 The indirect approach or exposure modeling determines the estimated exposure distributions within a population rather than the direct exposure an individual has experienced.The advantage is that process is minimally invasive to the population and is associated with lower costs than the direct approach.A disadvantage of the indirect approach is that the results were determined independently of any actual exposures, so the exposure distribution is open to errors from any inaccuracies in the assumptions made during the study, the time-activity data, or the measured pollutant concentrations.