A user-friendly computer model is being developed to estimate the distribution of Californians' indoor exposures to various pollutants. The model will be capable of operating at three levels. Level 1 will sum estimated inhalation exposures across multiple indoor locations to ultimately yield a population distribution of “total indoor air” exposures, that is, the portion of total (24-h) exposure associated with time spent indoors. Level 2 will link randomly selected location/activity profiles, collected in several California surveys, with measured or modeled concentration distributions, resulting in estimated exposure distributions for specific indoor locations such as residential environments. For each sampled location/activity profile, both time-integrated inhalation exposure and potential inhaled dose will be estimated. Level 3 will estimate concentration distributions for pollutants in locations for which measured concentrations are not available, based on the principle of conservation of mass. All information required for a model run will be supplied through a user interface. Options will be provided to save files relating to choices and inputs for a given run, so that all or selected parts of a run can be replicated. Providing this capability within Level 3 of the model will allow the user to make a series of related runs aimed at assessing the impact of one or more potential mitigation strategies.