The problem with using the ADA laws is that the ADA laws only require businesses to offer “reasonable accommodation” to those with disabilities. “Reasonable accommodation” could mean that a store offer to deliver groceries to your house for example. The better option is to use the actual health decree themselves. Every one of these health decrees provide exemptions for wearing a mask. Usually, one exemption is for people with a medical condition. My suggestion is to somehow “obtain” a Dr’s note (how you get one I’ll leave that up to your imagination) stating that you are exempted from wearing a mask. Carry the Dr’s note as well as a copy of the health decree with the exemptions highlighted with you at all times. If confronted by whatever store you’re in, give them the documents. There’s no way in hell that they are going to call every DR’s office or hospital for every letter they come across. Besides, with HIPPA laws, no Dr’s office or hospital will talk to anybody about your medical condition without your explicit permission.