Background
Pseudomonas aeruginosa in the sinuses plays a role in the lungs in cystic fibrosis (CF) patients, but little is known about the sinus environment where the bacteria adapt. Anoxic areas are found in the lower respiratory airways but it is unknown if the same conditions exist in the sinuses.

Methods
The oxygen tension (pO2) was measured, using a novel in vivo method, in the maxillary sinus in a group of 20 CF patients.

Results
The CF patients had a significant lower pO2 on the mucosa but not in the sinus lumen as compared with a control group of non-CF patients. Anoxic conditions were found in 7/39 (18%) of the sinuses from where we cultured P. aeruginosa, Stenotrophomonas maltophilia and/or coagulase negative staphylococci.

Conclusion
These findings support our hypothesis that P. aeruginosa can adapt or acclimate to the environment in the lungs, during growth in anoxic parts of the paranasal sinuses.