HVAC pros...why doesn't auto a/c use a natural refrigerant?

[timebuilder]

In a crash I'd be more worried about 20 gallons of gasoline then 10-12 oz of ammonia.

...especially if your air conditioned car was a Pinto or a Crown Victoria.

[double bogey]

I would worry about the ammonia dumping out of the evap into the cab while driving 70mph. You couldn't get out of the car soon enough.