Fire Safety · Mandatory · Pre-Construction
DSEAR Assessment — Dangerous Substances & Explosive Atmospheres
A DSEAR assessment addressing fire and explosion risk from dangerous substances on construction sites. Distinct from COSHH which addresses health risks. Covers LPG, diesel, flammable solvents, acetylene, and combustible wood dust. Requires hazardous zone classification (Zone 0/1/2 for gas; Zone 20/21/22 for dust), ignition source controls, and an Explosion Protection Document under DSEAR Reg 7.
Last reviewed: 2 April 2026 — This guide reflects UK law as of this date. Dangerous Substances and Explosive Atmospheres Regulations 2002 remains current with no amendments enacted as of 2 April 2026. Next scheduled review: 2 April 2027.
1. The Regulation Almost Every Construction Site Ignores
The Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) apply to every workplace where a dangerous substance is present — and that includes almost every construction site in the country. LPG cylinders for temporary heating and hot works, diesel and petrol for plant and generators, flammable solvents and adhesives, acetylene cylinders for cutting and welding, and combustible dusts from wood cutting and concrete grinding are all dangerous substances within the meaning of DSEAR. Yet most construction sites have no DSEAR assessment at all.
The confusion arises because many site managers assume that COSHH covers flammable substances. It does not — or rather, it covers a different aspect of the risk. COSHH addresses the health risk from exposure to hazardous substances: inhalation, skin absorption, ingestion. DSEAR addresses the fire, explosion, and thermal radiation risk from dangerous substances: what happens when they ignite, detonate, or undergo an exothermic reaction. The same substance may require both a COSHH assessment (for health effects) and a DSEAR assessment (for fire and explosion risk). A tin of solvent-based paint, for example, needs a COSHH assessment for the vapour inhalation hazard and a DSEAR assessment for the flammable vapour explosion hazard. These are separate risks requiring separate assessments under separate regulations.
The good news is that for most construction sites, the DSEAR assessment does not need to be a lengthy or complex document. Most sites need a single document covering LPG storage and use, fuel storage, solvent and adhesive storage, acetylene and oxygen cylinder management, and any dust-generating processes that create combustible atmospheres. The assessment identifies the dangerous substances present, evaluates the risk of fire or explosion, classifies any hazardous zones where explosive atmospheres may occur, and sets out the controls to prevent ignition and limit the consequences of any incident.
LPG cylinders stored inside buildings or in unventilated enclosures are a DSEAR-notifiable hazard
LPG (propane and butane) is heavier than air. If a cylinder leaks or a connection fails inside a building, basement, or enclosed storage area, the gas will not disperse upwards and dissipate — it will sink to the lowest point and accumulate at floor level, creating an explosive atmosphere that a single spark can ignite. The lower explosive limit (LEL) of propane is just 2.1% by volume in air, meaning that a very small quantity of leaked gas can create a lethal explosive atmosphere in an enclosed space. LPG cylinders must be stored outdoors in a well-ventilated location, away from ignition sources, drains, and below-ground openings. Any use of LPG inside buildings — for temporary heating, for example — requires specific controls including adequate ventilation, gas detection, and emergency shut-off procedures documented in the DSEAR assessment.
2. Content
The DSEAR assessment must systematically identify every dangerous substance on site, assess the fire and explosion risk from each, classify any hazardous zones, and document the controls in place. The following table sets out the key content areas.
