Inflight cabin ozone aboard long duration C-5 airlift missions: a historical issue revisited

Past research has shown that U.S. Air Force C-5 aircrews have been occasionally exposed to ambient, stratospheric ozone concentrations in excess of Federal Aviation Administration (FAA) ceiling and time-weighted average (TWA) standards. Although strategies such as steering around meteorological areas of high ozone concentrations or outfitting the C-5 with ventilation system modifications were recommended by past investigators, review has shown that such changes were not adopted. Informal interviews with current C-5 aircrews indicate mucosal dryness and irritation are routinely encountered during long duration flights. Such symptoms could be due to low humidity or perhaps, ozone-induced irritation. With the advent of miniaturized, highly accurate, electronic air quality monitoring equipment, a repeat study of cabin ozone was believed to be warranted to better clarify exposure risk and to look for other possible explanations for the encountered symptoms.

A portable, electronic air quality monitor was used to monitor breathing zone cabin air during 47 h of flight. Ozone, carbon dioxide, carbon monoxide, temperature, and relative humidity were recorded for the duration. Ozone levels were corrected for altitude using a calibration curve derived by earlier, unpublished hypobaric chamber work.

Ozone levels were not found to exceed the FAA ceiling limit. Two flights had 3-h time-weighted average values that minimally exceeded FAA standards. Carbon dioxide and carbon monoxide levels were well below health effect threshold. Relative humidity reached a nadir of 3% and could well have explained symptoms of mucosal irritation.

Current evidence suggests that C-5 aircrews are exposed to relatively low concentrations of ozone. Review of the literature regarding observed short-term and long-term toxicity to low dose ozone exposure would indicate that no further operational or aircraft design action is required.