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When performing Process Hazard Analysis (PHA), basic consequence evaluation methods are often sufficient. However, sometimes these approaches lead to overly conservative estimates, and we need to conduct more thorough dispersion modeling to refine the potential impact. Dow developed the Dose Adjusted Consequence Analysis (DACA) methodology, which incorporates dispersion modeling as well as the use of probits to estimate lethality thresholds so that the potential consequences of potential events can be more accurately predicted.

Probit functions are used to predict the lethality of a potential release following acute inhalation exposure from a toxic release. Developing probits to use in modeling can be challenging, for obvious reasons. In the absence of human or animal toxicological data, estimates of “pseudo probits” can be used in lieu of values derived from toxicological data.

The analysis makes use of the PHAST^TM and SAFETI^TM programs that uses probits as part of its basic tool set. Probits are not new to the industry, but this paper provides a simple method for estimating pseudo probits for materials that do not have readily available probit data.

An overview of Dow’s DACA methodology will be shared, which enables a more accurate assessment of the potential consequences arising from the potential release of a toxic material.