In the wake of the growing popularity of pedestrian-oriented community designs, it is timely to assess potential risk trade-offs of such urban planning strategies. Pedestrian-friendly designs are currently being called for and implemented in the US to tackle in particular problems associated with insufficient physical activity in the population. Unintended consequences may emerge, however, especially due to potential increases in the inhalation of pollutants as the population walking or cycling in polluted environments increases. A risk assessment of such built environment transformations was undertaken to evaluate quantitatively the competing risks and benefits of community design changes in active travel. A simulation model, built incorporating research from the fields of transportation, environmental sciences and exposure analysis, is applied to a case study area that undergoes hypothetical urban transformations. We find that the simulated population experiences roughly the same number of days in a year with decreases as number of days with increases in energy expenditure or inhalation of pollutants. In the 5% of days with greatest shifts, PM(10) inhalation was shown to increase by 175% or more, while the 5% of days of greatest decreases exhibited reductions of 45% or more (with similar results for ozone). Of particular concern, some individuals are shown to double their intake of the pollutants on high pollution days. However, uncertainty in the estimates is high. In particular, interpretations are very different according to the approach used to characterize year-long activity patterns. This innovative risk assessment uncovers critical gaps in the literature that must be further researched to allow essential comprehensive analyses of planning decisions.