The Control of Major Accident Hazards Regulations 2015 (COMAH) is the UK's legal regime for preventing major accidents at sites holding significant quantities of dangerous substances. Sites are classified as lower tier or upper tier based on the substances and quantities held, with progressively more demanding obligations at each tier.
Key points
- Lower-tier sites hold dangerous substances at or above Schedule 1 column 2 quantities; upper-tier sites hold amounts at or above the higher column 3 quantities.
- Example thresholds include 10 tonnes (lower tier) / 25 tonnes (upper tier) for chlorine gas, and 2,500 / 5,000 tonnes for ammonium nitrate.
- Lower-tier sites must have a Major Accident Prevention Policy (MAPP); upper-tier sites additionally prepare a detailed Safety Report.
- COMAH is enforced by a joint competent authority - HSE (or ONR) alongside the relevant environmental regulator for each GB nation.
What This Regulation Requires
COMAH requires sites above the relevant thresholds to prepare a Major Accident Prevention Policy, and for upper-tier sites, a more detailed Safety Report, alongside ongoing safety management system requirements proportionate to the site's tier.
Who It Applies To
COMAH applies to the operator of the site holding the dangerous substances - in contract manufacturing, this is the manufacturer's responsibility for their own facility, not something a buyer holds directly.
What This Means for Contract Manufacturing
If your product involves substances held at COMAH-relevant quantities, understanding your manufacturer's tier status helps set expectations around their safety management maturity and potentially their scheduling and onboarding processes for new products - see COMAH-site chemical manufacturing for the related capability.
How COMAH Status Arises
COMAH applies where quantities of specified dangerous substances present at a site reach defined thresholds. It is quantity-based and site-based, so it depends on what is held across the whole establishment rather than on any single product, and a site can move into or out of scope as its inventory changes. There are two tiers, with upper-tier sites carrying more extensive duties than lower-tier ones.
The duties are substantial: a major accident prevention policy, a safety management system, emergency planning, and for upper-tier sites a detailed safety report subject to assessment by the regulator. Compliance is resource-intensive and continuous rather than a one-off exercise, which is why relatively few sites operate at this level and why those that do tend to have mature safety management as a consequence.
What It Means When Choosing a Manufacturer
If your product involves substances that would be held in COMAH-relevant quantities, the shortlist of manufacturers able to take it is short by definition. That is worth establishing at the very start of a search, because it eliminates most of the market and there is no route around it - a site cannot take on the work and arrange the regulatory position afterwards.
If your product does not require a COMAH site, the status is neither necessary nor an automatic advantage. It indicates a site accustomed to rigorous safety management, which is a reasonable positive signal, but it may also mean higher overheads and more involved onboarding for new products. Judge it against what your product actually needs rather than treating it as a general mark of quality.
Practical Questions
Ask whether the site is within scope and at which tier, how new products are assessed against the site's inventory position, and what the onboarding process involves. Introducing a new substance to a COMAH site is not automatic - it may need assessing against the existing safety case, which affects lead time for a first batch. See COMAH-site chemical manufacturing.
Related Pages
Frequently Asked Questions
Does a COMAH site classification mean a manufacturer is less safe?
No - COMAH tier status reflects the quantity of dangerous substances held, not a judgement on safety performance. Many well-run, large-scale manufacturers operate as COMAH sites as a normal consequence of their scale.
Do I need to know my manufacturer's COMAH tier status?
It's not a legal requirement for you as the buyer, but it can be useful context for understanding their safety management approach and any scheduling implications for large-volume production.
Find a Manufacturer
Find a UK Contract Chemical Manufacturer
Search the directory of UK contract and toll manufacturers by process, industry and capability.
Search the DirectoryLast reviewed: August 2026. Regulation and guidance change — verify anything you rely on against the primary source. Tell us if something here is out of date.