Someone else pointed out LD50 is ~700ppm

It's unlikely you'd be exposed to this level for any period of time unless the battery ruptured into the car, underwater, with the windows up, in which case you have bigger problems. You're unlikely to to see 700ppm in an outdoor situation like a car wreck or battery malfunction on the highway during a rainstorm. Atmospheric CO2 is about 450ppm for comparison.

Ammonia is a superior refrigerant (widely used in industrial circles, and causes no ozone depletion, is biodegradable, etc) but not used in residential applications because it's highly toxic if there's a catastrophic seal failure and not vented outside, despite the fact that humans are very efficient at smelling even the slightest ammonia leak.