& 
The Control of Major Accident Hazards Regulations 2015 (COMAH) aim to prevent major accidents involving dangerous substances, such as fires, explosions and toxic releases, and to limit their consequences for people and the environment. They implement the Seveso III Directive and came into force on 1 June 2015. Disasters such as Buncefield in 2005 show why they matter.
Who do they apply to?
COMAH applies to establishments where dangerous substances are present, or likely to be present, at or above the threshold quantities in Schedule 1. These include chemical plants, fuel and oil storage depots, gas storage sites, some warehouses and distribution sites, and explosives and pharmaceutical sites. There are two tiers depending on the quantities held.
Lower tier and upper tier
| Duty | Lower tier | Upper tier |
|---|---|---|
| Take all measures necessary to prevent major accidents and limit their consequences | Yes | Yes |
| Notify the competent authority | Yes | Yes |
| Major accident prevention policy (MAPP) | Yes | Yes, implemented through a safety management system |
| Safety report, reviewed at least every five years | No | Yes |
| Internal emergency plan, tested | No | Yes |
| Information for the local authority’s external emergency plan | No | Yes |
| Public information | Yes | Yes, more detailed |
| Report major accidents to the competent authority | Yes | Yes |
The competent authority
COMAH is enforced by a competent authority made up of HSE and the environment agency for the area: the Environment Agency in England, Natural Resources Wales or the Scottish Environment Protection Agency. They inspect sites, assess safety reports and can prohibit operations.
Process safety leadership
Major accidents usually come from failures in process safety management rather than everyday hazards, so low lost time injury rates can hide serious risks. Senior leaders need leading indicators for the critical controls, such as alarms, trips, containment and permits. Our guides to DSEAR and permits to work cover related controls.
The ACoP and official guidance
| Publication | Status | What it covers |
|---|---|---|
| L111 A guide to the Control of Major Accident Hazards Regulations (COMAH) 2015 (3rd edition, 2015) | Guidance on Regulations, not an ACoP | Scope and thresholds, duties for lower and upper tier operators, safety reports, emergency plans and enforcement. |
An Approved Code of Practice (ACoP) has special legal status under section 16 of the Health and Safety at Work Act. If you are prosecuted and did not follow it, you must show that you met the law in some other way that was just as good. Guidance on Regulations has no special legal status, but inspectors refer to it and following it is normally enough to comply. Our guide to UK health and safety legislation lists every law and its ACoP or guidance in one place.
Training that helps
The NEBOSH National General Certificate and the NEBOSH International General Certificate cover major hazard and process safety principles. For senior leaders, the NEBOSH Health and Safety Leadership Excellence certificate and IOSH Leading Safely focus on leading safety from the top.
Frequently asked questions
What does COMAH stand for?
Control of Major Accident Hazards.
What is the difference between lower tier and upper tier COMAH?
It depends on the quantity of dangerous substances held. Upper tier sites have extra duties, including a safety report and an internal emergency plan.
Who enforces COMAH?
A competent authority made up of HSE and the relevant environment agency.
Is there an ACoP for COMAH?
No. HSE publishes L111, which is guidance on the regulations.
Read the source
The Control of Major Accident Hazards Regulations 2015 and HSE L111, A guide to the Control of Major Accident Hazards Regulations (COMAH) 2015, Guidance on Regulations (3rd edition). Facts checked on 7 October 2026.
Find a date that works
Every course, every centre, with live availability and prices.
On your site or ours, and group rates on request.
