Interventions to prevent occupational noise-induced hearing loss

Background Millions of workers worldwide are exposed to noise levels that increase their risk of hearing impairment. Little is known about the effectiveness of hearing loss prevention interventions. Objectives To assess the effectiveness of non-pharmaceutical interventions for preventing occupational noise exposure or occupational hearing loss compared to no intervention or alternative interventions. Search methods We searched the Cochrane Central Register of Controlled Trials (CENTRAL); PubMed; EMBASE; CINAHL; Web of Science; BIOSIS Previews; Cambridge Scientific Abstracts; and OSH update to 25 January 2012. Selection criteria We included randomised controlled trials (RCT), controlled before-after studies (CBA) and interrupted time-series (ITS) of non-clinical hearing loss prevention interventions under field conditions among workers exposed to noise. Data collection and analysis Two authors independently assessed study eligibility and risk of bias and extracted data. Main results We included 25 studies. We found no controlled studies on engineering controls for noise exposure but one study evaluated legislation to reduce noise exposure in a 12-year time-series analysis. Eight studies with 3,430 participants evaluated immediate and long-term effects of personal hearing protection devices (HPDs) and sixteen studies with 82,794 participants evaluated short and long-term effects of hearing loss prevention programmes (HLPPs). The overall quality of studies was low to very low. The one ITS study that evaluated the effect of new legislation in reducing noise exposure found that the median noise level decreased by 27.7 dB(A) (95% confidence interval (CI) -36.1 to -19.3 dB) immediately after the implementation of stricter legislation and that this was associated with a favourable downward trend in time of -2.1 dB per year (95% CI -4.9 to 0.7). Hearing protection devices attenuated noise with about 20 dB(A) with variation among brands and types but for ear plugs these findings depended almost completely on proper instruction of insertion. Noise attenuation ratings of hearing protection under field conditions were consistently lower than the ratings provided by the manufacturers. One cluster-RCT compared a three-year information campaign as part of a hearing loss prevention programme for agricultural students to audiometry only with three and 16-year follow-up but there were no significant differences in hearing loss. Another study compared a HLPP, which provided regular personal noise exposure information, to a programme without this information in a CBA design. Exposure information was associated with a favourable but non-significant reduction of the rate of hearing loss of -0.82 dB per year (95% CI -1.86 to 0.22). Another cluster-RCT evaluated the effect of extensive on-site training sessions and the use of personal noise-level indicators versus information only on noise levels but did not find a significant difference after four months follow-up (Mean Difference (MD) -0.30 dB(A) (95% CI -3.95 to 3.35). There was very low quality evidence in four very long-term studies, that better use of HPDs as part of a HLPP decreased the risk of hearing loss compared to less well used hearing protection in HLPPs. Other aspects of the HLPP such as training and education of workers or engineering controls did not show a similar effect. In four long-term studies, workers in a HLPP still had a 0.5 dB greater hearing loss at 4 kHz than workers that were not exposed to noise (95% CI -0.5 to 1.7) which is about the level of hearing loss caused by exposure to 85 dB(A). In addition, two other studies showed substantial risk of hearing loss in spite of the protection of a HLPP compared to non-exposed workers. Authors' conclusions There is low quality evidence that implementation of stricter legislation can reduce noise levels in workplaces. Even though case studies show that substantial reductions in noise levels in the workplace can be achieved, there are no controlled studies of the effectiveness of such measures. The effectiveness of hearing protection devices depends on training and their proper use. There is very low quality evidence that the better use of hearing protection devices as part of HLPPs reduces the risk of hearing loss, whereas for other programme components of HLPPs we did not find such an effect. Better implementation and reinforcement of HLPPs is needed. Better evaluations of technical interventions and long-term effects are needed