Accidental large toxic chemical release from 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 is concerning an incident in which a toxic gas is accidentally released from a Control of Major Accident Hazards (COMAH) site in the UK. There are two types (tiers) of establishment that are subject to COMAH Regulations, known as ‘Upper Tier’ and ‘Lower Tier’ Upper Tier establishments hold greater quantities of 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 operators fulfil their responsibilities effectively.


The reasonable worst-case scenario is based on an accidental large release of toxic chemical gas from an onshore COMAH site. The release may involve one of a number of hazardous chemicals and would not necessarily result in a fire or explosion. The site would be located near an urban area and could result in fatalities and casualties. There would also be some long-term health impacts to casualties, with some vulnerable groups disproportionately affected.

Key assumptions

This scenario assumes that the incident is accidental and that toxic chemicals would be released as a gas at a large industrial complex or bulk chemical storage site near an urban area.


The impacts of this event would depend on several factors, including the location of the site, the type and volume of gas released, the weather conditions, time of day and individual human responses to exposure. Other variations involve a larger toxic chemical release, which would generate similar impacts but on a larger scale, or a lower scale release of toxic gas with smaller impacts.

Response capability requirements

Site-specific risk assessments are carried out by the site operator, as this is a legal requirement under the COMAH regulations. These assessments inform specific capability requirements locally. Responders will require personal protective equipment (PPE) for recovery of no-notice mass fatalities and to treat no- notice mass casualties. Some temporary evacuation and shelter arrangements may also be required, as well as remediation and potential decontamination of the local environment.


The health effects arising from exposure to the toxic gas are likely to be acute, but some will continue beyond 5 years where chemicals pose longer-term hazards to health. Psychological support may need to be made available to those affected.