How healthy are horticultural plants cultivated in urban gardens in polluted cities? The case of Mexico City

Soil-to-plant metal(loid) accumulation has been highly evaluated recently due to its significant impact on human health. This study reports the concentrations of 15 metal(oid)s in four vegetables (tomato, onion, chili, and lettuce) cultivated in a greenhouse and three shade houses (two roofs of buildings and one in the UNAM Botanical Garden) in four locations in the metropolitan area of Mexico City. In this experiment, the contribution of metals and metalloids due to atmospheric deposition and irrigation may be negligible. Hence, the concentration of metals depends only on transfer from the soil. Soil analysis indicates that only vanadium in one location (a rooftop garden in the municipality of Venustiano Carranza) exceeds the permissible limits according to Mexican regulations. Lettuce shows the highest concentrations and transfer factors for most metals compared to the other vegetables studied. Transfer factors were exceptionally high for Cd (lettuce) and Hg (tomato), presenting values above 1. Concentrations of As, Cd, Hg, and Pb were compared with limit values recommended by different international agencies for vegetables. It was observed that, except for As, these values are exceeded in various vegetables harvested in this study. This information must be corroborated with more detailed studies evaluating the chemical species in which those metal(oid)s are present and identify the physical and chemical parameters of the soil that may have interfered with the soil-plant transfer factor.