Interventions to Prevent Occupational Noise Induced Hearing Loss - Hearing Loss Prevention Programmes

A Cochrane review [Abstract] 1 included 29 studies. Eleven studies (n=3725) evaluated effects of personal hearing protection devices and 17 studies (n=84 028) effects of hearing loss prevention programmes (HLPPs).Four studies used a randomised design, one study used a quasi-randomised design with alternation, two studies used an interrupted time-series (ITS) design and all remaining studies used a form of controlled before-after (CBA) design. In most studies only men were included or there were mostly male workers at the workplaces that were studied.

Effects on noise exposure

• Engineering interventions following legislation:One interrupted time-series (ITS) study found that new legislation in the mining industry reduced the median personal noise exposure dose in underground coal mining by 27.7 percentage points (95% CI -36.1 to -19.3 percentage points) immediately after the implementation of stricter legislation. This roughly translates to a 4.5 dB(A) decrease in noise level. The intervention was associated with a favourable but statistically non-significant downward trend in time of the noise dose of -2.1 percentage points per year (95% CI -4.9 to 0.7, 4 year follow-up).
• Engineering intervention case studies: Twelve studies, describing 107 uncontrolled case studies of immediate reductions in noise levels of machinery ranging from 11.1 to 19.7 dB(A) as a result of purchasing new equipment, segregating noise sources or installing panels or curtains around sources. However, the studies lacked long-term follow-up and dose measurements of workers, and we did not use these studies for our conclusions.
• Hearing protection devices:In general hearing protection devices reduced noise exposure on average by about 20 dB(A) in one RCT and 3 controlled before-after studies (CBA) (n=57). Two RCTs showed that, with instructions for insertion, the attenuation of noise by earplugs was 8.59 dB better (95% CI 6.92 dB to 10.25 dB) compared to no instruction (2 RCTs, n=140 participants).
• Administrative controls:information and noise exposure feedback:On-site training sessions did not have an effect on personal noise-exposure levels compared to information only in one cluster-RCT after 4 months' follow-up (MD 0.14 dB; 95% CI -2.66 to 2.38). Another arm of the same study found that personal noise exposure information had no effect on noise levels (MD 0.30 dB(A), 95% CI -2.31 to 2.91) compared to no such information (n=176 participants).

Effects on hearing loss

• Hearing protection devices: Two studies compared the effect of different devices on temporary threshold shifts at short-term follow-up but data were insufficient for analysis. In 2 CBA studies there was no difference in hearing loss from noise exposure above 89 dB(A) between muffs and earplugs at long-term follow-up (OR 0.8, 95% CI 0.63 to 1.03). In another CBA study wearing hearing protection more often resulted in less hearing loss at very long-term follow-up.
• Combination of interventions: hearing loss prevention programmes:One cluster-RCT found no difference in hearing loss at 3- or 16-year follow-up between an intensive HLPP for agricultural students and audiometry only. One CBA study found no reduction of the rate of hearing loss (MD -0.82 dB per year, 95% CI -1.86 to 0.22) for a HLPP that provided regular personal noise exposure information compared to a programme without this information.In 4 very long-term studies that better use of hearing protection devices as part of a HLPP decreased the risk of hearing loss compared to less well used hearing protection in HLPPs (OR 0.40, 95% CI 0.23 to 0.69). Other aspects of the HLPP such as training and education of workers or engineering controls did not show a similar effect.In 3 long-term CBA studies, workers in a HLPP had a statistically non-significant 1.8 dB (95% CI -0.6 to 4.2) greater hearing loss at 4 kHz than non-exposed workers and the confidence interval includes the 4.2 dB which is the level of hearing loss resulting from 5 years of exposure to 85 dB(A). In addition, of 3 other CBA studies that could not be included in the meta-analysis, 2 showed an increased risk of hearing loss in spite of the protection of a HLPP compared to non-exposed workers and one CBA did not.

Comment: The quality of evidence is downgraded by study quality (unclear allocation concealment, high drop-out rate, lack of information on the implementation level of the prevention measures), inconsistency (heterogeneity in study designs) and indirectness (most studies were retrospective, mostly men were included).

References

• Tikka C, Verbeek JH, Kateman E et al. Interventions to prevent occupational noise-induced hearing loss. Cochrane Database Syst Rev 2017;7():CD006396. [PubMed]