In order to have an explosion you need an ignition source. There really isn’t any under the hood. To prove this, take your unlit propane torch with the gas turned on and move it around the engine compartment. There is nothing that will cause it to ignite. Likewise, is this any more dangerous than high pressure fuel in your fuel injection? Don’t see too many fires from that, do we? Likewise, with the amount of air circulation in the engine compartment, remember the radiator fan(s) come on with the AC, any leak will be blown away very quickly.
Australia has been using propane as a refrigerant in their cars for quite some time. Guess how many fires have been reported due to propane. Zero. Duracool, a product you can purchase at auto parts stores and on the Internet has propane and butane in it.
From the Duracool Website:
FLAMMABILITY
Is DURACOOL® flammable?
Like all hydrocarbons, DURACOOL® is flammable, and must be handled accordingly. Practically all lubricants, additives and oils used in the engine of a car are flammable. Gasoline is flammable. Hair spray, deodorant and whipped cream aerosol propellants are flammable. All must be transported, handled and utilized (or installed) using safe practices and following instructions where possible.
Have there been studies to determine the potential level of incidence, in terms of ignited automobile compartment leakage?
Internationally respected risk assessors Arthur D. Little, in a detailed UK-based study, estimated the risk of an ignited refrigerant leak in the passenger compartment of a motor vehicle as being in the area of 3 in 10 million. Their findings, from a local perspective, mean that if every car in the USA (some 50 to 60 million vehicles) were to use a hydrocarbon refrigerant such an accident might occur once in every 50 years.
What would the impact of and "incident level, once every 50 years" ignited automobile compartment leakage actually be?
In terms of the "impact", it is important to recognize that automobiles generally have 12-15 ounces of DURACOOL® refrigerant. If there were a full amount leak into an automobile compartment and it ignited, it would theoretically create a "flash" which would last 1-1.5 seconds.