Simulated 4-h flights were carried out in a realistic model of a three-row, 21-seat section of an aircraft cabin that was reconstructed inside a climate chamber. Twenty-nine female subjects, age 19-27 years, were split into two groups; each group was exposed to four conditions: two levels of ozone (<2 and 60-80 p.p.b.) at two outside air supply rates (2.4 and 4.7 l/s per person). A companion study measured the chemicals present in the cabin air during each of the simulated flights. The subjects completed questionnaires to provide subjective assessments of air quality and symptoms typical of complaints experienced during actual flight. Additionally, the subjects' visual acuity, nasal peak flow and skin dryness were measured. Based on self-recorded responses after 3(1/4) h in the simulated aircraft cabin, they judged the air quality and 12 of the symptoms (including eye and nasal irritation, lip and skin dryness, headache, dizziness, mental tension, claustrophobia) to be significantly worse (P<0.05) for the "ozone" condition compared to the "no ozone" condition. The results indicate that ozone and products of ozone-initiated chemistry are contributing to such complaints, and imply previously unappreciated benefits when ozone is removed from the ventilation air supplied to an aircraft cabin.