Based on this study, the prevalence of hearing impairment is comparable in metal and woodwork worker, but higher noise exposure level is recorded in woodwork industry and the majority of the employees exposed to the noise level above the permissible exposure limit value. Listening to music using earphones for more than two hours per day and playing music with loud volume was associated with hearing impairments.
In this study, the prevalence of self-reported hearing impairments among wood and metalwork workers was 20.7%, which is in line with a study done in the United States among wood manufacturing industry workers 20.86% (20) and the study done in Ethiopia, among metalwork industry 22% (22). This similarity may be due to similar methods followed(cross-sectional and retrospective cross-section in the case of US )and characteristics of the worker. However, this study result is lower than a study done in Rwanda among the wood and metalwork industry (36%)(23), and 31% in Nepal among woodworkers (9). This difference might be attributed to differences in methods followed (audiogram test in Rwanda may be an increased prevalence), utilization of hearing protective device (0.5% in Rwanda, 6.6% in our study). Also, this difference could be justified by the duration of exposure or work experience. On the other hand, this finding is higher than the study conducted in a metalworking company in Brazil 15.9% (24) and this difference may be because enforcement of occupational health and safety regulation is very weak in Ethiopia and the other may be due to high noise exposure level in Ethiopia.
In this study worker who listens to music using earphone for more than two hours per day were more likely to develop noise-induced hearing impairments when compared to their counterparts, and this finding is consistent with research done in Singapore which reported that one in six young person’s is at risk of developing leisure NIHL from music delivered via earphones(25), and another study also indicated that listening music through headphone for 3 hours at their usual maximum level showed transient shifts of 10 dB and 30 dB and returned to normal within 24 hours and this can cause noise-induced hearing impairment if not reduced duration of exposure(26).
In this study, other factors that had a significant relation with noise-induced hearing impairment were listening to music with loud volume, and workers who were listening to music with loud volume were more likely to develop noise-induced hearing impairments than workers who didn’t listen to music at loud volume. This finding is supported by research finding that reported exposure to loud leisure noise is correlated with hearing loss and tinnitus and the risk rises as noise exposure increases (27). And another study done among young individuals showed that listening to music too loud over a lengthy time on personal listening devices (PLDs) such as CDs, iPods, and other MP3 players has been shown as a potential contributor to NIHL (28).
Another finding of this study is the noise level of the wood and metalwork industry, which is not a significant factor for noise-induced hearing impairments and the finding is similar to a study done among the metalwork industry in Ethiopia, which was not significant for noise-induced hearing impairments (22). But in another study, the level of noise was one of the determining factors for NIHI (11, 29, 30) but the possible reason for noise level was not significant in this study is may be due to the short duration of exposure or work experience and the work nature is entertainments and not fully exposed for 8 hours for a noisy machine. And another reason may be the total noise intensity level may not be reached to workers as workers are mobile in the working environment.
According to this study, prevalence of self-reported hearing impairment was comparable among woodwork (20.8%) and metalwork(20.5%) industry workers and this finding is different from a study done among metal and woodwork industry workers and prevalence of hearing impairments was higher among metalwork (40%) workers than woodwork (30%) industry workers (23).
Regarding the noise exposure level of workers in their current occupation nearly three-fourth (72%) of respondents exposed to noise levels greater than 95 dBA, which is above OSHA permissible exposure limit value for eight hours working time as it is considered hazardous (31). This finding shows that workers in the wood and metalwork industry work on hazardous noise condition for a long time (8 hours) as the country set occupational exposure limit to continuous noise at 90 dBA,92dBA,95 dBA,95dBA, 97 dBA,100 dBA,102 dBA,105 dBA,110 dBA,115dBA to a period of 8,6,4,3,2,1 and ½, 1, ½, 1/4, hours respectively (32).
Limitation of the study
The inability of measuring the personal noise exposure level using a noise dosimeter and not using an audiogram to assess the hearing impairments workers.