ooooooh no my friend. The lack of oxygen will slow things down a little bit, but it will also promote the growth of anaerobic bacteria - germs that don't need oxygen. Normally they are out-competed by their oxygen loving neighbors, but when those die because of the lack of oxygen, they take over, and they are NASTY. They can be highly toxic too (ever heard of botulism?)
Real life story that happened to me: I had a summer job as a university student in a government lab that did safety testing on shellfish. We would get dozens of samples every day to analyse for phytotoxins. About half a dozen were in the lab that morning. It wouldn't be uncommon that we would get one or two dead shellfish in the lot, and mud, and various critters, we were used to funky smells.
But that morning, I had one shellfish - a mussel - that had died. This was nothing unusual, but what was VERY unusual was that it had, somehow, managed to seal itself shut before dying. So there was no oxygen in there, ie the bacteria that were eating and rotting the mussel, were anaerobic. Unknowing of this, thinking it was healthy, I opened it and this goo just came out. And the smell was ....
vile.
PANDEMONIUM erupted in the lab. Grown-ass adults fled screaming. My immediate supervisor
shrieked at me "DO SOMETHING". Gagging, I started spraying the air with alcohol in an attempt to kill the smell. It was revolting - this happened more than 20 years ago and I still remember it vividly.
And that's from what, an ounce (or few dozen grams) of flesh? Bodies going into anaerobic decomposition? The horror, the
absolute horror.
So from both scientific knowledge
and personal experience, that is essentially how you make the stinking cloud spell without casting a spell.
edit: I somehow missed a lot of other replies mentioning this, but I hope that this story will entertain and illustrate