14 CFR 1251.110: Direct threat.

(a) This part does not require a recipient to permit an individual to participate in or benefit from the services, programs, or activities of that recipient when that individual poses a direct threat to the health or safety of others.

(b) In determining whether an individual poses a direct threat to the health or safety of others, a recipient must make an individualized assessment, based on reasonable judgment that relies on current medical knowledge or on the best available objective evidence, to ascertain: The nature, duration, and severity of the risk; the probability that the potential injury will actually occur; and whether reasonable accommodations in policies, practices, or procedures or the provision of auxiliary aids or services will mitigate the risk.