They find masks very difficult. They're fine for the first 10 minutes or so, but after that they feel very dizzy and nauseous and like they can't breathe properly. It sounds like a lot of people have the same problem*.
A discussion elsewhere suggested that it's not depletion of oxygen or buildup of CO2, but just buildup of humidity from the water vapour in your exhaled breath.
So they tried tucking a couple of sachets of silica gel into the mask (the ones you get in packets of electronics and some mail-order clothes), and that seems to work! Now they can last through an hour-long supermarket trip with only mild discomfort.
Spread the word to your mask-sensitive friends.
* I get a very minor version of this myself. I dislike having a hot lower face, and it can make me feel a bit claustrophobic but it doesn't make me feel nauseous/dizzy.
Original post on Dreamwidth - there are comments there.