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Explosive atmospheres can be caused by flammable gases, mists or vapours or by combustible dusts. If there is enough of a substance, mixed with air, then all it needs is a source of ignition to cause an explosion. There are separate regulations that cover this hazard called the Dangerous Substances and Explosive Atmospheres Regulations 2002 or shorted to DSEAR. These regulations are beyond the scope of this course if you need more information the HSE website has full details and there is a link in the student download area. 

Each year people are injured at work by flammable substances accidentally catching fire or exploding. Work which involves using or creating chemicals, vapours, liquids, gases, solids or dusts that can readily burn or explode is hazardous. 

The effects of an explosion or a fire in the workplace can be devastating in terms of lives lost, injuries, significant damage to property and the environment, and to the business community. 

Most fires are preventable, dealing with workplace process fire safety is important and those responsible for workplaces and other non-domestic premises to which the public have access can avoid them by taking responsibility for and adopting fire safe behaviours and procedures. 

Some materials either in their standard form or when mixed can become explosive. The different forms are liquids, Dust, gasses and solids. 

Liquids such as petrol and other fuels and solvents in industrial products such as paint, ink, adhesives and cleaning fluids, give off flammable vapour which, when mixed with air, can ignite or explode. The ease by which liquids give off flammable vapours is linked to a simple physical test called Flashpoint which allows them to be classed according to the fire hazard they present in normal use. 

Flammable liquids are classed as: 

  • Extremely flammable where liquids which have a flashpoint lower than 0°C and a boiling point lower than or equal to 35°C. 
  • Highly flammable is where liquids which have a flashpoint below 21°C but which are not extremely flammable. 
  • Flammable is where liquids which have a flashpoint equal to or greater than 21°C and less than or equal to 55°C and which support combustion. 

Dusts which can form explosive atmospheres are also classed as dangerous substances. Dusts can be produced from many everyday materials such as coal, wood, flour, grain, sugar, certain metals and synthetic organic chemicals. They are found in many industries such as food or animal feed, chemicals, woodworking, rubber and plastic processing and metal powders. They may be raw materials, intermediates, finished or waste products. A cloud of combustible dust in the air can explode violently if there is a source of ignition.

Gases, such as liquefied petroleum gas (LPG) or methane, which are usually stored under pressure in cylinders and bulk containers. Uncontrolled releases can readily ignite or cause the cylinder to become a missile. 

Solids include materials such as plastic foam, packaging, and textiles which can burn fiercely and give off dense black smoke, sometimes poisonous. 

Many chemical substances can give rise to harmful heat and pressure effects because they are unstable or because they can react violently with other materials. Chemicals need to be stored correctly and when reacted together sufficient information obtained to ensure that correct process controls can be used to prevent dangerous exothermic runaway reactions. 

The flammable gases and oxygen used as a fuel for hot work and flame cutting can give rise to fire and explosion risks on their own without any involvement of any other dangerous or combustible substances. A risk assessment carried out according to DSEAR will help to identify the correct controls and equipment before the work is carried out. 

The Dangerous Substances and Explosive Atmospheres Regulations 2002, DSEAR and ATEX, require employers to assess the risk of fires and explosions arising from work activities involving dangerous substances, and to eliminate or reduce these risks. 

HSE and local authorities are responsible for enforcing those workplaces covered by the legislation on working in potentially explosive atmospheres.