When can Diesel form a Hazardous Area?

Diesel begins to form a flammable vapour when heated to its flashpoint, which is typically 55°C. Precautions to prevent ignition of vapour should be introduced whenever it is possible for diesel to be heated to within 5-10 °C of its flashpoint (or when it can be released as a mist or spray, see below). This includes instances where diesel can be released within an enclosure, bund or vehicle tank, which could be, or is, exposed to extreme sunlight. Without adequate ventilation, this can create a flammable atmosphere, which can easily ignite and propagate its flame, igniting the remaining liquid, and resulting in an explosion.

Diesel vapour is flammable when mixed with air in concentrations from 0.6% up to 7.5%; too low and the mix is considered lean and too high, the mixture is too rich to burn. These values are known as the lower and upper explosive levels (LEL & UEL).

When diesel is released under pressure, it can form a flammable mist. Mists can occur at pressures as low as 1 bar and are generated in many instances, including pressurised leaks and during the filling/dispensing processes.  

Mists can have an LEL as low as 10% of the equivalent vapour limit, meaning that it can only take 0.06% of diesel mist within the surrounding atmosphere to create a flammable, hazardous area.

Diesel is also likely to form a hazardous area when spilt thinly over an area, for example, within the dispensing area, onto concrete, brickwork or storage tank/equipment surfaces.

As a result of these facts, where Diesel is used within the workplace, it is necessary to undertake a risk assessment under DSEAR (regulation 5) and identify where/if these hazardous areas could form (regulation 7) and then implement documented Control Measures to ensure that the storage and dispensing apparatus is;

  • Fit for purpose
  • Suitably maintained
  • Protected from unauthorised access
  • Operated by trained individuals with documented procedures


Observing the information provided within our DSEAR toolkit will enable you to evaluate existing measures and identify where to introduce control measures and procedures which will mitigate risk. By evaluating working practices and encouraging safety improvement policies within the workplace, it is often possible to satisfy the requirements of the DSEAR risk assessment for safe diesel storage without the expense of replacing equipment unnecessarily.