Simplified Methods Of Using Probit Analysis In Consequence Modeling

The probit function provides a link between probability of expected response and the total exposure of a population to a specific event.  In the case of chemical facilities, probit analysis can provide an estimate of the percentage of occupants inside of a structure who may have an adverse response to a specific chemical release.  Based upon available data, probit analysis can be used to estimate duration of exposure required for probability of nuisance-level response, loss of consciousness, or even fatal exposures. 

Dispersion modeling tools readily provide data on probability of response to long-term exposure (over one hour).  However, these tools do not define the maximum time that occupants can shelter before exposure inside of the structure may have severe consequences.  This paper provides methodology for determining this critical exposure duration.

While data on probit values for some chemicals is readily available, there is minimal guidance in the open literature on developing estimates of probit constants for compounds where they do not currently exist, or are not published.  This paper presents a methodology for generating probit constant estimates based on existing toxicological data for situations where published probit constants do not exist.  This paper also provides methodology on using existing MS Excel formulas in measuring probits and, along with data from dispersion modeling tools, how to arrive at probability of specified response to a toxic release.

Once the probability is understood, a recommended inclusion of this data into a Layers of Protection Analysis is provided.