Continual increase in biologically effective dose of solar UV radiation determined by spore dosimetry from 1980 to 1993 in Tokyo

The genotoxic activity of solar UV radiation was monitored by measuring the inactivation of UV-sensitive spores of Bacillus subtilis; this has been performed since 1980 in Tokyo. The relationship between the cumulative values of the half-day inactivation dose and the half-day global insolation was determined by regression analysis. Data collected in 127 afternoons were grouped into three successive periods ( 1980-1986, 1989-1991 and 1992-1993). The ratio of the UV dose to the insolation of the last group was the highest, and that of the first group was the lowest. The ratio of the observed half-day inactivation dose to that expected from the relationship was regressed to daily values of the ozone column recorded at the nearest Tsukuba observatory. The relative inactivation dose was related to the ozone column by a power function with a constant exponent of -2.1. However, the inclusion of this relationship in the regression analysis between the inactivation dose and insolation could not explain completely the differences observed between the three groups. It was suggested that certain atmospheric factors other than the change in the stratospheric ozone concentration over the Kanto plain were responsible for this continual increase.