SleepyGas - CO maybe? Also from holy boots (with matching exhausts) though I read one medical student that wrote CO never killed anyone (so it must be safe).
Otherwise good old CO2 (is that at 4% concentration - you black out without warning - but you want the Yawning).
I agree with a "master" post approach, but everything about this has been on the net for years yet nobody sees it. The only "growth" caused by the net is that of the WRONG info.
Even Wiki is a good source for a lot (see their "car battery" - it quashes a few good spouse tales).
But that's why I tend to not respond or write less. Repeating is a waste. (Rewording or enhancement may b an improvement.)
Like here I'm wondering why block boot vents.... to keep hydrogen gases in? To keep oxygen out? (That's based on a classic Aussie site moderator that reckoned a certain car can never have a boot fire - the holes in the boot are large(sic) enough to let hydrogen out but small enough to prevent oxygen entering. Ergo, smouldering maybe, but never a fire.... Mind you, that site reckoned they'd rather extinguish a boot fire than an engine bay fire and I suspect they are still none the wiser.)
But as per the poor fool that tried to educate the aforementioned Aussie site, I face a similar dilemna... help someone continue breaking the law or provide advice that may aid their death etc? Or laugh as fumes and acid eat through the boot and rest of the vehicle.
Or figure out why vents are closed off despite the OP's inquiry whether remaining vents are enough? (For what - escaping petrol fumes?)
Of course if I was inquisitive, I might find that it is indeed a fuel tank in the floor of the boot....
Internally arcing gassing battery, acid holed or otherwise venting fuel tank....
Whilst hydrogen isn't a destructive explosion, petrol is a bit different (after all, they tried to ban petrol didn't they?)
Maybe someone will alert the anti-terrorist mob. It's about time they leaned how many are making "natural" looking bombs. They took long enough to figure out water bombs and butane lighters.... And in Australia they are yet to lean of --- ----- -- -------....