134a, there's a reason the manufacturers use it... best thing other than r12 given the parameters, cost, safety, effectivenes. From what I read, it's a hyrocarbon like propane and that does not seem too safe to me. Same thing with freeze12 and alcohol based systems. Having a highly presurized flamable substance cycling around with potential venting right in your face should the evaporator spring a leak does not sound good.