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|Year : 2012
: 14 | Issue : 59 | Page
|Noise and hand-arm vibration exposure in relation to the risk of hearing loss
Hans Pettersson1, Lage Burström1, Mats Hagberg2, Ronnie Lundström1, Tohr Nilsson3
1 Department of Public Health and Clinical Medicine, Occupational and Environmental Medicine, Umeå University, Umeå, Sweden
2 Department of Occupational and Environmental Medicine, University of Gothenburg, Gothenburg, Sweden
3 Department of Occupational and Environmental Medicine, Sundsvall Hospital, Sundsvall, Sweden
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|Date of Web Publication||18-Aug-2012|
The aim of this study was to examine the possible association of combined exposure of noise and hand-arm vibration (HAV) and the risk of noise-induced hearing loss. Workers in a heavy engineering industry were part of a dynamic cohort. Of these workers, 189 had HAV exposure, and their age and hearing status were recorded in the same year and were, therefore, included in the analysis. Data on HAV duration and acceleration was gathered through questionnaires, observations, and measurements. All available audiograms were categorized into normal and hearing loss. The first exposure variable included the lifetime HAV exposure. The lifetime HAV exposure was multiplied by the acceleration of HAV for the second and third exposure variable. Logistic regression using the Generalized Estimation Equations method was chosen to analyze the data to account for the repeated measurements. The analysis was performed with both continuous exposure variables and with exposure variables grouped into exposure quartiles with hearing loss as an outcome and age as a covariate. With continuous exposure variables, the odds ratio (OR) with a 95% confidence interval (CI) for hearing loss was equal to or greater than one for all exposure variables. When the exposure variables were grouped into quartiles, the OR with a 95% CI was greater than one at the third and fourth quartile. The results show that working with vibrating machines in an environment with noise exposure increases the risk of hearing loss, supporting an association between exposure to noise and HAV, and the noise-induced hearing loss.
Keywords: Combined exposure, hand-arm vibration, hearing loss, noise
|How to cite this article:|
Pettersson H, Burström L, Hagberg M, Lundström R, Nilsson T. Noise and hand-arm vibration exposure in relation to the risk of hearing loss. Noise Health 2012;14:159-65
|How to cite this URL:|
Pettersson H, Burström L, Hagberg M, Lundström R, Nilsson T. Noise and hand-arm vibration exposure in relation to the risk of hearing loss. Noise Health [serial online] 2012 [cited 2020 Sep 25];14:159-65. Available from: http://www.noiseandhealth.org/text.asp?2012/14/59/159/99887
| Introduction|| |
Workers in industries such as heavy engineering, construction, mining, and forestry are exposed to hazardous levels of both hand-arm vibrations (HAV) and noise. This is particularly true when using hand-held vibrating tools such as angular grinders, hammers, or drills. If the noise levels are high and/or the duration of the noise exposure is long enough, it could cause hearing loss.  The risk of hearing loss is confounded by several factors such as age, medical, chemical, and genetic factors. ,,, Another factor that could confound and also increase the risk of hearing loss is vibrations. ,,,, A study by Pyykko et al.  suggested a common mechanism behind the association between exposure to noise and HAV and their combined effect on hearing. They suggest that vibration exposure from hand-held tools reduces the blood flow in the cochlea by activating the sympathetic nervous system, leading to increased risk of hearing loss.
Animal studies show an enhanced noise-induced hearing loss when vibrations are present.  Experimental studies on HAV, noise, and temporary effects on hearing provide conflicting results. Some support an association between noise, HAV, and hearing loss and some do not. ,, Longitudinal studies on the relationship between combined exposure to noise and HAV in relation to hearing loss did not find any excess risk of hearing loss when HAV was present. , However, longitudinal and case-control studies on subjects who have contracted vibration-related disorders found that subjects with vibration white fingers (VWF) have an increased risk of developing hearing loss. ,, To gain further knowledge, a new longitudinal study is presented.
The aim of the present study is to examine the possible association of combined exposure of noise and HAV in the relation to the risk of hearing loss.
| Methods|| |
The cohort was established in 1987, and follow-ups were done in 1992, 1997, and 2008. In 2002, only a questionnaire was used to collect information. It is a dynamic cohort that consisted of 276 male workers in 2008. All participants worked in a heavy engineering production workshop that constructs paper and pulp-mill machinery. The workers were welders/grinders, engineers, supervisors, salesmen, and administrators. This cohort study was approved by the Regional Board of Ethics for Medical Research in Umeå, Sweden.
