A study for the simplification of permissible criteria for noise exposure recommended by Japan Association of Industrial Health (author's transl)

Abstract

The Japan Association of Industrial Health (J.A.I.H.) recommends the permissible criteria for noise exposure for hearing conservation as is shown in Fig. 1 and Table 1. In the criteria, permissible octave band levels or permissible one third octave band levels are given as a function of exposure time. J.A.I.H. also recommends a provisional permissible sound level of 90 dBA for 480 minute exposure. The criteria is based on the critical band theory of temporary threshold shift (TTS) and the empirical equations for predicting TTS; in the sense that the criteria has been rationally determined.According to the survey conducted by A rchitectural Institute of Japan, however, permissible criteria are determined in terms of the sound level in most of the countries (Table 2). This is considered to be due to the fact that simpler methods are needed than those which request frequency analyses. In an effort to meet this need, a simplification of the permissible criteria for noise exposure recommended by J.A.I.H. has been undertaken. A comprehensive noise survey of the plants provided in Labor Safety and Sanitation Ordinance furnished 265 measurements that were used to relate A-weighted sound levels to the octave band contours of Fig. 1. Octave band levels are averaged on center frequency and the mean values are shown in Fig. 2. In the figure, the ranges of standard deviations are also shown. The noises are sorted into groups by their octave band levels (Table 3). Examples of noise spectra sorted into the groups are shown in Fig. 3. The permissible sound levels of the sound which has a typical industrial noise spectrum as is expressed by the curve a of Fig. 2 are given by the A-weighted sound levels when the curve a of Fig. 2 osculates the curves of Fig. 1. The permissible sound levels so obtained are shown in the row a of Talbe 4. The permissible sound level obtained in the same ay for the curve b of Fig. 2 is also shown in the row b of Table 4. The distribution of permissible sound levels obtained as above stated for each of 265 industrial noises are shown in Fig. 4. The means of the distribution and the values of means minus one standard deviation are presented in the rows c and d of Table 4 respectively. If thepermissible sound levels are determined as the values in the row a or c of Table 4, about a half of the noises are placed, on the basis of their A-weighted sound levels, into a group where noise hazard is underestimated. If they are determined as the values in the row b or d of the talbe, about 17 percent noises are grouped unsafe. In order to determine he permissible sound levels with a reasonable proportion of noises grouped unsafe, the permissible sound levels for proportions from 10 to 90 percent of the noises grouped unsafe are calculated and presented in Table 5. The levels for 10, 50, and 90 percent are plotted against exposure time in Fig. 5, which shows the trading relations between the sound level and the exposure time, the proportion being constant. Though the curves expressing the trading relation are hyperbolic, it would be more appropriate to apply a linear trading relation for simplification. In Fig. 5, EPA's criteria and OSHA's standard are shown for comparison. EPA adopts 85 dBA for 8 hour exposure with 3 dB rule and OSHA does 90 dBA with 5 dB rule. From the figure one can see that the trading relation is approximated better by 3 dB rule than by 5 dB rule except the shorter exposure time than 60 minutes. The permissible level of 90 dBA for 8 hour exposure seems to be unsafe, because the percentage of the noises grouped unsafe exceeds 50 percent. Hence the present authors propose the permissible sound level of 85 dBA for 8 hour exposure with the trading relation of 3 dB rule. The proportions of the noises grouped unsafe and those grouped overprotected, when the permissible sound level is applied, are shown in Table 6. A noise is grouped overprotected when the sound level is equal to the permissible sound level of a