The relationship between risk of death and risk of dangerous dose for toxic substances

The relationship between risk of dangerous dose or worse and risk of 'death' (expressed as LD50 or worse, or LD95 or worse) has been studied for acutely toxic substances using the Health & Safety Executive's (HSE) Risk Assessment Tool, RISKAT. The analysis was carried out for chlorine, ammonia, hydrogen fluoride and sulphur dioxide, at a range of major hazard sites. Consistent relationships, of the form y = x(a)/b, were found between individual risk of dangerous dose or worse and individual risk of 'death'. These were y = x(1.11)/2.59 for LD50 and y = x(1.23)/5.62 for LD95, where y is the risk of 'death' and x is the risk of dangerous dose or worse. It is thought that the form of this relationship arises from the way hazard ranges vary with distance and the way in which large releases of toxic material tend to have lower frequencies of occurrence than smaller releases. The existence of a consistent relationship between different substances is thought to be due to the way in which HSE selects the toxicological data it uses and the fact that all of the substances studied have a similar mechanism of toxic action.