 exact measurements or modeling of human exposures to environmental pollutants are of crucial importance for a realistic evaluation of public health risks . 
 current concepts , however , often use assumptions that result in overly conservative assessments of public health risks . too frequently the dose of the pollutant retained in the body is approximated by oversimplified predictions assuming that all that is inhaled remains in the organism , that pollutant concentrations in various microenvironments are identical to those recorded by remote monitors , that the residence indicates the site where people spend all their time , and that the urban population is continuously exposed to outdoor air for 24 hr / day and 70 years / lifetime . 
 the review shows that in intermittent exposures only a fraction of inhaled toxicants remains in the body , that pollutant concentrations differ largely from one microenvironment to another , and that human activity patterns must be incorporated in every realistic exposure assessments . specifically , the probability of being exposed to a short peak of ozone is predetermined in variable urban concentrations primarily by the coincidence of exercising outdoors at the time and site of elevated ozone levels . when combined with a physiologically based exposure evaluation , 
 this probabilistic approach provides a scientifically sound estimate of actual occurrences of adverse exposures and a realistic assessment of potential health hazards.imagesfigure 2.figure 4.figure 5 . 
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