Which rules apply to major-hazard chemical sites in the United Kingdom?

Major-hazard chemical sites in Great Britain are regulated by the Control of Major Accident Hazards Regulations 2015 (COMAH), which implement most of the EU Seveso III Directive 2012/18/EU. A site is a lower-tier or upper-tier establishment depending on whether the dangerous substances present reach the thresholds in columns 2 or 3 of Schedule 1. All operators must notify the competent authority and keep a major accident prevention policy; upper-tier operators must also submit a safety report and prepare and test an internal emergency plan.

Short answer

• Law: COMAH 2015 (SI 2015/483), in force since 1 June 2015; in Northern Ireland, the COMAH Regulations (Northern Ireland) 2015.
• Competent authority: HSE (or ONR for nuclear establishments) acting jointly with the Environment Agency in England, Natural Resources Wales or the Scottish Environment Protection Agency.
• All establishments: notification (regulation 6), a written major accident prevention policy (MAPP) implemented by a safety management system and reviewed at least every five years (regulation 7), action after a major accident (regulation 26).
• Upper tier in addition: safety report reviewed at least every five years and after changes (regulations 8 to 10), internal emergency plan reviewed and tested at intervals not exceeding three years (regulation 12), information to people likely to be affected (regulation 18); the local authority prepares and tests an external plan (regulations 13 and 14).
• Charges: the competent authority recovers its costs; HSE lists £218 an hour for COMAH regulatory activity (2026 rates).
• Other laws still apply: HSWA, MHSWR, DSEAR, COSHH and RIDDOR, and land-use planning controls on hazardous substances.

Lower tier and upper tier duties compared

The MAPP must be proportionate to the major accident hazards, set out the operator's aims and principles, and define management's role and commitment to continuous improvement (regulation 7(2)). When preparing the internal emergency plan, the operator must consult the people working at the establishment, the environment agency, the emergency services, the health authority and the local authority (regulation 12(5)).

Enforcement and after a major accident

The competent authority examines safety reports, inspects establishments and must prohibit the operation of any establishment or part of it where the prevention and mitigation measures are seriously deficient (regulation 23). It also identifies domino groups where neighbouring sites can increase each other's risk (regulation 24). After a major accident, the operator must inform the competent authority as soon as practicable and supply the circumstances, the substances involved, the consequence data, the emergency measures taken and the steps to prevent recurrence (regulation 26). Deaths, specified injuries and dangerous occurrences are also reported under RIDDOR.

How the UK compares with the EU and Türkiye

COMAH follows the Seveso III structure used across the EU, so tiers, MAPP, safety report and emergency plan requirements are closely aligned. Türkiye applies the same Seveso-based model through its own major industrial accidents regulation (known as BEKRA). For the qualifying quantities, operators must use the Schedule 1 tables in the current UK text rather than EU or Turkish lists.

Common mistakes

• Counting only one tank or one substance. Thresholds apply to the whole establishment, including aggregation rules in Schedule 1.
• Letting the emergency plan test lapse. Upper-tier plans must be reviewed and tested at intervals not exceeding three years.
• Treating COMAH as a substitute for DSEAR. Workplace explosion risks still need a DSEAR assessment and zoning.

Related on excodex

• Occupational safety and health in United Kingdom: law, authorities and employer duties
• Seveso III and major-accident prevention: tiers, MAPP, safety report
• BEKRA: major industrial accidents regulation in Türkiye
• How are explosive atmospheres and Ex equipment regulated in the United Kingdom?

Standards and further reading

• Control of Major Accident Hazards Regulations 2015 (SI 2015/483): https://www.legislation.gov.uk/uksi/2015/483
• Control of Major Accident Hazards Regulations (Northern Ireland) 2015: https://www.legislation.gov.uk/nisr/2015/325
• HSE, Introduction to COMAH: https://www.hse.gov.uk/comah/introduction.htm
• HSE, Fees and charges, current rates: https://www.hse.gov.uk/enforce/charging/rates.htm
• Directive 2012/18/EU (Seveso III) on the control of major-accident hazards involving dangerous substances

This article is general occupational safety and health information prepared by the excodex editorial team. It does not replace the applicable legislation, standards, manufacturers' instructions or the assessment of a competent OSH professional or occupational physician.