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Control of Industrial Major Accident Hazards (CIMAH) Report Page 15
Ranhill SAJ Sdn. Bhd. Revision 01
Part D: Information on the Potential Major Accident Date Aug 2019

For this study, the Probit constants used are a = -6.35, b = 0.5 and c = 2.75. The
constants are from the database of substances in the ‘Purple Book’. The Probit value
can be calculated for each potential dose and then converted to a probability of
fatality using the following relation where Pr is the Probit value and ERF is the
mathematical error function:

Table D.2.5.1 shows the fatality probabilities with corresponding Probit value and
concentration of chlorine for fifteen (15) minutes of exposure.

Fatality Concentration (15 minutes)
Probit
Probability mg/m ppm
3
5.00 50 % 1436 495
8.09 99.9 % 14000 4827

Table D.2.5.1: Fatality Probabilities

It is very important that the full spectrum of casualties is calculated, not only for risk
evaluation, but also for emergency planning purposes. Therefore, dose that relates
to a value considered Immediately Dangerous to Life and Health (IDLH) which is 10
ppm (US NIOSH) taken into account for consequence analysis.

It is assumed that the population exposed to the most vulnerable condition, i.e. they
are staying outdoor (indoor concentration may be lower) in the toxic gas cloud within
the breathing zone and cannot move out of the way, exposed for fifteen (15) minutes
before the cloud passes by.