Among the “toxins” is carbon dioxide. People who are prone to hypoxia can have that condition exacerbated by wearing a mask.
You've just reminded me that one of the DUMBEST things I've seen is people driving with their windows up - while wearing a mask - and they're the only one in the car.
The second dumbest thing I've seen was someone standing outside a grocery store smoking a cigarette while wearing a mask. They'd pull the mask down, take a drag off their cigarette, exhale, then put the mask back up. Unbelievable.