Measures of the risk to building occupants posed by indoor contaminant exposures must account for the combined impact of concentration (C) and duration of exposure (t). A common assumption embodied in Haber's law is that the outcome of an exposure is proportional to C × t, i.e., the effects of concentration and duration are equivalent. However, it is well known that the effects of many chemicals do not follow this simple relationship and are better approximated by a relationship of the form Cn × t. This is explored using a building-specific exposure metric that measures the relative impact of a chemical release event using the results of multizone simulations. Metrics are computed with various toxic load exponents, and the resulting metrics are then compared. In particular, these results are examined in terms of suitability for the evaluation of security improvements as might be done as part of new building design or a retrofit project.