R-12, used in mobile A/C systems, is not flammable.
 R-22, used in stationary A/C systems, is not flammable.
 R134a, the more ozone friendly stuff used in cars since ~1994, is not
flammable.
       The refrigerant oil mixed in with all of these IS flammable, but only
if you do something really silly, like blow off the relief valve at 500psi
directly onto the exhaust manifold (usually from gross overcharging).
         The off brand substitutes for R-12 that are NOT labeled " r-12,
r-134a, r-22" May be *very* flammable, because they may contain Isobutane,
Butane, Propane, and other nasty stuff. Nobody reputable sells or installs
this stuff. Most shops have identifiers on their expensive equipment to avoid
contaminating their tanks. Butane contaminated refrigerant can cause massive
damage if vented at high pressure into a hot engine compartment (think
flamethrower/bomb)
  R-12/22/134a are *not* interchangable or compatible. They use different
lubricating oils which don't agree and will fall out of suspension, causing
loss of lubrication and compressor failure. The dessicant in the
reciever-dryers are not the same either.
  R-12 hoses will allow 134a to migrate through them due to smaller molecule
size. R134a hoses are "barrier" style which will contain 134a without
leakage. The O-rings are different too.
  Operating pressures are different for all 3. Condensers and evaporators
have to be a bit larger, and working pressures higher,  for r134a than r12.
 Hope this clears up some confusion. - Ed