons [PAHs]). Lettuce, (Lactuca sativa), is a particular health issue among the commonly grown vegetables due to its high transpiration rate, strong tendency to uptake the contaminants in its edible parts and being eaten raw. This review summarizes the most recent science available on the human health risks from eating lettuce that is cultivated in contaminated urban soils. The various components of the human health risk assessment (HHRA) framework for applying it to this pathway are examined systematically: contaminant sources in urban soils, bioavailability and soil-to-plant transfer mechanisms (quantified by bioconcentration factors and translocation factors), exposure assessment (including ingestion rates, body weight parameters), risk characterization (Hazard Quotient (HQ), Hazard Index (HI) and cancer risk (CR) models). Results from a synthesis of lead, cadmium and arsenic values in global HQ/CR case-study cities across North America, Europe, Asia and Africa suggest a high prevalence of exposures above safe limits (HI > 1; CR > 10-4), especially for children who have lower body weights and consume more per kg of body weight. Most importantly, risk outcomes are influenced by soil conditions (pH, organic matter), cultivar of lettuce, and post-harvest conditions (washing and cooking). The review reveals that there are important gaps in knowledge, such as the lack of standardized protocols that consider contaminant bio accessibility and mixture toxicity. We believe that there is an urgent need to develop urban specific soil guideline values and evidence-based mitigation strategies to protect public health based on current evidence.