My friend posited that the amount in the sewers would be likely to be less than that in the ambient air and therefore worth attempting if desperate.
I don't know much of anything about sewers, but you don't really smell things around manholes, do you? (My anosmia prevents me from answering this with confidence.) If not, it would make sense to presume there's some kind of seal on them. If that's the case, the outlet would be the primary inlet for gas. I don't know enough about gas attacks or drainage to say whether that's a problem.
If I'm ever unable to breathe ambient air or escape the area, I'll try to remember to attempt this and get back to you.
In a previous house, the sound insulation wasn't great due to the necessity of a window AC unit. Outside of my window apparently lived a bird who had learned to imitate my alarm and had no regard for timing.
I agree that birdsong can be quite lovely. I do not agree that it is universally so.
It wasn't as bad as some - no one was offended AFAIK - but recently a friend's wife asked if I was close with my in-laws. I had recently moved geographically much closer to them, so I described the distance between us, about four miles.
I didn't even realize I had misunderstood until her husband clarified later in the conversation.
You have done a much better job of explaining than anticipated and I hereby express my gratitude.