Occupational noise and age : a longitudinal study of hearing sensitivity

Abstract

The interaction between age and noise and their individual effects on hearing is complex, making it difficult to distinguish between noise-induced hearing loss and agerelated hearing loss at an individual level. This retrospective cohort study aimed to determine the decline in hearing sensitivity in noise-exposed mineworkers as opposed to non-noise-exposed mineworkers over time to evaluate the combined and individual contributions of noise exposure and age. A mixed effects regression analysis was applied to longitudinal audiological data from a group of mineworkers in South Africa. Four serial (annual) audiograms of 2,583 mineworkers were utilised. Data of a non-noise-exposed group (n=951) and a group exposed to underground noise (≥85 dBA) (n=1632) were included. Results indicated that base values were significantly higher for the noise-exposed versus non-noise-exposed group, for the low-frequency average (LFA) of 0.5, 1 and 2 KHz (16.1 dB HL versus 11.1 dB HL), and high-frequency average (HFA) of 3, 4 and 6 KHz (25.7 dB HL versus 18.5 dB HL). These results may be indicative of previous noise damage in the noise-exposed group obtained before occupational noise limitations and personal hearing protection use were enforced through legislation in 2001. All year-to-year increases in mean hearing thresholds were statistically significant (p-values of <0.01). When correcting for age, year-to-year increases in mean hearing thresholds were higher for the noise-exposed group than for the nonnoise- exposed group when comparing HFA (3.5 dB versus 2.9 dB decline over a fouryear period) but similar when comparing LFA (0.6 dB versus 0.7dB decline over a fouryear period). Mixed effects regression analyses were repeated without correcting for age to represent year-to-year increases in hearing thresholds including noise and age influences. Year-to-year increases in mean hearing thresholds were significantly higher for the noise-exposed group than for the non-noise-exposed group in comparing HFA (4.0 dB versus 3.5 dB decline over a four-year period), and when comparing LFA (1.5 dB versus 1.1 dB decline over a four-year period). Year-to-year increases in mean hearing thresholds were between 0.4 and 0.9 dB higher in the analysis where age was uncorrected for, for both exposure groups across LFA and HFA, identifying age as a factor in year-to-year decrease in mean hearing thresholds. It is evident that occupational noise exposure and aging are significant contributors to hearing loss over time for mineworkers, even in a setting where noise exposure is controlled through legislation, and use of personal hearing protection is enforced. Ongoing effort is necessary to increase the effectiveness of noise management programmes in order to counter the prevailing negative effects of noise exposure on hearing thresholds, as well as the possible additive interaction between noise exposure and age-related hearing loss.