In The UK
If someone asks why you are mask exempt.
They do not, under Section 26 of the Equality Act 2010 have the right to ask that; and doing so, under Paragraph1, Section B, constitutes a charge of ‘harassment’ which you can bring against them and their employers.
If they are getting aggressive.
You can say that you claim it under Exemption (ii), Section A, Paragraph 1, Regulation 4 of the Health Protection (Coronavirus, Wearing of Face Coverings in a Relevant Place) Regulations 2020.
If they ask what Exemption (ii) is.
Ask them why they are trying to enforce regulations they have not read and do not understand.
Exemption (ii) is that wearing a mask causes ‘severe distress’. No-one has the right to ask you that. NO-ONE.