Both Phillips et al. (2001) and Elgethun et al. (2003) reported limitations of the technology as a sole source of space–time data for an exposure assessment study. Both studies found the reception of the satellite signals to be adversely impacted by shielding from buildings of certain materials (concrete, steel), electrical power stations, and to some extent vehicle body panels. Signal blockage continues to be an issue with GPS today. Phillips et al. (2001) also reported extensive failure including battery failure, data-logging failure, and data storage limitations, which resulted in capturing only about 30% of the total monitoring time attempted in 25 trials. Elgethun et al. (2003) reported reception efficiencies of 79% outdoors, 20% in homes, 12% in vehicles, and 6–9% in schools and businesses. These findings indicate that although GIS using GPS technology hold promise in terms of integrating study population activity data with measured or predicted levels of environmental contaminants in the exposure assessment process, their use is still very much in the developmental research stage for use in epidemiology studies.