災害と疲労 (II) : 作業時flicker値の低下限界とこれからみた災害発生の可能性

Abstract

Though it is comprehensible from our routine experience that fatigue should be an important cause of accident, the proving of it is next to impossible. As far as every hour's accident rate is concerned, accidents caused by fatigue seem to be very few in common factories. But, in accident curve of heavy workers the peaks tend to thrive toward the end of work, this means the increase of accidents of fatigue origin. This tendency is also found in bus driving, an increase in accident rate several hours after the start (figs. 1, 2). We saw that drivers' fatigue remarkably varied with the condition of roads. FF (critical flicker frequency) curve, which is based on flicker test performed every 15-20 minutes during run, seem to have four patterns in it (fig. 3). Pattern B is obtained from a bad road, C from a good one, A worse than B and D better than C. In A and B FF heavily drops sometime during drive, but not below a certain level as long as the driving is kept on, which is presumably due to the fact that the situation requires a driver to be excited interoceptively or exteroceptively above the level. The level is, therefore, thought to be the lowest bottom of FF he is allowed to take, in other words, the physiological criterion for safety drive. We next examined FF change in accordance with consciousness disorder during the experimental inhalation of low oxygen air and ascertained that when the initial disorder (drowsy state) appeared in brain wave (fig. 4, 5) the ability in mental work such as a simple adding showed a quick drop (fig. 6). FF at this period when normal mental activity begins to get impaired I call a critical level (CL). CL is variable individually 85-92 per cent of the resting level, but almost constant with the same person. And it appears nearly equal to the safety limit during drive as seen from the diminution rate of the two. When FF during run surpasses the critical level normal mental activity must be in disorder, thus increasing the possibilty of accidents. Hence the relation between fatigue and accidents can be discussed with regard to the FF curve and critical level mentioned above.