NOPE! Pure Nitrogen. Easier to make and less toxic. Also its almost impossible to detect.

https://en.wikipedia.org/wiki/Pressure_swing_adsorption

You know those air tank replacement devices the advertise on TV. All that is is a battery operated device the removes most of the nitrogen using a small scroll compressor. Exact same tech can be used to make pure nitrogen on the spot from regular air. Pump the pure nitrogen into the tunnel and it displaces the oxygen. They just pass out and never wake up.

Bah I just looked down and someone else already suggested nitrogen.