Accidental fire or explosion at an onshore major hazard (COMAH) site

Impact 5
upper risk error bar
upper likelihood error bar risk indicator
lower likelihood error bar
lower impact error bar
Impact & Likelihood
Impact key
5 Catastrophic
4 Significant
3 Moderate
2 Limited
1 Minor
Likelihood key
5 >25%
4 5-25%
3 1-5%
2 0.2-1%
1 <0.2%


This risk involves a Control of Major Accident Hazards (COMAH) site that produces, stores or uses significant amounts of flammable or explosive substances. There are two types (tiers) of establishment that are subject to COMAH, known as ‘Upper Tier’ and ‘Lower Tier’. Upper Tier establishments hold greater quantities and/or more dangerous substances compared to Lower Tier establishments, meaning that additional requirements are placed on them. Operators of these sites have a legal duty to prevent accidents from occurring and to mitigate their consequences. The Health and Safety Executive develops and enforces legislation, standards, codes of practice and guidance to ensure that operators fulfil these responsibilities effectively.


The reasonable worst-case scenario for this risk concerns a major fire and/or explosion occurring at an onshore COMAH site, potentially causing building damage and possible collapse close to the site. The fire would generate a visible plume of smoke that may travel to nearby areas. The accident could result in casualties and fatalities. Other impacts include short-term local transport disruption and economic impacts in the order of hundreds of millions of pounds.

Key assumptions

The incident is accidental and occurs at a large industrial complex storing or using flammable substances. The incident produces a cloud of gas or vapour or a spill of flammable liquid, which ignites causing a fire or explosion.


The impacts of the scenario will vary depending on the number of people working on site at the time, how far the nearest population is, the time of day, how long the event lasts, what the site is used for and volume and type of substances involved.

Response capability requirements

Capability requirements would include temporary evacuation and shelter for displaced people. There would be a need for specialist treatment, surge capacities and appropriate recovery and storage for no-notice mass fatalities and casualties. As a result of rubble and debris, the site would need to be cleaned up and possibly decontaminated and the response would involve search and rescue teams.


The health effects arising from exposure to the effects of fire and explosion are likely to be acute but some will continue beyond 5 years. Psychological support may need to be made available to those affected. The duration of environmental contamination could be short (less than one year) or long term (more than 5 years), depending on the site location and inventory.