"since when has R-134 been considered harmful to the environment?"

SINCE ALWAYS!!!

R-134a is "better" than what it replaced, R-12, but it is still harmfull.

Refrigerants are harmful in two major ways:

1) Ozone Depletion Potential (ODP)
2) Global Warming Potential (GWP)

R-12 was bad on both ODP and GWP. R-134a is great on ODP (because it does not contain any chlorine), but it is still bad on GWP.

To quote from EPA regulations:

"Under section 608 of the Clean Air Act, it has been illegal since November 15, 1995, to knowingly vent substitutes for CFC and HCFC refrigerants during the maintenance, service, repair and disposal of air-conditioning and refrigeration equipment."

R-134a is one of these substitutes.

You may be asking, well than, how come I can buy this stuff at the local discount store. Well, this is because the EPA has not yet been able to muster a prohibition on the sale of R-134a. It is still illegal to "release" R-134a, it is just not illegal to buy R-134a.

This is one of those stupid government rules where it might be legal to possess something, but not to sell it (actually, the reverse logic).

Here is a link to some information on refrigerant substitutes.

http://www.ucsusa.org/environment/ozone.faq.html (look at paragraph 6)

But the bottom line is that it is illegal, AND MORE IMPORTANTLY, WRONG, to vent R-134a, BECAUSE IT IS HARMFUL TO THE ENVIRONMENT.