The efficiency of the Racal Airstream helmet respirator in improving peak expiratory flow rates (PEFR) and symptoms (dyspnea, wheezing, and cough) in aluminum potroom workers with respiratory complaints was assessed in 19 workers. Peak expiratory flow readings and symptom recording from a 2-week working period with use of the respirator were compared with a period when the 3M 9906 disposable mask was used. The study was designed as a randomized, parallel, cross-over study with five or six daily measurements of PEFR and daily symptom recording. A significant number of workers (15) had a higher mean peak flow in the helmet period than in the nonhelmet period (p less than 0.01); symptoms did not improve significantly in the helmet period. Objective evidence of respiratory protection was observed for the group of workers as a whole, but the effect on symptoms as well as individual effect on peak flow was minor in the majority of the workers.