Either way, you get caught outside without your oxygen supply and you're a goner. The difference between toxic or unsupportive atmospheres and non-atmospheres is a matter of pressure. Blow open the bulkhead of a space station, and air comes whipping out, dragging with it anyone and anything in the ruptured compartment. Most installations have doors which automatically close, isolating a compromised compartment. If you are, you still get sucked outside and have to deal with a swift change of place.
Airlocks allow entry and exit without the pressure problem.
It takes one turn for an airlock to change pressure so as to admit entry or permit exit. In toxic space, the pressure problem doesn't happen. Instead, all air in the compartment goes toxic, killing all inside who aren't wearing suits. And in unsupportive atmospheres, those inside have one move to make it to pressurized rooms or they suffocate. This also applies to space ship compartments, satellites, orbiting stations and planetary installations.