Buildings as a means of protection from toxic gas exposures—comparing chlorine Toxic Load indoors and outdoors

Acute health effects resulting from exposures to toxic gases can scale with the time integral of concentration raised to a power – an empirically determined toxic load exponent (Health Effects α ∫[Concentration^Toxic Load Exponent] dt). This integrated quantity is called a toxic load. For many toxic industrial chemicals, including chlorine and ammonia, the toxic load exponent is significantly greater than one for many health effects of interest – implying that health outcomes are disproportionately determined by exposure to high chemical concentrations. Due to limited air exchange between the indoor and outdoor environments and the strong loss mechanisms (e.g. deposition, sorption) expected for many toxic gases in the indoor environment, peak indoor concentrations (and thus acute health effects) are expected to be significantly lower than those outside.

In this study, we provide an illustrative example of the protection that buildings are expected to provide their occupants against exposures to outdoor chlorine plumes. We first model the outdoor concentration-time profile for a hypothetical release of chlorine. Then the outdoor concentration-time profile is used as an input to a building protection model to provide indoor concentration time profiles for each of several different general classes of building, e.g., residential, commercial. Subsequent processing of the indoor and outdoor concentration time series provides estimates of building protection (defined as the ratio of outdoor to indoor toxic load). The model used in this analysis uses a range of parameters to estimate the variability in the predicted building protection factors.

Our analysis shows that the range of expected building protection factors varies widely and can exceed 1,000 – illustrating the need to incorporate the role of buildings in hazard assessments and in consequence planning (sheltering). We also discuss these results in the context of hazard extents – the distance downwind over which significant health effects are expected to occur.