n. An area distinguished by characteristics that provide freedom from danger, risk or injury. (Beighley 1995)
The primary distinguishing characteristic of a safety zone is lack of available fuel. A safety zone may already be free of fuel (a rock outcropping, for example), or firefighters may need to clear away fuel to create the safety zone. Butler and Cohen (1998) modeled radiant heat flux from flames to determine the minimum separation distance a properly outfitted firefighter (with personal protective equipment) must maintain to avoid radiant heat injury.
Their model, however, does not consider the potential for injury due to convective heat transfer, which would occur is a firefighter inhales hot combustion gasses.