You don't need to be in a regulated "confined space" to run into problems.
Asphyxiation: If you’re emptying nitrogen or carbon dioxide tanks for transport back to the rental place, vent them outside. If you’re running a drill rig inside a building, open the place up and set up fans.
Drain bamage: If you’re collecting water samples in a manufacturing facility and you hear a commotion nearby and your little air quality meter starts spooling up, take a break. Outside. If you’re sitting inside a vehicle because the weather’s bad and you’re labeling stuff with a big sharpie and you start giggling uncontrollably, go outside.
Personal protective equipment: If you’re in a space with low oxygen, the fresh cartridges in your gas mask aren’t going to be a big help.
Common sense? The problem is that once you’re in a low oxygen environment or you’re breathing funny fumes, you may not have common sense. So you’ve got to think about what you’re doing before you start.