Inert gas asphyxiation is a danger when welding and so on.
Many sources explain the dangers of inert gases, e.g. a quick search found this:
Hazards of Welding with Argon Gas
Even though argon gas is non-toxic, there are hazards associated with using it. The primary health hazard is asphyxiation or suffocation by displacement of oxygen
The danger is explained further in Wikipedia's Inert gas asphyxiation page (quote follows) where it is also explained that the danger can be mitigated by the presence of CO2, relying on the hypercapnic alarm to make the person do something about the danger.
the human subject detects little abnormal sensation as the oxygen level falls. This leads to asphyxiation (death from lack of oxygen) without the painful and traumatic feeling of suffocation (the hypercapnic alarm response, which in humans arises mostly from carbon dioxide levels rising)
Are there alternatives CO2 that can still trigger the same response but perhaps in a low concentration? The hint I'm working from is the phrase "arises mostly from carbon dioxide levels rising".