Acceptable levels of risks

Very high levels of perceived risk are associated with actions to reduce risk - when people think the risk of the volcano erupting is too high, they move. At some level the risk becomes unacceptable. What level constitutes an acceptable risk is always a complex issue. It is a matter of political discussion and public comfort. The concept of risk tolerance and the thresholds of unacceptability are what determine, ultimately, whether public money is voted through for a flood dike project or whether people comply with building regulations to make their houses earthquake resistant.

Many risks are also associated with benefits. Living close to a volcano may bring the benefit of fertile igneous soils for good agriculture. The risks associated with chest X-rays and driving to work are generally considered acceptable because the benefits are immediately obvious. Generally, the exposure to natural and environmental hazards does not have any specific benefit associated with it: the exposure is a simple consequence of living or working in a particular location. This can have the effect of making such risks less acceptable than those from which some benefit is obtained. Generally the acceptable levels of risk appear to increase according to the benefits derived from being exposed to it.

Some risks are entered into voluntarily and a distinction is sometimes made between voluntary and involuntary risk. Many recreational activities and sports, involve considerable levels of personal risk entered into voluntarily. Indeed the thrill of the risk is part of the enjoyment of the recreation. The benefits of the risk outweigh the costs and so the perception of the risk is reduced; i.e. the level that is deemed acceptable is much higher than a risk that is imposed from outside or involuntary.

Studies of what people actually do about risks - accepted levels of risk in society - have been carried out to try to derive an understanding about the acceptability of risk.¹⁰ An example from the U.S. is shown in figure 3.

The figure suggests three things. First it indicates that the level of risk accepted increases with the benefit involved. Secondly that tolerance of so-called voluntary risks may be as much as 1,000 times higher than that of involuntary risks. Thirdly it suggests that the background risk of death from disease in society as a whole may provide a yardstick from which acceptability of involuntary risks may be judged. Subsequent research has shown that this notion is rather simplistic and that acceptable levels of risk are rather more complex to determine, but the research does identify some important factors involved.

Another concept derived from research studies is that of the comparability of risks: the notion is that classes of similar risks may have approximately the same level of acceptability. Thus the acceptable level of risk from wind hazards would be expected to be similar to the acceptable level of risk from floods, but need not necessarily be comparable with transportation risks, for which other value systems would operate.

The important point to emphasize from this discussion is that the judgement that a risk is acceptable is not something that depends on actual risk level so much as subjective determination using value judgements. Factual information about risk, if it is believed, can affect the acceptability of a risk.