Control of Major Accident Hazards Regulations 1999. The EC Directive 96/82/EC (the so-called Seveso II Directive) has been implemented in Great Britain as the Control of Major Accident Hazards Regulations (1999), known as COMAH [1]. Application of the Regulations depends on the quantities of dangerous substances present (or likely to be present) at an establishment. Two levels (or 'tiers') of duty are specified within the Regulations, corresponding to two different quantities (or thresholds) of dangerous substances. Sites exceeding the higher, 'upper tier' thresholds are subject to more onerous requirements than those that only qualify as 'lower tier'. The Regulations contain a general duty (Reg. 4), which is applicable to both lower tier and upper tier establishments: "Every operator shall take all measures necessary to prevent major accidents and limit their consequences to persons and the environment". HSE have provided the following interpretation of this general duty: "By requiring measures both for prevention and mitigation, the wording of the duty recognises that risk cannot be completely eliminated. This in turn implies that there must be some proportionality between the risk and the measures taken to control the risk". Amongst the duties placed on upper tier sites is the requirement to produce a Safety Report. One of the purposes of the Safety Report is to provide a demonstration that the measures for prevention and mitigation employed by the establishment result in a level of risk that is as low as reasonably practicable (ALARP)