The illegal to retrofit R152a thing is mostly about old systems not being designed to ensure that in the event of a leak people are not exposed to high levels (

The illegal to retrofit R152a thing is mostly about old systems not being designed to ensure that in the event of a leak people are not exposed to high levels (concentrations of 3.7% v/v or above) for more than 15 seconds. Old designed for R12 were not engineered to meet those guarentees. R152a can be huffed to get high, and doing so can be lethal. Thus we know that prolonged exposures to high enough concentrations of R152a can cause fatal cardiac arrhythmia. So it is fundamentally a safety concern. Just an unfortunate one, considering how good it otherwise is as an R12 replacement.