At KVM Forum 2019 [1] there was a particular theatre where the enclosed space with many people and lack of ventilation was very obvious. I felt dizzy and sleepy a few times at lectures there (and nowhere else). No training needed! Edit: It was the room shown in this picture: [2]

[1] https://events19.linuxfoundation.org/events/kvm-forum-2019/a...

[2] https://locations.filmfrance.net/location/lyon-convention-ce...