Examination and questionnaire
All participants were medically and clinically examined by a physician and had to complete questionnaires on basic data about themselves. The questionnaires asked about the participant's subjective assessment of their daily HAV exposure, type of tool used, type of work, and years at work as well as individual data. In 2008, they were also asked about their use of hearing protectors. For each participant in the cohort, a database has been developed to collect all the data from questionnaires, health monitoring, and technical measurements during each follow-up period. ,
All available audiograms obtained on the participants in the cohort between 1987 and 2008 were analyzed. The audiograms were obtained in cooperation with the occupational health service and the County Councils central archive in Härnösand. A total of 476 audiograms were collected.
All audiograms were performed with screening pure-tone audiometry using a calibrated Entomed SA 202 automatic audiometer in a soundproof booth. The participant has to focus on low sound levels of discrete frequency pure-tones and respond by pressing a button. Only hearing levels above the screening level at 10 dB hearing level were registered. The hearing levels were determined for every discrete frequency in 5 dB steps by the shortened ascending method. Each hearing threshold level was determined by two of three responses.  The same operator was used between 2001-2008 and was educated in audiometry.
We classified all audiograms for the left ear according to Klockhoff.  This classification uses the measured hearing levels at 500, 1000, 2000, 3000, 4000, and 6000 Hz. Normal hearing was defined as hearing threshold levels under 30 dB at 500 Hz and under 25 dB for 1000, 2000, 3000, 4000, and 6000 Hz. Noise-induced hearing loss was defined as a maximum of two hearing thresholds above 30 dB at 500 Hz or above 25 dB at 1000-2000 Hz. Also, at least one of the hearing thresholds at 3000, 4000, and 6000 Hz must be above 25 dB. Other types of hearing loss such as conductive, sensorineural, or mixed type with hearing levels not meeting the criteria to be classified as normal or noise-induced hearing loss according to Klockhoff were excluded. 
Hand-arm vibration exposure measurement
The vibration acceleration exposure from hand-held vibrating tools was measured under normal working conditions at all relevant workstations. The measurements were defined in an earlier study.  The vibration acceleration from a random number of vibrating hand-held tools was also measured. If the hand-held vibrating tool had two handles, then both handles were measured and the highest vibration acceleration was then used in the analysis. All measurements were performed according to international standards. , 50% to 90% of all hand-held vibrating tools used at the company were measured at each follow-up period, and 306 tools were measured for vibration acceleration in total. The most frequently used hand-held vibrating tools were grinders and hammers. From 1987 to 2008, the mean frequency-weighted acceleration of grinders had decreased from 11.0 to 7.6 m/s 2 and for hammers, it decreased from 5.8 to 4.5 m/s 2 . 
At every follow-up, each participant estimated their daily exposure duration of HAV in a questionnaire. Each participant estimated the HAV exposure duration in minutes per day for each hand-held vibrating tool used during their last working day. They also answered how many months and years they had used hand-held vibrating tools. The total time each participant had used each vibrating tool was then calculated (in hours). To get a better assessment of the duration of HAV exposure, several observation studies were done. , The total duration of HAV exposure during a working day decreased from 108 min in 1987 to 52 min in 2008 for a working day. 
Noise exposure measurement
In 2008, personal noise exposure measurements were done on a sample of the participants (n = 15) working as welders/grinders and supervisors. By measuring supervisors who worked at the heavy engineering production workshop, the background noise exposure could be measured. Measurements were performed under normal working conditions at all relevant workstations with personal noise dosimeters (Larson Davis Spark 706). The most frequently used kinds of tools were grinders and hammers. Some of these tools were measured for HAV in 2008 (n = 78) and also measured for their noise exposure with a sound level meter (Brüel and Kjær 2240). All measurements were performed according to international standards. 
The measured noise exposure levels from the selected grinders and hammers varied between 77 and 109 dB (A) with a mean of 95 and standard deviation 7 dB (A). The background noise exposure levels varied between 75 and 88 dB (A) with a mean of 85 dB (A). In 2008, the average work time with hand-held vibrating tools during a working day was 52 min. The average 8 hour equivalent noise level is, therefore, 88 dB (A) with a range of 75 to 99 dB (A) for 2008, calculated according to international standards. 
Earlier follow-up periods did not include the measurement of noise levels at the company. The manufacturer's own noise exposure measurements of the grinder and hammer models used at the company in 1987 and 2008 were collected. Noise exposure levels were measured according to international standards.  All the measurements were made on grinders and hammers running on idle. In 1987, the mean and standard deviation of noise levels were 81 and 72 dB (A), respectively, for grinders and hammers. The total noise exposure level for both hammers and grinders was 76 dB (A). In 2008, the mean noise exposure level was 79 dB (A) for grinders and 73 dB (A) for hammers. The total mean noise exposure level was 79 dB (A).
