COMAH Overview

The main aim of the COMAH regulations is to prevent, control and mitigate the effects of accidents involving chemicals that could cause serious harm to people and/or the environment. Typical types of Major Accidents that are covered by COMAH include fires, explosions, toxic gas releases, and leaks of hazardous substances to rivers and land.

The COMAH Regulations are based on the EU Seveso Directives, and have been in force since 1999, when they replaced CIMAH, the existing UK major accident hazard legislation.  The regulations were significantly updated in 2005 to align with Seveso II, and in 2015 they have been changed again to meet the requirements of Seveso III.  This takes into account the change in hazardous substance classification from CHIP to CLP, and also brings in some other changes such as new Named Substances.

There are two levels of COMAH site: Lower Tier sites, which hold a smaller hazardous inventory; and Upper Tier sites, which have larger hazardous inventories, and are considered to be more potentially hazardous.  The regulatory requirements are more stringent for Upper Tier sites.

A new requirement, from the EU, is that all COMAH sites must provide limited information to a public database held by the HSE.  The HSE have written an online system which ensures that UK sites enter the minimum information in a standardised way.

As it is recognised that the change from Seveso II to Seveso III is quite complicated, the COMAH regulations include a phase-in period running from 1st June 2015 to 31st May 2016.

The phase-in period is to enable all existing COMAH sites to re-notify their COMAH status to the HSE, as required in COMAH 2015.  It is anticipated that, due to changes in classification thresholds caused by the move from CHIP to CLP, some sites may drop out of COMAH or change status between Lower and Upper Tier.

During the phase in period, there are several deadlines which define when new COMAH entrants should notify HSE.  Briefly, this depends on the reasons why a site is entering COMAH.  For more details see our COMAH Notification page.

TT Environmental’s COMAH consultancy and advice can help you:

  • Work out whether you are COMAH-liable
  • Devise strategies to help you keep out of COMAH
  • Help compile your Lower Tier or Top Tier Application
  • Carry out environmental consequence modelling for incidents such as spillage, fire and flood
  • Write COMAH Environmental Risk Assessments.

If you need COMAH advice, contact Janet on 01422 24 22 22 or email Janet.

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COMAH Factsheet

This 10 page pdf is free to download (168kb).
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SevaluateTM is an Excel based program which runs your COMAH (Seveso III) calculations for you, provides printable reports showing the main contributors to your COMAH liability, and allows you to predict the impact of changing your COMAH inventory on your COMAH liability.
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COMAH Liability Toolkit toolkit

The COMAH liability toolkit contains 5 useful documents:
  • COMAH Factsheet
  • COMAH Calculation Factsheet
  • Top 16 COMAH calculation mistakes
  • When to notify under COMAH 2015-16
  • Should you control your inventory to avoid COMAH or stay in Lower Tier?
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External Links

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COMAH Regulation 2015

This pdf is housed on It is 44 pages long and the pdfs size is 268 kb.
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Seveso III Directive

This pdf is housed on It is 37 pages long and the pdfs size is 1 mb.
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HSE COMAH guidance

HSE landing page for Control of Major Accident Hazards (COMAH).

HSE public database information

These notes are for operators of COMAH establishments and describe the arrangements for making certain information available to the public. They explain what is happening to ensure provision of this information, when and how that will happen and what operators will need to do.