just say you have a medical condition and if they ask what either point to HIPPA or tell them your Dr and OSHA said you're required to breathe air with an oxygen content of 19.5% minimum and that wearing a mask reduces this concentration to well below this %.

I honestly have hardly ever had anyone hound me as to what the condition is. I usually just say it quickly and matter factly and go about my business not looking at them, leaving ot to them to be assertive if they so choose. the fact that they are wearing a mask still and asking you to wear one should suggest they are a sheep or a cuck and aren't likely to even push back.

it's when someone is trying to "protect" their business from being shutdown by health dept or some shit that their back hair may get up. just try explaining to them that per HIPPA they aren't allowed to ask you what your condition is and by having asked you they are in compliance like a good sheep.

if all else fails, leave. no use getting confrontational but you should not wear a mask unless you find yourself with no other option such as when you fly these days...even then I make sure that I'm very very slowly sipping water or nibbling on sunflower seeds for hours