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Control of major accident hazards (COMAH)

Control of major accident hazards (COMAH)

The COMAH  Regulations are applicable to any establishment storing, or otherwise handling, large quantities of chemicals or substances of a hazardous nature, including production facilities, warehouses, and some distributors.

This section outlines our main duties and explains what operators need to do if the regulations apply to them.

In Scotland the competent authority is SEPA and the Health and Safety Executive (HSE), or SEPA and Office for Nuclear Regulation (ONR)  at nuclear sites. There are separate competent authorities in England and Wales comprising Environment Agency (EA)  and HSE or ONR, and Natural Resource Wales (NRW)and HSE or ONR respectively.

What are the COMAH regulations for?

The principal aim of the regulations is to reduce the risks of potential major accidents involving dangerous substances, such as toxic substances (eg chlorine), flammable substances (eg liquefied petroleum gas), substances that are environmentally hazardous, and explosives.

If dangerous substances are used or stored at the site in quantities above certain thresholds, COMAH requires operators to take all measures necessary to prevent major accidents and limit the consequences for human health and the environment.

A major accident could involve a release of substance, fire or explosion resulting from uncontrolled developments involving one or more dangerous substance that causes serious danger to human health or the environment, whether immediate or delayed, inside or outside the site.

As part of Scotland’s competent authority, it is SEPA’s duty to ensure that business and industry adhere to the regulations, and to investigate and report on any major incidents.

Do the COMAH regulations apply to me?

The regulations mainly apply to the chemical and petrochemical industries, but also some storage and distribution activities, alcohol production and storage (whisky maturation), explosives and other industries (eg agrochemicals, aerosols), where dangerous substances identified in the regulations are kept or used in quantities above the prescribed thresholds.

Dangerous substances covered by the COMAH Regulations include named substances (eg hydrogen, ammonium nitrate), and generic categories with:

  • health hazards, including acute toxics;
  • physical hazards, including explosives and flammable liquids and gases;
  • environmental hazards, acute and chronic hazards to the aquatic environment;
  • others that react with water, including those that evolve toxic or flammable gases.

A full list of dangerous substances and relevant threshold quantities can be found in Schedule 1 of the COMAH Regulations 2015, which is separated into Part 1 (generic categories) and Part 2 (named dangerous substances).

There are two thresholds for dangerous substances under COMAH. These threshold quantities vary for different categories of substances.

If you store or use more than the lower threshold for a dangerous substance your site is classed as a lower tier establishment. If you store or use more than the higher threshold your site is an upper tier establishment.

The exact requirements differ according to the size of your operation, however all operators must do the following:

  • notify  the competent authority, of the basic details or your operation, such as the address of the establishment, site activities and the dangerous substances on site (a breakdown is required for petroleum products);
  • prepare a major accident prevention policy (MAPP); and
  • develop a safety management system (SMS).

In addition, upper-tier operators must:

  • prepare a safety report and update it every five years or following any significant changes or new knowledge about safety matters;
  • prepare and test an internal emergency plan for the site;
  • supply information to the local authority for external emergency planning purposes;
  • provide certain information to the public about the activities.

Further information is available from the HSE and Netregs

What guidance is available?

The HSE hosts most of the applicable guidance; however, the following guidance is specifically relevant for assessing the environmental hazards or impacts of major accidents.

Guidance on assessing the risks associated with "Natural Hazards Triggering Technological Disasters" (Natech), including those that could result from a changing climate, is available to download from the Process Safety Forum's website:

In addition, specific guidance relating to secondary and tertiary containment is available:

The Scotland’s environment websiteis a useful source of information to assist in the preparation of your environmental risk assessment.

Contact us

If you have any questions or require any further information or advice on any aspect of COMAH, please contact us.