Don't lie. By lying you are not only committing a sin, you are strengthening the fraud which you are trying to extract yourself from. The people wearing masks are perpetuating the lie that masks prevent the spread of airborn virus. The people who lied to get out of wearing a mask are also tacitly supporting the same lie with the addition of another lie (that they have a condition which would prevent them from wearing a mask). If you can wear a mask/ventilator to avoid toxic fumes, you can wear a surgical mask too. There certainly are medical reasons for not wearing a mask all the time but that's not the reason you are giving for not wearing a mask. You are specifically saying that you have a medical condition which prevents you from wearing a mask altogether. That actually is the case for some people. They simply cannot wear a mask without severe and immediate consequences to their health and maybe risk death. But I know people who have been medically diagnosed with asthma who regularly wear masks without any apparent problem. So you can see why the medical exemption is barely believable. Don't lie. Just tell them flat out that you refuse to wear a mask. Let the chips fall where they may. Why do you want to give business to these tyrants anyway? There are plenty of people who would love to have your business, mask or no mask.