Pb isotope composition of tree rings (Celtis Australis) and urban aerosols have been determined to assess whether arboreal species can be used as bio-geochemical tracers of the evolution of heavy metal pollution to the environment. Particular care was paid to setting up a high quality analytical technique to work with arboreal species with low Pb content. The Pb isotope composition of tree rings from 1950 to 1995 is within the range of European aerosols and is correlated with the temporal evolution of Pb isotopes measured in air particulates from Firenze. The entire data set (tree rings and air particulates) demonstrate that Pb isotope composition of tree rings can be used successfully as a proxy of the atmospheric Pb isotope composition of urban areas. This, in turn, suggests that tree rings are potentially a powerful bio-geochemical tracer for monitoring air pollution history due to human activities. (C) 2000 Elsevier Science Ltd.