The Control of Major Accident Hazards Regulations 2015 (COMAH) apply in Great Britain where dangerous substances are present at or above specified qualifying quantities. Sites are classified as lower-tier or upper-tier establishments depending on the substances and quantities involved. Operators must take all necessary measures to prevent major accidents and limit their consequences for people and the environment.
COMAH stands for the Control of Major Accident Hazards Regulations 2015.
The Regulations came into force on 1 June 2015 and replaced the previous COMAH 1999 regime.
COMAH implements most of the requirements of the Seveso III Directive in Great Britain and is intended to prevent major accidents involving dangerous substances and limit their consequences where accidents do occur.
The Regulations apply in England, Scotland and Wales. Northern Ireland has separate COMAH legislation.
No. Storing hazardous substances does not automatically make a site a COMAH establishment.
COMAH applies where dangerous substances covered by Schedule 1 of the Regulations are present, or are liable to be present, at or above the relevant qualifying quantities.
Whether a site falls within COMAH therefore depends on:
A complete inventory of relevant dangerous substances is therefore essential when determining whether COMAH applies.
Schedule 1 of the COMAH Regulations identifies the dangerous substances and categories of dangerous substances used to determine whether an establishment falls within scope.
It contains:
Schedule 1 specifies separate qualifying quantities for lower-tier and upper-tier establishments.
Where a specifically named substance is also covered by a general hazard category, the qualifying quantities assigned to the named substance are used.
COMAH status is not always determined simply by checking whether one substance exceeds one threshold.
Where several dangerous substances are present below their individual qualifying quantities, the Schedule 1 addition rule may still bring the establishment within COMAH.
Operators should therefore consider:
Sites close to a threshold should obtain competent advice if there is uncertainty about the calculation.
COMAH establishes two levels of establishment according to the quantities of dangerous substances present.
An establishment becomes lower-tier where the applicable lower-tier qualifying quantities in Schedule 1 are reached.
Lower-tier operators are subject to a number of duties including notification, major-accident prevention arrangements and the general duty to prevent major accidents and limit their consequences.
An establishment becomes upper-tier where the higher qualifying quantities specified in Schedule 1 are reached.
Upper-tier operators are subject to additional requirements because greater quantities of dangerous substances are involved.
These include preparation of a safety report and internal emergency plan, together with additional information and emergency-planning duties.
All operators of establishments within COMAH must take all measures necessary to prevent major accidents involving dangerous substances and to limit their consequences for people and the environment.
General duties include:
The measures required should be proportionate to the hazards and risks associated with the establishment.
A Major Accident Prevention Policy, commonly known as a MAPP, sets out how an operator intends to prevent major accidents and limit their consequences.
The MAPP should address the operator’s overall aims and principles of action relating to major-accident control.
The associated safety management arrangements should cover areas including:
Lower-tier establishments are required to prepare and retain a MAPP. Upper-tier establishments must also have a MAPP, with the required information forming part of their wider safety-report arrangements.
Operators of upper-tier establishments must prepare a safety report demonstrating that major-accident hazards have been identified and that suitable prevention and mitigation measures are in place.
The safety report includes information about matters such as:
The purpose is not simply to produce documentation, but to demonstrate that major-accident risks are being systematically understood and controlled.
An upper-tier safety report must be kept up to date.
It should be reviewed when significant changes, new information or other circumstances could affect the major-accident risk profile.
HSE also requires the safety report to be reviewed after five years even where no significant change has occurred.
Where the review shows that revisions are required, the report must be updated, and the Competent Authority notified in accordance with the Regulations.
Upper-tier establishments must prepare an internal emergency plan for dealing with the on-site consequences of major accidents.
The plan should address credible major-accident scenarios, including low-probability events capable of producing serious consequences.
Emergency arrangements may include:
Local authorities also have responsibilities relating to external emergency planning for upper-tier establishments.
Chemical storage can be a significant part of a COMAH establishment’s major-accident prevention and mitigation arrangements.
