['To characterize and control the potential interference to P(O2) determinations when blood contaminated with air is sent via a pneumatic tube system (PTS).', 'Both tonometered blood at P(O2)s of 65, 75, 142, and 339 mm Hg and arterial blood gas samples from patients with baseline P(O2)s from 70 to 400 mm Hg were analyzed for P(O2) to determine possible effects of air contamination from PTS transport.', 'A large teaching hospital in which a variety of personnel routinely send samples to the laboratory by PTS transport.', 'Twenty patients under anesthesia for elective surgery and 21 patients in an intensive care unit who had a wide range of P(O2)s. Several additional patients with a preexisting lung pathology likely to cause hypoxemia were selected to provide samples with low P(O2)s.', 'Measurement of bias in P(O2) between samples sent via PTS and samples walked to the laboratory.', 'Interference from air contamination was worse after PTS transport compared with manual transport of the specimen. Over a wide range, the P(O2) in specimens after PTS transport tended toward 160 mm Hg. Samples from hypoxemic patients were prone to errors in P(O2) that could have resulted in clinical misinterpretation; 5 of 10 samples with a baseline P(O2) less than 85 mm Hg had increases of 10 mm Hg or more when contaminated with air. Cooling samples with high P(O2)s minimized changes to P(O2), probably by increasing the solubility of oxygen. Mechanical buffering by various liners used in the carriers did little to alleviate the interference. Decreasing the speed of pneumatic transport by 50% lessened the effect on P(O2).'] Does pneumatic transport exacerbate interference of room air contamination in blood gas samples?