The ADA Protects You from
Mandatory Masking if
Masks Interfere with Your Breathing
When Congress adopted the Americans with Disabilities Act (ADA) in 1990 it explained the purposes: 
It is the purpose of this chapter
(1) to provide a clear and comprehensive national mandate for the elimination of discrimination against individuals with disabilities;
(2) to provide clear, strong, consistent, enforceable standards addressing discrimination against individuals with disabilities;
(3) to ensure that the Federal Government plays a central role in enforcing the standards established in this chapter on behalf of individuals with disabilities; and
(4) to invoke the sweep of congressional authority, including the power to enforce the fourteenth amendment and to regulate commerce, in order to address the major areas of discrimination faced day-to-day by people with disabilities.
The law defines “disability” as:
“a physical or mental impairment that substantially limits one or more major life activities of such individual…” and further, “For purposes of paragraph (1), major life activities include, but are not limited to, caring for oneself, performing manual tasks, seeing, hearing, eating, sleeping, walking, standing, lifting, bending, speaking, breathing, learning, reading, concentrating, thinking, communicating, and working.” Congress also required, “The definition of disability in this chapter shall be construed in favor of broad coverage of individuals…” (Sec. 12102. Definition of Disability) [Emphasis added.]
The Department of Labow notes,
“The Americans with Disabilities Act (ADA) prohibits discrimination against people with disabilities in several areas, including employment, transportation, public accommodations, communications and access to state and local government’ programs and services.” 
A fair reading of the statute suggests that the mis-use of sanitary masking as a sign of compliance with Declared Pandemic rules which interferes with “breathing” to the extent of impacting major life activities is prohibited by the Law.
While sanitary masks have a proper use, during surgery, for example, the labeling of such masks make it clear that they are not designed to prevent disease, just to limit the spreading of droplets from the mouth and nose. Whether masks “work” or not, however, is not the issue.
It is my opinion that the Law protects individuals who cannot mask due to breathing difficulties when they attempt to mask. The phrase, “Can’t mask — Don’t ask.” is appropriate for such persons. There is no requirement in the wording of the Law that requires a person to be diagnosed with a specific disability. The law does not permit others to demand to know what disability is involved.
If wearing a mask interferes with breathing, being required to wear a mask violates the Law.
Sen. Tammy Duckworth (D-Ill.), first disabled woman in the US Senate recently told The Hill,
“Disability rights are human rights, and these civil rights must never become optional benefits that can be taken away whenever it’s convenient or cheaper for employers and those who are in power…” 
While no are no formal requirements to assert your rights under the ADA, you may want to obtain the Advance Sanitary Masking Directive, an Advance Medical Directive Card that asserts your right to refrain from masking as an Informed Consent medical decision.
More here: https://tinyurl.com/maskexemption