The Age Variation of Hearing among Chinese Farmers

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To clarify the effects of environmental noise on the age-related changes in hearing, the joint research between Wakayama Medical University in Japan and Shandong Medical University in China was conducted. The subjects were 1064 male and 1273 female Chinese who were occupationally unexposed to noise ranging in age from 5 to 83 years. Audiometric threshold testing was performed at the audiometric frequencies of 0.5,1,2,4 and 8 kHz. The upper limit of hearing was also measured. The ears of subjects with no evidence of hearing impairment were selected to analyze the hearing level and the upper limit of hearing. The subjects finally accepted comprised 1604 male and 2223 female normal ears. Normal aging curves of hearing level at each frequency and upper limit of hearing were established by calculating the 25th, 50th and 75th percentiles for each age group. The hearing level gradually increased with age and rapidly over 50 years. Deterioration of the upper limit of hearing was observed from early childhood and slight between age 25 and 49 but at ages over 50 it was accelerated. The age-related changes in hearing of Chinese were compared with those of Japanese normal group. Those of Chinese were slower than those of Japanese. When the age-related changes in hearing of urban Chinese were compared with those of rural Chinese. Those of urban Chinese were faster than those of rural Chinese. These differences may be the effects of environmental noise. In this study, 370 subjects (16% subjects) were excluded from the normal groups because of hearing impairment, 223 of them showed sensory hearing loss. Logistic regression analysis was used to analyze the items related to hearing impairment. Odds ratio of otitis media was the highest. This might come from the effect of aminoglycoside antibiotics commonly used to treat otitis media.