The two classes of gases that electricians are likely to
encounter are asphyxiants and oxygen binders.  Nitrogen is an
example of an asphyxiant. Nitrogen makes up about 78% of the air
we breathe. Although it’s essential to human life, getting too
much of a good thing is lethal. So what's the problem? Nitrogen
suffocates you by displacing oxygen, as it does in vessels being
purged.  Carbon monoxide (CO) is an example of an oxygen binder.
Carbon dioxide (CO2) is one carbon atom bound with two oxygen
atoms; that's the stable state that carbon and oxygen seek when
present together. Electrical generators emit CO and nitrous
oxide (another oxygen binder).  Asphyxiant defense. Always test
the air before entering (or sticking your head into) any vessel.
Follow confined entry procedures as if your life depends on it,
because it does.  Oxygen binder defense. If you can smell
exhaust gas, your ventilation is faulty. Ventilate more.