Comment further done the thread about HFC-152a. Seems to work much better, and is more friendly than 134a. I think the issue for many manufacturers, and the government, is the potential for a flammable gas, even in small amounts, leaking into the passenger compartment. Most of the design work on new A/C systems using HC refrigerants are a secondary loop system. Keeps the loop for the HC seperate from the loop that runs in the passenger cpmpartment. Did a Google on HFC-152a and one of the first hits was an SAE paper written by a Volvo engineer on design an implementation of just such a system.