Which air contaminant is primarily recognized as a depressant?

Acetylene is primarily recognized as a depressant among the options listed. In toxicology, a depressant is typically a substance that slows down physiological functions, particularly the nervous system. Acetylene, while commonly used as a fuel gas, can act as a central nervous system depressant under certain conditions, leading to effects such as drowsiness or impaired cognitive function when inhaled in high concentrations.

Benzene is primarily known for its carcinogenic properties and does not exhibit depressant effects. Carbon dioxide, although it can influence the respiratory drive at high concentrations, is not classified as a depressant but rather as a simple asphyxiant at lower levels. Helium, while it can affect voice and breathing when inhaled in large amounts, is not recognized for causing depressant effects on the central nervous system. Instead, it is generally considered to be inert and non-toxic.

Thus, acetylene stands out as the correct answer due to its classification and known effects on the body.