From 1908 to 1914, Douglas did research with John Scott Haldane on human respiration. In 1910 Nathan Zuntz organised a high-altitude expedition to Tenerife in the Canary Islands; the participants included Douglas from Oxford, Joseph Barcroft from Cambridge, and Arnold Durig from Berlin. In 1911 J. S. Haldane, Douglas, Yandell Henderson, and Edward C. Schneider were the four principal members of an Anglo-American expedition to Pikes Peak in Colorado to do research on high-altitude breathing. The four principal members, but no other expedition participants, went to the summit house of Pikes Peak. Mabel Purefoy FitzGerald joined the expedition to conduct research of mining town residents at lower altitudes. Gerald B. Webb, J. Richards and J. E. Fuller also did research for the Pikes Peak expedition.