Vibration exposure estimate
The vibration exposure was estimated in three ways (Exp1- 3) [Table 1]. It was calculated by the lifetime duration of HAV and acceleration for every tool each participant used. The first estimate was the cumulative time of HAV exposure calculated by using the measured lifetime HAV duration in hours. The second estimate was calculated by the cumulative time of HAV multiplied by HAV acceleration, and the third estimate was calculated by the cumulative time of HAV multiplied by the square of the acceleration. , The estimates of exposure were calculated for every year each participant was part of the cohort and had their HAV exposure duration and acceleration measured.
|Table 1: All three exposure variables are calculated by first summarizing the lifetime duration of HAV exposure. For the second and third exposure estimates, the lifetime HAV duration exposure is multiplied by the acceleration of HAV and the square of the acceleration, respectively|
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The analysis included 189 participants. The total number of years from the participants with registered audiometric measurement, age, and the exposure variable, Exp1, in the same year was 1077, while for Exp2-3, it was 1076. The analyzes of the data were performed using PASW statistics software version 18.0 (SPSS Inc., Chicago, Illinois).
Logistic regression was used to assess the longitudinal effects from the calculated exposure variables and age on hearing loss. The Generalized Estimating Equations procedure was used with a first-order autoregressive correlation structure to take into account the repeated measurements. This structure was used because measurements adjacent to one another were assumed to be more correlated than measurements several years apart. The dependent or outcome variable was hearing status. Normal hearing was classified as 0 and for impaired hearing, it was classified as 1. The independent or explanatory variables were age (in years) and the calculated exposure variable. The analysis was performed with both continuous exposure variables and with exposure variables grouped into exposure quartiles. When the three exposure variables are continuous, it was assumed that the risk of hearing loss was the same for the entire range of exposure variables levels. This assumption might be false; therefore, an analysis with exposure variables divided into quartiles was also done. Every exposure variable was divided by 1,000, so that each unit of exposure variable shared a greater change. The number of years from the 189 participants in the analysis with HAV exposure, hearing audiogram, and age in the same year were grouped into quartiles by the range of the exposure variables. The second and third continuous exposure variables were also normalized to the mean value of the first exposure variable. This was done by first dividing the mean values for both the second and third exposure variable with the mean value of the first exposure variable. Then, the individual data for both the second and third exposure variable was divided with the calculated ratios. If one of the exposure variables increased between two follow-up periods, then it was assumed that the exposure variables increased linearly for each year between these follow-up periods.
| Results|| |
In the analysis with continuous exposure variables, the odds ratio (OR) and 95% Confidence Interval (CI) for hearing loss were equal to or greater than one for all exposure variables [Table 2].
|Table 2: Odds ratios with 95% confidence intervals (95% CI) for hearing loss among workers exposed to HAV (adjusted for age) by the lifetime exposure duration of HAV (Exp1), or the lifetime exposure duration of HAV multiplied by the acceleration of HAV (Exp2) or multiplied by the square of the acceleration (Exp3), or normalized second and third exposure variable (Exp2N and Exp3N)|
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The cumulative exposure time (Exp1), the normalized cumulative time multiplied by the acceleration (Exp2N), and the cumulative time multiplied by the square of the acceleration (Exp3N) are presented in [Table 2]. If the cumulative time increases by one thousand units, the OR for hearing loss increases by 12%. If the normalized cumulative time multiplied by the acceleration or the square of the acceleration increases by one thousand units, the OR increases 9% and 8%, respectively. The increase is only significant for the normalized cumulative time multiplied by the acceleration.
The OR with 95% CI and the numbers of years with normal hearing or with a hearing loss for each quartile of the exposure variables are presented in [Table 3]. The lowest quartile is the reference category. For the cumulative time and cumulative time multiplied by the acceleration, there is a small increase in the number of hearing-impaired subjects between the first and second quartile. From the second to third quartile, the numbers almost double for all exposure variables. The numbers of years with hearing loss increases even more between the third and fourth quartile for all exposure variables.
|Table 3: Odds ratios with 95% confidence intervals (95% CI) and the number of years with normal hearing and hearing loss among workers exposed to HAV grouped into quartiles (Q1-4) (adjusted for age) by the lifetime exposure duration of HAV (Exp1), or the lifetime exposure duration and acceleration of HAV (Exp2-3)|
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For all exposure variables, the OR increases from the first to third quartile and then decreases for the fourth quartile compared with the third quartile. For the second quartile, only the cumulative time had an OR with 95% CI greater than one of 2.6 (1.0 - 6.9). All exposure variables had an OR with 95% CI greater than one for the third and fourth quartile. For the third quartile, the OR was at its highest level for all exposure variables with OR of 4.6 (1.9-11) associated with the cumulative time, 5.6 (2.2-14) associated with the cumulative time multiplied by the acceleration, and 3.9 (1.6-9.1) associated with the cumulative time multiplied by the square of the acceleration.
