ALARP
1 It provides a framework for evaluating risk (i.e. deciding on need for reducing risk). This requires explicit description and analysis of risk tolerability.
2 It involves a method of determining if the cost of a risk reduction measure is disproportionate to the benefits it will provide, and hence if the measure should be implemented.
When using the ALARP principle, the risk is divided into three levels, as shown in Figure 5.1 (HSE 2001):
1 An unacceptable region, where the risk is intolerable except in extraordinary circumstances, and risk reduction measures are mandatory.
2 A middle band, or ALARP region, where risk reduction measures are desirable but may not be implemented if their cost is grossly disproportionate to the benefit gained.
3 A broadly acceptable region, where no further risk reduction measures are needed. In this region, further risk reduction is uneconomical and resources could be spent better elsewhere to reduce the total risk.
We thus have to state two risk limits: an upper limit (i.e. between the unacceptable region and the ALARP region) over which the risk cannot be justified on any grounds, and a lower limit (i.e. between the ALARP region and the broadly acceptable region) under which the risk is considered generally acceptable.
If we look back at the three “pure” principles of risk acceptance, ALARP incorporates all three of these:
The equity principle is incorporated through the upper limit above which risk is unacceptable.
The utility principle is incorporated in the ALARP region, where risk is accepted if the cost of reducing risk is too high.
The technology principle is in practical use incorporated in the ALARP region because there is an expectation that “good practice” is followed when decisions are made regarding what measures to implement and not.
The ALARP principle was introduced in the framework on tolerability of risk (TOR) from UK nuclear stations (HSE 1992), and has subsequently been adapted for general applications in HSE (2001).
The ALARP principle is relevant primarily for risk to humans. The vertical axis in Figure 5.1 (not shown) is thus a measure of individual risk, such as AIR. The UK HSE has defined the upper and lower limits for AIR in Table 5.1 .
Table 5.1 ALARP limits in the United Kingdom.
| Limit | Probability per year | For whom | |
| Upper | 1 in 1000 |
|
The workforce |
| 1 in 10 000 |
|
The public (for existing industrial plants) | |
| 1 in 100 000 |
|
The public (for new industrial plants) | |
| Lower | 1 in 1 000 000 |
|
The public |
The risk is usually in the ALARP region, which is the region where the risk must be reduced to an ALARP level. The principle as such does not define what is “reasonably practicable.” Additional guidance is therefore required. An important first principle in ALARP is that risk reduction measures should be implemented, unless there are good arguments for not doing so. This means that if no evidence is available to show that a risk reduction measure is not practicable, it should be implemented. Some other aspects that are important are the following:
1 Good practice should be adhered to when deciding on what risk reduction measures to implement. The argument that it is not practicable (due to cost or other reasons) to implement a measure cannot be used if it is common practice to do it. Good practice can be found in standards, industry guidelines, regulations, and other types of documents.
2 Systematic identification of possible risk reduction measures should be undertaken. This needs to be based on a good understanding of the accident scenarios and how they can be prevented or mitigated.
3 The severity of the hazardous event in question. The more serious the event is, the more is expected to be done to reduce risk.
4 The state of knowledge about the hazardous event, and the availability and suitability of ways of preventing or mitigating its effects. If it is practically difficult to implement measures to reduce risk, this can be taken into account.
5 The cost of preventing the hazardous events or mitigating its effects.
In practical application of the ALARP principle, there tends to be focus on the final item in this list and how cost–benefit analysis supports decisions about whether to reduce risk or not. The other points clearly show that many other aspects need to be considered and that cost–benefit assessment by no means is the only input into the decisions.
5.3.1.1 Cost–Benefit Assessment
ALARP implies that a cost–benefit approach can be adopted to decide what constitutes a “practicable” level of risk. In the ALARP region, the notion grossly disproportionate is central. It requires that a risk reduction measure be implemented if the cost of the measure is not grossly disproportionate to the benefit gained. A disproportion factor
(5.2)
The cost of the risk reduction measure is an estimate of the total cost, which covers not only purchase, installation, and training but also cost implications related to the operation of the system, such as reduced productivity. If reduced productivity has a strong influence on a decision not to implement, the company should show that phasing or scheduling the work to coincide with planned downtimes (e.g. for maintenance) would not change the balance. In the cost estimation, it is common to take a societal view on the costs. For a company, cost associated with accidents can often (at least partially) be covered by insurance. There may also be other effects that reduce the net cost. For society at large, this is not relevant, because insurance is just a redistribution of the costs on many instead of just one.
The
