A model has been developed to describe the temporal behaviour of the concentrations of a pollutant tracer within the urban environment of Barrow-in-Furness, NW England. The tracer used was 137Cs derived primarily from wet deposition, on 3 May 1986, of the radioactive cloud from the Chernobyl reactor accident (28 April 1986). The 137Cs activity deposited during this primary event was supplemented by a small secondary atmospheric deposition input of resuspended activity. The model was validated against the measured temporal behaviour of 137Cs in urban dust for two outdoor reservoirs in which the only observed input of dust and activity was by atmospheric deposition. Further modelling studies on other reservoirs (both outdoors and indoors) confirmed the existence of additional input fluxes of dust and activity; the most significant of these being the mechanical transport of soil and degradation of construction material (outdoors) and the mechanical transport of outdoor dust and soil and incorporation of organic material (indoors). The model enabled estimates of the magnitudes of these additional fluxes to be made and mean dust mass residence times to be calculated (outdoors, 250 ± 110 d; indoors 29 ± 1 d, indicating the length of time that conservative dust-bound pollutants would be expected to remain in urban reservoirs with no secondary inputs following the primary contamination event. These residence times correspond to environmental half-lives of 170 ± 70 d outdoors and 20 ± 1 d indoors, for reservoirs which only receive a single primary input of a contaminant. Where secondary inputs of pollutants occur, such as the atmospheric deposition of resuspended activity or the mechanical transport of contaminated dust indoors, the mean environmental half-lives of the pollutants increase by 50% for outdoor dust reservoirs and over 18-times for indoor reservoirs. This re-contamination of indoor dusts has implications for decontamination programmes in that attention should be paid to reducing outdoor contamination levels to ensure that attempts to reduce the levels of a pollutant indoors by cleaning are effective.