The neat thing about propane as a refrigerant is that it's a really powerful one, actually rather better than R12 in practical applications.

If you replaced an R134a fill with R290 you'd need about half as much gas and if it got out, it's got a Global Warming Potential of 3, making it three times as bad as carbon dioxide and about 475 times better than the same amount of R134a.

Even in a very large air conditioning system like in my elderly Range Rover, the 500g or so of propane it would need is about the same as the engine sucks up in about four miles.

The effect of that escaping into the atmosphere is about as bad as the amount of petrol you spilled the last time you refilled your lawnmower.

If you're worried about the ZOMG IT IS GAS IT WILL MAKE A GINORMOUS EXPLODISHION aspect like a lot of people are, ask yourself this - how many aerosol cans are in your car right now? Tin of deicer, tin of magic tyre inflater, maybe a tin of air freshener or WD40 or something? You're already carrying more propane around with you, rattling around loose at your feet.