The ground-level air concentrations of supported and total 234Th were determined over a period of 1 year at four sites in the vicinity of a nuclear fuel fabrication plant in northwest England. Both supported and unsupported 234Th were found, with mean monthly concentrations of total 234Th 13 times those observed at a control site 50 km north of the factory. Bulk deposition rates of total 234Th were found to be 2–3 times the background values. Analysis of mosses revealed a systematic decrease in supported 234Th concentrations with distance from the factory, with background values reached at a distance of about 1.5 km. Road dust samples showed a similar decrease in supported 234Th concentrations with distance, but with significant departures from this trend, possibly being evidence for the mechanical transport of contaminated dust from the factory. A Gaussian plume dispersion model was able to predict 85% of the observed air concentrations within a factor of four, despite large uncertainties in some source parameters.