Some patients have noted a foul odor during radiation therapy sessions, but the cause of the odor remains unknown. Since we suspected that this phenomenon is due to ozone generated by ionizing radiation, this experimental study measured ozone concentrations in the treatment room and in a coiled polyvinyl chloride (PVC) tube placed within the radiation field. We measured ozone concentrations using an ultraviolet absorption method and an ozone monitor. A PVC tube (inner diameter 7 mm, outer diameter 10 mm) was used to mimic the environment of the nasal cavity. The tube (790 cm) was coiled and set between two 4-cm-thick (for X-rays) or 2-cm-thick (for electron beams) water-equivalent solid phantoms. The sampling tube of the ozone monitor was inserted into the PVC tube, and the joint was sealed to prevent environmental air contamination. To measure ozone concentrations in the atmosphere, the sampling tube supplied with the unit was used. A linac was used on a full-sized treatment field (40 cm × 40 cm at a source-to-axis distance of 100 cm). The effect of an electron beam on ozone concentrations was also evaluated with a full-sized treatment field (40 cm × 40 cm at a source-to-surface distance of 100 cm). Ozone levels in the treatment room were undetectable before the start of daily treatment but reached 0.008 parts per million (ppm) or more at 1 h after the start of treatment. Concentrations then remained nearly constant at 0.010-0.015 ppm throughout the day. The maximum ozone concentration in the PVC tube was only 0.006 ppm, even when it was irradiated at 2400 monitor units/min. Depending on the X-ray dose rate, the concentration increased to a maximum of 0.010 ppm with oxygen flowing into the other end of the tube at 1.5 L/min. Ozone concentrations in the PVC tube did not differ significantly between X-ray and electron-beam irradiation. Only traces of ozone were found in the PVC tube that was used to mimic the nasal passages during radiation, these concentrations were too low for human perception. However, ozone concentrations did reach potentially detectable levels in the treatment room.