| Discussion|| |
HAV and noise exposure co-exist when workers use hand-held vibrating tools. To gain further knowledge on the effect of HAV on hearing, we, therefore, included HAV acceleration exposure in the analysis. The results from the analysis suggest an association between the risk of hearing loss and combined exposure to noise and HAV.
The risk of hearing loss calculated as OR increases when the cumulative exposure time increases. Because noise and HAV exposure co-exist from hand-held vibrating tools, the results could be due to an increased duration of noise exposure alone. The analysis of the cumulative time multiplied by the vibration acceleration also increases the OR significantly. The OR of hearing loss did not increase significantly when cumulative time was multiplied by the square of the vibration acceleration. The OR is larger for the cumulative time than for the cumulative time multiplied by the acceleration. However, these two variables differ since the cumulative time multiplied by the acceleration also consists of the HAV acceleration and has a different exposure range.
By normalizing the cumulative time multiplied by the acceleration and the cumulative time multiplied by the square of acceleration, these measures will be recalculated to be of more comparable units with the cumulative time mean and range. If the normalized cumulative time multiplied by the acceleration increases by one thousand units, the OR of hearing loss is 1.09 compared with 1.12 for the cumulative time.
With continuous exposure variables, it is assumed that the risk of hearing loss is the same for the entire range of exposure variable levels. The same increase from low levels of the exposure variables is assumed to be an equal risk as if the same increase happens for high levels of the exposure estimates. By dividing the continuous exposure variables into quartiles, the results showed that this was not the case. The risk of hearing loss increases from the first quartile up to the third exposure quartile and decreases from the third to the fourth quartile for all three exposure variables. These results suggest that increased total lifetime exposure duration of HAV with or without acceleration increases the risk of hearing loss. The results from this analysis suggest an association between the risk of hearing loss and combined exposure to noise and HAV from hand-held vibrating tools. However, the association between hearing loss and combined exposure to noise and HAV must be interpreted with some caution.
Extensive noise exposure measurements were only performed in 2008, while there is a lack of noise exposure measurements from 1987. However, noise emission values from manufacturers of hand-held vibrating tools on a majority of grinders and hammers used in 1987 could be used for a rough estimation of noise levels. In 1987, the average noise exposure from grinders and hammers were 81 dB (A) and 72 dB (A), respectively, and in 2008, the average was 79 dB (A) for grinders and 73 dB (A) for hammers. The exposure duration from hand-held vibrating tools decreased from 108 min per day in 1987 to 52 min per day in 2008. The noise exposure levels from grinders and hammers were more or less the same in 1987 and 2008. The small decrease in duration of working with hand-held vibrating tools also suggests that the average 8 hour equivalent noise levels were more or less the same for 1987 and 2008. From these noise measurements and HAV measurements, we can conclude that participants using hand-held vibrating tools are exposed to both high levels of HAV acceleration and noise. The lack of noise measurement from earlier years made it impossible to separate the noise exposure duration from HAV exposure duration since they co-exist. To gain further knowledge of how HAV might affect hearing loss, the HAV acceleration was multiplied by the lifetime exposure of HAV for the exposure estimate cumulative time multiplied by the acceleration and cumulative time multiplied by the square of the acceleration.
Earlier longitudinal studies by Pyykko et al. and Starck et al. did not find any association between hearing loss and the exposure to both noise and HAV. There are some differences between these earlier studies and this study. Both Pyykko et al. and Starck et al. tried to predict the hearing for each individual by calculation according to the Robinson model.  The difference between measured and estimated hearing levels would then be related to HAV. Since our study did not have extensive measured noise levels or information on the use of hearing protectors from years prior to 2008, we used a different approach. We analyzed if noise duration exposure and the total lifetime HAV exposure with vibration acceleration are associated with measured hearing loss.
