This got me to thinking about a new helmet, and Speedway is having a sale of sorts.
The inner liners - often Nomex, or what's called a Nomex blend - are fire-retardant, but yeah, helmet vents supply oxygen...
So, hard to quantify, but that's what head socks are supposed to do.
But those have their limits too, as anyone who knows who Niki Lauda was can tell.

Anyway, I see Speedway's selling Zamp helmets - never heard of them, but the company has a Meridian, ID address...a couple miles from my new place.
Supposedly a Euro-centric helmet maker, owned by Bell.