Last edit: 02/08/2023
The Alarp principle recognizes that there are three broad categories of risks:
1. Negligible risk: Broadly accepted by most people as they go about their everyday lives, these would include the risk of being struck by lightning or of having brake failure in a car.
2. Tolerable risk: We would rather not have the risk but it is tolerable in view of the benefits obtained by accepting it. The cost in inconvenience or in money is balanced against the scale of risk and a compromise is accepted. This would
apply to traveling in a car, we accept that accidents happen but we do our best to minimize our chances of disaster. Does it apply to bungee jumping?
3. Unacceptable risk: The risk level is so high that we are not prepared to tolerate it. The losses far outweigh any possible benefits in the situation.
One of the difficult aspects in implementing the ALARP principle is to define the three level of risks. Here the approach from HSE (Health and Safety Executive, in UK).
HSE believes that an individual risk of death of one in a million per annum for both workers and the public corresponds to a very low level of risk and should be used as a guideline for the boundary between the broadly acceptable and tolerable regions.
Nevertheless, in our document on the tolerability of risks in nuclear power stations, we suggested that an individual risk of death of one in a thousand per annum should on its own represent the dividing line between what could be just tolerable for any substantial category of workers for any large part of a working life, and what is unacceptable for any but fairly exceptional groups. For members of the public who have a risk imposed on them ‘in the wider interest of society’ this limit is judged to be an order of magnitude lower – at 1 in 10 000 per annum.
Essentially this principle guides the design engineer and the safety specialist into setting tolerable risk targets for a hazardous situation. This is the first step in setting up a standard of performance for any safety system.
Here an example on the application of the ALARP Principle.
A £1,000,000 cost per life saved target is used in a particular industry. A maximum tolerable risk target of 10-4 pa has been set for a particular hazard which is likely to cause 2 fatalities. The proposed system has been assessed and a predicted risk of 8×10-5 pa obtained. Given that the negligible risk is taken as 10-6pa then the application of ALARP is required. For a cost of £3,000, additional instrumentation and redundancy will reduce the risk to just above the negligible region (2×10-6 pa). The plant life is 30 years.
1.000.000 × (8×10-5 – 2×10-6) × 2 × 30 = £4680 > £3.000. Therefore the proposal should be adopted.