Stated in Lahmann’s pamphlet Das Luftbad als Heil- und Abhärtungsmittel, first published in German in 1898 (Deutsche Biographie records the first edition as 1898 and a third edition in 1904); the Wellcome-catalogued English translation, The airbath as a means of healing and hardening the body, is a later, second edition dated 1901. The claim is classified as a surrogate-endpoint claim (general hardening or resistance to disease, not a measured clinical outcome) and unreplicated: no controlled study in the cited sources tests whether systematic air-bathing specifically, as opposed to general fresh air and moderate sun exposure, confers the broad protective effect Lahmann attributed to it.