New CDOIF Risk Assessment Guidance
Understanding Major Hazard Human Harm Risk Assessment (MHHHRA) for COMAH
25th September 2026
What COMAH operators need to know, why the guidance matters and how to prepare
New guidance, Understanding Major Hazard Human Harm Risk Assessment (MHHHRA) for COMAH, has been developed through the Chemical and Downstream Oil Industries Forum (CDOIF) to provide a more consistent approach to assessing major accident risks to people at COMAH establishments.
Following its release, it is expected to become an important industry good-practice reference point for major-hazard human-harm risk assessment across a wide range of COMAH establishments, from relatively straightforward storage facilities to complex chemical processing sites.
Work on the guidance followed industry requests for greater clarification of expectations around Major Hazard Human Harm Risk Assessment. In the absence of a single consolidated approach in some areas, different interpretations and risk assessment practices have developed across industry.
The guidance brings established principles and good practice together within a common framework for operators, risk assessment practitioners and regulators. Its purpose is to support greater consistency in the identification, analysis, assessment and management of major accident hazards affecting people.
What does MHHHRA cover?
Identification: Identify the full range of credible major accident hazard scenarios and, where appropriate, select representative scenarios. This includes the requirement for both ‘Top Down’ assessment of the substance harm potential and ‘Bottom Up’ studies such as HAZOP and HAZID.
Analysis: Understand scenarios in suitable detail, including consequences, initiating events, safeguards, relevant good practice, likelihood and potential risk gaps.
Assessment: Consider the overall and cumulative risk presented by major accident scenarios, including risks to people both on and off site.
Further measures & ALARP: Consider risk tolerability, whether risks have been reduced As Low As Reasonably Practicable (ALARP), and whether further risk-reduction measures are required.
What does this mean for COMAH operators?
The guidance is intended to be relevant across the spectrum of COMAH establishments, but the practical implications will vary according to the site’s hazards, complexity, existing risk assessment arrangements and regulatory tier.
Lower Tier operators
Lower Tier establishments do not have the same COMAH Safety Report requirement as Upper Tier establishments. However, MHHHRA remains relevant to the way major accident hazards and human-harm risks are identified, assessed and managed. Lower Tier operators should therefore consider how well existing major accident risk assessments align with the principles and good practice described by the new framework.
Upper Tier operators
For Upper Tier establishments, the implications may be more immediate, particularly in relation to the preparation, review and revision of COMAH Safety Reports and the demonstration that risks are tolerable and ALARP. Upper Tier operators are required to submit a Safety Report that demonstrates that all measures necessary have been taken to prevent major accidents and limit their consequences. MHHHRA is therefore expected to provide an important reference point for the human-harm risk assessment supporting that demonstration.
You do not necessarily need to start again.
For many operators, the sensible first step is to review existing risk assessments against the new MHHHRA framework and establish where current arrangements already align and where improvements may be needed.
- What already aligns with the new guidance and can be retained.
- Where information, evidence or documentation may need to be strengthened.
- Whether major accident scenarios have been identified and assessed with sufficient breadth and consistency.
- Whether cumulative individual and societal risk has been adequately considered.
- Whether the site’s demonstration of ALARP remains sufficiently robust.
A structured MHHHRA gap assessment can provide a proportionate starting point, allowing improvements to be prioritised according to the site’s hazards, complexity and existing arrangements rather than assuming established assessments need to be recreated from scratch.
A practical preparation pathway
Understand
Establish what the final guidance means for the establishment.
Gap assess
Compare existing major-hazard human-harm risk assessment arrangements with the new framework.
Prioritise
Identify and rank material gaps according to risk, regulatory significance and effort.
Implement
Strengthen assessments, evidence and ALARP demonstrations where required.
A Multidisciplinary Approach Will Be Key
Arthian can support COMAH operators in understanding what the new guidance means for their establishment, assessing existing arrangements against MHHHRA good practice, identifying gaps and developing a proportionate programme of improvement.