4. Flammable atmospheres are present. Such atmospheres are
created by flammable materials in the optimum concentration
in air, ranging between their lower explosive limit (LEL) and
upper explosive limit (UEL). At concentrations in air above
the UEL, the mixture air–flammable material is too poor in
oxygen to start combustion; at concentrations in air below the
LEL, there is not enough flammable material to sustain combustion.
Flammable atmospheres can be continuously present
in the area during normal operations, or momentarily due to
accidents (e.g., rupture of a tank).