Abstract
Experimenting with steady exposure numerous studies were made on the physiologically induced responses of exposure to noise. The result concluded was that the central location of physiological damage by noise quite possibly is in a higher centre.
Next the metabolic change under noisy environmental conditions was investigated. It was noticeable that the ammonia content in the brain was significantly elevated in the case of noise exposure. This fact could not be observed in other stress exposures. It was brought about by the disturbonce in the removal process of the ammonia not by the acceleration of the ammonia production.
In this study, white noise was used at 80 or 90dB (C) from 2 to 4 hours. Intermittent exposure was made under 2 conditions, i.e., one the condition that repeated a cycle formed by ON-time of 30 seconds and OFF-time of 30 seconds (ON-OFF 30sec.), another the condition that repeated a cycle formed by ON-time of 300 seconds and OFF-time of 300 seconds (ON-OFF 300sec.). The ON/OFF ratioes were 1 in both intermittent exposures.
Adult male rats were exposed to noise, then were killed by decapitation. The brain was rapidly removed, and homogenized in ice-cold 10% trichloroacetic acid with glass homogenizers. The estimation of ammonia was made by Conway's method using Seligson-Hirahata's apparatus.
The following results were obtained:
At 80dB exposure, the central ammonia concentration was elevated significantly under steady and ON-OFF 300sec. intermittent exposure for 2 and 4 hours, but not under ON-OFF 30sec. intermittent exposure for 2 and 4 hours.
Under ON-OFF 30sec. intermittent exposure to 90dB, the central ammonia content was elevated significantly during a 4 hour period but not during a 2 hour period.
From the above mentioned results, the following concepts may be considered;
1) Injury by steady exposure to noise will be higher than that caused by the intermittent exposure.
2) From the view-point of biological damage, intermittent exposure may be characterized by the following 2 factors;
a: duration of one duty cycle
b: total ON-time during the exposed period.
