A person with a disability who cannot wear a mask, or cannot safely wear a mask, for reasons related to the disability https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/cloth-face-cover-guidance.html

From https://www.ada.gov/regs2010/titleIII_2010/titleIII_2010_regulations.htm#a105:

(1) Disability means, with respect to an individual: (i) A physical or mental impairment that substantially limits one or more of the major life activities of such individual; (ii) A record of such an impairment; or (iii) Being regarded as having such an impairment as described in paragraph (f) of this section.

(f) Is regarded as having such an impairment. The following principles apply under the “regarded” as prong of the definition of “disability” (paragraph (a)(1)(iii) of this section): (1) Except as set forth in paragraph (f)(2) of this section, an individual is “regarded as having such an impairment” if the individual is subjected to a prohibited action because of an actual or perceived physical or mental impairment, whether or not that impairment substantially limits, or is perceived to substantially limit, a major life activity, even if the public accommodation asserts, or may or does ultimately establish, a defense to the action prohibited by the ADA. (2) An individual is not “regarded as having such an impairment” if the public accommodation demonstrates that the impairment is, objectively, both “transitory” and “minor.” A public accommodation may not defeat “regarded as” coverage of an individual simply by demonstrating that it subjectively believed the impairment was transitory and minor; rather, the public accommodation must demonstrate that the impairment is (in the case of an actual impairment) or would be (in the case of a perceived impairment), objectively, both “transitory” and “minor.” For purposes of this section, “transitory” is defined as lasting or expected to last six months or less. (3) Establishing that an individual is “regarded as having such an impairment” does not, by itself, establish liability. Liability is established under title III of the ADA only when an individual proves that a public accommodation discriminated on the basis of disability within the meaning of title III of the ADA, 42 U.S.C. 12181–12189.