Box 30A Societal risk criteria: the Dutch national limits

Societal risk limits provide a convenient tool for setting criteria for the acceptability of hazardous activities to society as a whole rather than just individuals who may be affected. They take account of the fact that, for a particular industrial installation, a whole range of accidents are possible, each with a probability of happening and a range of possible impacts on the surrounding population. As with any hazardous event, a higher probability of occurrence can be tolerated for those events with lower resulting impacts. As the consequences become more serious the 'acceptable' frequency of occurrence becomes lower.

Figure 30.1 illustrates the societal risk limits, quantified in such terms, which have been adopted by The Netherlands. They specify, for example, that the probability of an accident resulting in ten deaths must not exceed 1 in 100 000 years (or 10^-5 per year). A higher frequency is unacceptable. A frequency of 1 in 10 million years (10^-7 per year) or less is negligible. An n-times larger impact should correspond with an n-squared times smaller probability of occurrence. Thus, the maximum permissible frequency for 1000 deaths is 10^-9 per year, while a frequency of 10^-11 or less is considered negligible.

A frequency in between these values is considered to warrant investigation of options to reduce the frequency. This large 'grey' area represents a margin between acceptability and unacceptability, to allow for the uncertainties associated with risk assessments, to allow for the consequences of cumulative exposure and to allow the two levels to be distinguished properly. This is in accordance with the overall approach taken towards setting risk limits in The Netherlands ­ the negligible risk level is considered to be 1 per cent of the maximum permissible level.