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debated. Also, for the same reasons, it is critically important that the criteria are applied cautiously and that decision makers consider them together with traditional deterministic assessments and in light of consultation with the affected public. MAIN FEATURES OF THE 1994 CRITERIA Sub-section 2.7 of the 1994 Guidelines provides the rationale for the risk to life criteria proposed in that document and Guideline G.12 gives the criteria. Individual Risk Individual Risk is the increment of risk imposed on particular persons by the existence of a dam. People live with a background risk of death throughout their lives. Figure 2.4 of the 1994 Guidelines is reproduced from the work of Chicken (1975). It shows that the average background risk of death in Britain, at that time, started at about 2E-02 per annum at birth and declined rapidly to a minimum value of 5E-04 per annum at ages eight through fifteen (later referred to as the "young person" risk). Thereafter the risk increased steadily with age, reaching IE-Olper annum at age eighty for example. Individual Risk criteria aim to limit the increment of risk imposed by a facility such as a dam to a small fraction of the average background risk level for young people in the age bracket eight to fifteen. The Individual Risk criteria were expressed as a Limit of tolerability (higher risks are unacceptable in all circumstances) and an Objective that owners of dams should aim to meet (lower risks are negligible and are generally acceptable). Between these bounds the acceptability of risk was to be judged according to the ALARP (As Low As Reasonably Practicable) principle. The ALARP principle basically states that risks should be regarded as tolerable only if risk reduction is impracticable or if the cost is grossly disproportionate to the improvement gained. Consideration of ALARP is meant to be a stem test and is well described by HSE (1992). In the 1994 Guidelines, the criteria required that new dams, and modifications of existing dams, meet the Objective risk levels. Thus, for existing dams, the ALARP question was: "Is it impracticable or disproportionately costly to reduce the risks of this dam to the Objective level, given that the present risk level is lower than the Limit value?" If the answer was "yes", the existing situation would be acceptable. If the answer was "no", the risk was to be reduced to the Objective level. If the existing risk was higher than the Limit value, the risk was to be reduced to the Objective value and ALARP did not enter into consideration. Individual Risk is the total increment of risk imposed on individuals by a dam. Thus the risks contributed by all failure modes and scenarios need to be combined to obtain the overall risk. The chance of death per annum contributed by each flood and earthquake failure mode, is the product of: . the annual probability of the flood or earthquake event . the conditional probability of dam failure, given the event . the conditional probability of the individual's death, given dam failure ~gaFcli~l1c:e o!aeat!!.Eontr1Diif~Q'li.rraillms'iillde:C~f!::3I'~eratii;g~~iti?.ll~.::.i=- the 'product Qf:7 ~~~.l!~J;~r.o~ability?~ ~.~an~erou: c~~~i~ni s~~~s. pip~~~ o.:a .banks.lide, occurrin? . "_ !7.'"~.tl!e.s:.<>r\(httonal probablhty ofllam fallure, gIVen the plpmg or the shde or othenlangerons congltt.o]tf t. tl1.eConrlitional probability of the individual's death, given damfailllr.:.J ...... ...._.-::.:7 A difficulty arises with the estimation of Individual Risk where only one aspect of dam safety, such as spillway adequacy, is under examination, The updated guidelines will provide some suggestions for dealing with such situations in a practical way.