COMAH is an abbreviation for the Control of Major Accident Hazards Regulations 1999, as amended in 2005. They implement Council Directive 96/82/EC known as the Seveso II Directive (as amended by Directive 2003/105/EC), with the exception of the land-use planning requirements (which are implemented by changes to planning legislation.)

The aim of COMAH is to prevent (and mitigate) the effects of major accidents involving dangerous substances that can cause serious harm to people and damage to the environment.

A Major accident is defined in the regulations as:

“an occurrence (including in particular, a major emission, fire or explosion) resulting from uncontrolled developments in the course of the operation of any establishment and leading to serious danger to human health or the environment, immediate or delayed, inside or outside the establishment, and involving one or more dangerous substances.”

COMAH is enforced by a “Competent Authority”. In England and Wales this is the Health and Safety Executive (HSE) and the Environment Agency, whilst in Scotland it is the HSE and the Scottish Environment Protection Agency (SEPA).