All audiograms in our study were classified according to Klockhoff et al.  By using this classification, more information on the noise-induced hearing loss for all frequencies was used in the analysis. A problem with this classification is that a small change in hearing for one frequency can result in a noise-induced hearing loss classification. Similarly, a larger noise-induced hearing loss for several frequencies might appear in the normal hearing range and may not affect the hearing classification. Moreover, other causes of hearing loss cannot be ruled out. Another problem is that different classification methods might classify an audiogram differently.  Pyykko et al. and our study only used audiometric measurement of the left ear. Starck et al. calculated an average of both ears (binaurally measurement). The left ear was chosen since epidemiological and experimental studies have shown an asymmetry in hearing loss between the left and right ear after noise exposure. The left ear has a greater hearing loss than the right ear and the difference increases with poorer thresholds. ,,, The asymmetry is independent of handedness.  This asymmetry is also present for subjects without any experience of firing a weapon in military service or hunting in leisure time. , By using the left ear, we should not underestimate the hearing loss. Both Pyykko et al. and Starck et al. studied hearing at 4000 Hz as a response variable. Both Pyykko et al. and Starck et al. gathered information on workers' use of hearing protectors. The forest workers in the Pyykko et al. study used hearing protectors 60% of their sawing time on average. In the Starck et al. study, the average use of hearing protectors for forest workers was 79% and 93% for shipyard workers. In our study, only information on the use of ear protection for 2008 was available. Among participants who used hand-held vibrating tools during their latest working day, the average use of hearing protectors as a percentage of the working day was 82% with a range of 0-100% in 2008. Even a short duration of noise exposure without hearing protectors reduces much of the dampening effect from the protectors. As mentioned earlier, the average noise exposure from hand-held vibrating tools is 95 dB (A) for 52 min each working day with a background noise exposure level of 85 dB (A). In-ear hearing protector's used by workers dampen noise by about 29 dB. Using hearing protectors the whole working day, the average 8-hour equivalent noise level is 59 dB (A). 
Experimental studies on humans are restricted to safe noise and vibration levels to prevent permanent hearing loss. To experimentally study the effect of HAV and noise exposure on hearing with exposure levels common to hand-held vibrating tools, animal studies can be used. Animal studies by Hamernik et al. have shown that vibrations combined with noise exposure produce more hearing loss than exposure to just noise. However, they also found that by increasing the vibration's acceleration while still using the same noise level did not produce a higher hearing loss. Therefore, the vibrations might increase the risk of hearing loss, but only up to a certain level. In our study, the results indicated a decrease of OR for the fourth quartile compared to the third quartile of the exposure variables. There might, therefore, be a threshold where an increased HAV exposure duration and/or acceleration would not increase the risk of hearing loss. Another explanation for the decrease in OR for the fourth quartile could be due to the healthy worker effect. Workers with injuries might have ended their employment and no longer be part of the study. Only healthy workers would then be present in the last quartile. Furthermore, supervisors, salesmen, and administrators were included since some of them had an earlier career as a welder/grinder within the company and had been exposed to HAV and noise. Also, by categorizing all hearing audiograms in normal hearing and those with a hearing loss, no information is available about whether the participants with a hearing loss have an increased loss of hearing with increased lifetime exposure to noise and HAV. A study on the same cohort found a higher occurrence of vibration white finger (VWF) in participants exposed to HAV acceleration and to longer durations.  Earlier longitudinal and case-control studies have found an increased risk of hearing loss for workers with VWF. ,, The present study did not include VWF in the analysis.
As mentioned earlier, there is a lack of noise exposure measurement from years prior to 2008. There are no measurements of how much of the noise exposure is continuous and/or impulsive. Nor do we have information on the participants' leisure noise exposure such as the use of fire arms during hunting. An exposure to HAV in leisure time was not included in the study. The participants had to estimate their own exposure duration. Studies on self-estimation have shown that people tend to overestimate their self-estimation of exposure duration when they have used hand-held vibrating tools. This could create a biased relation between HAV (and, in this study, noise) exposure duration and the outcome variable (in this study, the hearing loss). , Tobacco usage was not controlled for since an earlier study on the effect of HAV and noise exposure on hearing loss did not find excess risk of smoking.  The same study also found that blood pressure might affect hearing loss, but this was not measured in the present study.
| Conclusion|| |
The results show that working with vibrating machines in an environment with noise exposure increases the risk of hearing loss, supporting an association between exposure to noise and HAV, and noise-induced hearing loss.
| Acknowledgement|| |
We gratefully acknowledge the financial support of AFA Insurance (Project 2007-0104). The authors would also like to thank Leif Nilsson for his support in the statistical analysis of the data.
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Department of Public Health and Clinical Medicine, Occupational and Environmental Medicine, Umeå University, SE-901 87, Umeå
Source of Support: AFA Insurance (Project 2007-0104), Conflict of Interest: None
[Table 1], [Table 2], [Table 3]
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|[Pubmed] | [DOI]|