Time course of changes in the concentration of CD gas in each chamber. CD gas concentration in each chamber was measured continuously throughout the exposure period for six months. To avoid the Figure looking too busy, symbols show the mean of weekly CD gas concentration (open square: control (air); open circle: 0.05 ppm; closed circle: 0.1 ppm), and error bars represent standard deviations. Throughout the exposure period for six months, there was no tendency of increase or decrease of CD gas concentration, and the fluctuation of the CD gas level was kept within ± 25%, irrespective of the CD dose level. The mean ± standard deviation of CD gas concentration during the exposure period for the low concentration chamber was 0.054 ± 0.007 ppm, and that for the high concentration chamber was 0.103 ± 0.011 ppm.