Ways of Specifying Risk

The way in which risk is specified is important, both for communicating it to others and also for thinking about it oneself. It is helpful to classify risk into two main types : risk to human life, and all other kinds, such as economic risk or environmental risk. There are various ways of describing risk to human life, and all of them have their place. We will look at three, though note that we are considerably simplifying the issue. Firstly, there is individual risk, the risk to an individual person who might be affected. This could be expressed as lives lost per 10 000 per year,

sometimes it is transformed to a reduction in life span by so many days. One source has given the annual risk of death per 10000 as 3.3 for coal mining and 12 for flying as aircrew. These measures show the general level of risk of an activity or situation.

As a measure of the riskiness of a particular activity to an individual, the fatal accident rate (FAR), sometimes also known as the fatal accident frequency rate (FAFR), is used. This is defined as the number of deaths per , hour of exposure divided by the number of people at risk multiplied by lo8.

The FAR is intended as an instantaneous measure, just as, say, miles per 1 hour is a measure of velocity. It will differ throughout the day for an individual, depending on the activity. For example, FAR values for sleeping, eating, driving a car or riding a motorcycle are 1,2.5,57 and 660 respectively. For work in the chemical industry the FAR value is about 5 and in the construction industry about 67. A locomotive engineer has an FAR of about 13 while driving. However, society is even more concerned with preventing major disasters than it is with providing safety for individuals.

A risk measure reflecting this is the societal risk graph or F-N curve, introduced briefly in the previous section. An example is given in Fig. 11.2. Economic risk can be handled using cost-benefit analysis and expected (i.e. average) values of risk, though the quantification of costs and benefits is often difficult.

However, this approach cannot easily be applied to questions of safety and loss of life because of the moral issues involved, even though a weighing of costs and benefits must be implicit in any risk situation. A limiting risk approach is better for safety issues. It is often more appropriate for environmental risk as well.