Leave, and stop anyone else going in. That is the whole of the first minute. The instinct to check on a colleague is what turns a single casualty into two, and it is the pattern that recurs in asphyxiation incidents across every industry that handles inert gas.
The procedure needs to be written so that it does not rely on judgement at the door. Evacuate the room and the immediately adjoining spaces, close the door behind you, raise the alarm, and post someone at the entrance so that nobody enters to investigate. Nobody re-enters until the monitor reads normal and the cause is understood, and nobody enters to effect a rescue without breathing apparatus and trained rescuers.
If a person is inside and unresponsive, this is an emergency services call, not a colleague’s job. Rescue from an oxygen deficient atmosphere without air supplied breathing apparatus is how rescuers become casualties.
Two practical points. Ventilation should be running before anyone re-enters, and the incident should be logged even where nobody was hurt, because a spurious alarm and a genuine release look identical until the cause is established.
