The U.S. Environmental Protection Agency (EPA) is conducting field measurements in Philadelphia, Pennsylvania to: (1) characterize the spatial and temporal variations of acid aerosol species in an urban environment, (2) investigate the complex chemistry of acid aerosols and other acidic species, including formation/removal mechanisms, and (3) provide acidic aerosol and particulate data base for exposure modeling and a study of pollution health effects. One of the goals of the EPA's aerosol acidity study is to develop models of human exposure to acid aerosol species. Exposure models will be used to estimate the distribution of human exposures to acid aerosols. The models would be an important planning tool for assessing exposures by: (1) determining acid aerosol exposures in high-risk groups; (2) facilitating planning of subsequent sampling strategies; and (3) evaluating the effectiveness of proposed or implemented mitigation efforts on reducing human exposures to acid aerosols. The paper focuses on issues to be addressed in developing models of human exposure to acid aerosols. The intent is to describe a sampling scheme that provides the information needed for development of an acid aerosol exposure model.