Storage design may need to consider:
The appropriate measures should arise from the site’s major-accident hazard assessment rather than from a generic specification applied to every COMAH establishment.
No. COMAH does not prescribe one universal chemical store design suitable for every establishment.
Operators must instead demonstrate that they have identified their major-accident hazards and adopted all necessary prevention and mitigation measures.
The required storage specification may therefore vary considerably depending on the substances, quantities and scenarios involved.
Depending on the site, features could include:
The actual specification should be determined from the site’s risk assessment and major-accident scenarios.
No. COMAH compliance applies to the establishment and its management of major-accident hazards, not simply to an individual storage product.
A storage facility can form an important part of the prevention and mitigation measures at a COMAH establishment, but compliance can also involve:
Claims that an individual chemical store alone makes a site “COMAH compliant” should therefore be avoided.
COMAH and DSEAR address different but potentially overlapping risks from dangerous substances.
DSEAR applies broadly to workplace risks involving dangerous substances, including fire and explosion.
COMAH applies only when specified dangerous substances are present at or above the qualifying quantities in Schedule 1 and addresses major-accident hazards affecting people and the environment.
A COMAH establishment may therefore also have duties under DSEAR.
See our DSEAR Regulations 2002 guide for further information.
COSHH principally addresses risks to health from exposure to hazardous substances, while COMAH addresses major accidents involving dangerous substances.
The same substance can therefore give rise to duties under both regimes.
For example, routine employee exposure to a hazardous chemical may need to be controlled under COSHH, while a sufficiently large inventory could also contribute to COMAH status and major-accident scenarios.
See our COSHH Regulations guide for further information.
COMAH is enforced by a joint Competent Authority.
For most establishments this consists of HSE together with the relevant environmental regulator:
For nuclear establishments, the Office for Nuclear Regulation may perform the health and safety role alongside the relevant environmental regulator.
The Competent Authority inspects COMAH establishments and assesses the arrangements operators have put in place to prevent and mitigate major accidents.
Yes. HSE maintains a public information system for operational establishments subject to the COMAH Regulations.
The public information includes details supplied by operators about the establishment, its activities, dangerous substances and measures for controlling major-accident hazards.
Upper-tier establishments are required to provide additional information reflecting their greater major-accident potential.
The Control of Major Accident Hazards Regulations 2015 discussed on this page apply in Great Britain.
Northern Ireland has separate legislation: the Control of Major Accident Hazards Regulations (Northern Ireland) 2015.
In Northern Ireland, a joint Competent Authority enforces COMAH, comprising the Health and Safety Executive for Northern Ireland and the Northern Ireland Environment Agency.
Businesses operating in Northern Ireland should therefore check the HSENI requirements applicable to their establishment.
Chemstore designs and manufactures hazardous material storage solutions that can form part of the prevention and mitigation measures identified by COMAH operators and their competent advisers.
For larger or more complex inventories, Chemstore provides bespoke chemical warehouses that can be configured to site-specific requirements, including segregation, bunding, fire-rated construction, ventilation, temperature control, monitoring, and access arrangements.
For flammable inventories, the Firevault range provides dedicated fire-rated hazardous material storage options.
The required storage specification should be determined from the dangerous substances, quantities, credible accident scenarios and the site’s wider major-hazard assessment.
Chemstore storage equipment does not replace the operator’s duties under COMAH and should not be treated as making an establishment automatically compliant.
The Health and Safety Executive and UK legislation website provide the primary sources for COMAH requirements in Great Britain.
This page provides general information about the Control of Major Accident Hazards Regulations 2015 in Great Britain. It does not replace the Regulations, HSE guidance, Schedule 1 calculations, a Major Accident Prevention Policy, safety report, site-specific risk assessment or advice from a competent person.
Whether COMAH applies, the tier of an establishment and the measures required should always be determined from the dangerous substances, classifications, quantities, processes and major-accident hazards associated with the individual site.
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