Pseudo-total (i.e. aqua regia extractable) and gastric-bioaccessible (i.e. glycine+HCl extractable) concentrations of Ca, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn were determined in a total of 48 samples collected from six community urban gardens of different characteristics in the city of Madrid (Spain). Calcium carbonate appears to be the soil property that determines the bioaccessibility of a majority of those elements, and the lack of influence of organic matter, pH and texture can be explained by their low levels in the samples (organic matter) or their narrow range of variation (pH and texture). A conservative risk assessment with bioaccessible concentrations in two scenarios, i.e. adult urban farmers and children playing in urban gardens, revealed acceptable levels of risk, but with large differences between urban gardens depending on their history of land use and their proximity to busy areas in the city center. Only in a worst-case scenario in which children who use urban gardens as recreational areas also eat the produce grown in them would the risk exceed the limits of acceptability.

In order to characterize the influence of environmental factors in dust metal loadings inside homes in an urban environment and to evaluate the associated potential health risks, samples of settled indoor dust from 10 apartments in the urban area of Madrid (Spain) were collected with wet wipes. Cd, Cr, Cu, Pb, Zn, Ni, and Mn loads were determined by Atomic Absorption Spectroscopy (AAS) after a HNO₃ + H₂O₂ digestion. The environmental factors evaluated were load distribution between rooms, number of residents, presence of smokers, traffic intensity, apartment elevation, and frequency of house cleaning. Tukey’s range test and stepwise multiple linear regression analysis revealed that metal dust loadings present two prevailing origins: (1) They present higher loadings in the entry hall, which suggest that dust is tracked indoors adhered to footwear and clothing and (2) they arise from tobacco smoking. Significant correlations were also observed between metal loadings and traffic intensity (Cr), number of residents (Cr, Pb, and Cu), number of days between cleaning (Ni), and flat height (Mn). A human health risk assessment considering a mechanistic hand-to-mouth model for dust ingestion and dermal absorption revealed that urban children are not expected to develop adverse health effects from exposure to trace elements in household dust. The contribution of this exposure scenario to the overall received dose should be included when assessing the background exposure of children to trace elements. A more precise assessment should attempt to reduce the significant uncertainty of the risk model output associated with estimates of exposure variables, deposition rates, and metal bioaccessibility.