Statistical Investigation of Working Pressure and Decompression Sickness at Compressed-air Works

Recently compressed-air engineering work mostly employs pneumatic caisson method and tends to use higher working pressures in accordance with mechanization of shield works. Under the circumstances, the present author totaled results of five years' surveys on development of decompression sickness (DCS) and compared the results with former research findings. For this comparison, the control data was chosen among those which had been comprised by the same procedures as ours (Mano & Shibayama, 1987). It is commonly said that DCS could not be developed under a condition of working pressures less than 1.0 kg/cm^2. Although we experienced seven patients with DCS development under 1.0 kg/cm^2 of the pressure, no case was observed in the present survey conducted with a total of 12,503 exposures of compressed-air works. Total number of compressed-air works with over 1.0 kg/cm^2 pressures include: 3,822 more exposures found in 1986 or later than before 1986; 9,649 more exposures observed in 1985 or former with working pressures of 1.0-2.0 kg/cm^2; and, with higher pressures over 2.0 kg/cm^2, the number of the works showed rapid increase in 1986 or later (Table 1). This means that deep excavation works, which need higher working pressures than before, are becoming popular for today's compressed-air works. In other words, working circumstances became more strict in these years, therefore, more precise safety supervision should be required. When decompression control is carried out basing on the current decompression table, development rate of DCS increases in line with enlargement of working pressures. This was also observed in the present study. Comparing onset rates of DCS during these five years with those observed before the years, the results of these five years show lower rates, based on the working pressures divided by each 1.0 kg/cm^2 group, than those seen before the five years (Table 1). This supposedly indicates that more strict decompression control is carrying out today than before