| Area | Detail required |
|---|---|
| Dangerous substances inventory | A complete list of all dangerous substances present on site. For construction sites this typically includes: LPG (propane/butane cylinders for heating, brazing, roofing), petrol and diesel (plant fuel, generator fuel, fuel stores), flammable solvents (thinners, cleaners, adhesive solvents, paint strippers), acetylene and oxygen (oxy-fuel cutting and welding), and combustible dust (wood dust from carpentry, concrete dust from grinding, MDF dust) |
| Risk assessment — release scenarios | For each dangerous substance: how could it be released? Cylinder leak, hose failure, spillage during decanting, vapour emission from open containers, dust generation during cutting. The assessment must consider both normal operating conditions and foreseeable abnormal conditions (dropped cylinder, vehicle impact on fuel store, container left open) |
| Risk assessment — ignition sources | For each release scenario: what ignition sources are present? Hot works (welding, grinding, cutting), electrical equipment, static electricity, smoking, vehicle engines, lightning. The assessment must map ignition sources against release locations to identify where an explosive atmosphere could encounter an ignition source |
| Risk assessment — likelihood | For each substance and scenario: what is the likelihood of an explosive or flammable atmosphere forming and being ignited? Consider the quantity of substance, the ventilation conditions, the proximity of ignition sources, and the frequency and duration of the activity |
| Risk assessment — consequences | For each substance and scenario: what are the consequences of ignition? Flash fire, pool fire, explosion, BLEVE (boiling liquid expanding vapour explosion for LPG), toxic combustion products. Consider the number of workers in the affected area and the potential for escalation |
| Zone classification | Classification of any areas where an explosive atmosphere may occur: Zone 0 (explosive atmosphere continuously or for long periods), Zone 1 (explosive atmosphere likely in normal operation), Zone 2 (explosive atmosphere not likely in normal operation but may occur for a short period). Most construction site scenarios produce Zone 2 classifications — typically around LPG storage, fuel storage areas, and spray painting enclosures |
| Ignition source controls | Controls to prevent ignition sources in classified zones: no smoking signs and enforcement, hot works permit requirements, use of ATEX-rated electrical equipment in Zone 1 and Zone 2 areas, earthing and bonding to prevent static discharge, vehicle engine exclusion zones around fuel and LPG stores |
| Storage controls | Controls for safe storage of dangerous substances: LPG cylinder cages (ventilated, locked, signed, located away from drains and below-ground openings), fuel storage bunds (≥ 110% capacity of the largest container), solvent cabinets (fire-resistant, self-closing, ventilated), acetylene cylinder separation from oxygen cylinders (minimum 3 m or fireproof barrier) |
| Explosion protection document | DSEAR Regulation 7 requires an explosion protection document (EPD) where explosive atmospheres may occur. The EPD must set out: the hazards identified, the measures taken to meet DSEAR requirements, the areas classified into zones, the areas where zone controls apply, the equipment and protective systems to be used in classified zones |
| Emergency procedures | Procedures for dealing with dangerous substance incidents: LPG leak response (evacuate, do not operate switches, ventilate from a safe distance), fuel spill response (stop the source, contain with spill kit, do not wash into drains), solvent fire response (appropriate extinguishing media, evacuation routes, assembly points) |
| Worker information | Information provided to workers about the dangerous substances they work with or near: hazard properties, safe handling procedures, emergency procedures, location of safety data sheets, zone classifications and their meaning, prohibited activities in classified zones |
| Assessor | Name, qualifications, and signature of the competent person carrying out the DSEAR assessment, and the date of the assessment |
3. Common Mistakes
1.Treating DSEAR as the same as the COSHH assessment
DSEAR and COSHH are separate regulations addressing different risks from the same substances. COSHH addresses health risks from exposure — inhalation of vapours, skin contact with solvents, ingestion of contaminated materials. DSEAR addresses fire and explosion risks — what happens when a flammable substance ignites or an explosive atmosphere detonates. A COSHH assessment for a flammable solvent will cover ventilation to reduce inhalation exposure, RPE selection, skin protection, and health surveillance. A DSEAR assessment for the same solvent will cover vapour accumulation in confined areas, ignition source elimination, zone classification, storage requirements, and emergency procedures for fire or explosion. Both assessments are legally required, and one does not satisfy the other.
2.Not classifying hazardous zones even when explosive atmospheres are possible
DSEAR Regulation 7 requires the employer to classify places where explosive atmospheres may occur into zones, and to document this in an explosion protection document. Many construction DSEAR assessments identify the dangerous substances and list generic controls but fail to classify any hazardous zones. This is a specific non-compliance. On a typical construction site, Zone 2 classifications are likely to apply around LPG cylinder storage areas (within 1–3 metres depending on the configuration), fuel storage compounds, generator refuelling points, and areas where spray painting with flammable coatings is carried out. The zone classification determines what equipment can be used in those areas (ATEX-rated equipment in Zone 1 and Zone 2), what activities are prohibited (hot works, smoking), and what signage is required.
4. Frequently Asked Questions
Does DSEAR apply to small quantities of flammable materials on a construction site?▾
Yes. Unlike the COMAH Regulations (Control of Major Accident Hazards), which have specific threshold quantities below which they do not apply, DSEAR has no minimum quantity threshold. If a dangerous substance is present on site in any quantity and there is a risk of fire, explosion, or corrosion to metals, DSEAR applies and an assessment must be carried out. However, the assessment must be proportionate to the risk. A construction site with a few LPG cylinders, a diesel bowser, and some tins of solvent-based paint does not need the same level of assessment as a petrochemical refinery. Most construction sites need a relatively brief DSEAR document that identifies the dangerous substances present (LPG, fuel, solvents, acetylene), describes the standard storage and handling controls, classifies the hazardous zones around storage areas, and sets out the emergency procedures. The key is that the assessment exists, is specific to the site, and addresses the actual substances and quantities present.
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