Rocco M, Dell'Utri D, Morelli A, Spadetta G, Conti G, Antonelli M, Pietropaoli P.Chest. 2004 Nov; 126(5):1508-15
Case-control study of 19 immunocompromised patients treated with NPPV through a helmet vs 19 controls treated through a face mask. Authors conclude helmet represents a 'valid alternative' to face mask in this scenario.
Link to abstract.