A Cross-Sectional Study of the Time of Onset of Hearing Loss in Bus Drivers Following Exposure to Occupational Noise
Article information
Abstract
Background and Objectives
Occupational noise exposure is a significant risk factor for hearing loss, affecting approximately 5% of the global population. Although noise-induced hearing loss is commonly associated with reduced sensitivity to pure tones, there is limited information regarding when this hearing loss begins after noise exposure. This study aimed to investigate the time of onset of hearing loss in bus drivers exposed to noise for varying durations.
Subjects and Methods
The study involved 102 bus drivers aged 25-40 years who had been exposed to noise for periods ranging from 6 months to over 10 years. A control group comprising 102 age-matched individuals without noise exposure was also included. Pure-tone audiometry was performed to assess hearing loss, and time of onset of hearing loss relative to noise exposure duration was evaluated.
Results
Bus drivers experienced only “slight” hearing loss, even after 10 years of noise exposure. By contrast, reduced sensitivity to pure tones was observed after 25 to 48 months of exposure.
Conclusions
This study confirms that prolonged noise exposure leads to “slight” hearing loss, which can appear as early as 25-48 months after exposure to noise. Among individuals under 40 years of age, significant hearing loss appeared to occur only after 10 or more years of exposure. These findings suggest that the impact of noise on hearing acuity may have been overestimated in previous studies.
Introduction
Noise is defined as any unwanted or disruptive sound that interferes with the normal auditory environment. While everyone encounters noise in daily life, individuals exposed to high levels of noise in their work environments—known as occupational noise—are of particular concern. Occupational noise exposure involves prolonged, excessive exposure to loud sounds in the workplace and is widely recognized as a major risk factor for hearing impairment [1-3]. Noise-induced hearing loss (NIHL) is a key consequence of such exposure, characterized by a gradual decline in hearing sensitivity [3]. Globally, NIHL affects approximately 5% of the population, impacting physical, mental, social, and economic well-being [4].
Mohammadi [5] identified traffic noise as a major source of environmental pollution in developing nations. Professional bus drivers are particularly vulnerable to NIHL due to their consistent exposure to high levels of engine noise over extended periods [6-14]. In addition to engine noise, poor road conditions contribute to elevated traffic noise [15]. Drivers of heavy vehicles face additional exposure to engine vibrations and wind noise [16].
While there is substantial evidence that prolonged noise exposure results in reduced hearing sensitivity [3,8], little information exists on the precise time of onset of hearing loss following noise exposure.
Pure tone sensitivity and noise exposure
Janghorbani, et al. [8] reported bilateral NIHL in 18.1% of 4,300 long-distance professional bus drivers in a retrospective study. However, this study lacked a control group for comparison, and all participants were over 50 years old. Age, a well-known factor in hearing loss, particularly at high frequencies [17], was not considered.
Majumder, et al. [9] compared hearing sensitivity between two groups of drivers—those exposed to noise for less than 10 years and those exposed for more than 10 years—with a control group of office workers. As expected, the office workers had better hearing thresholds (below 25 dB) compared to both driver groups (above 25 dB at 0.5, 1, 2, and 3 kHz). However, the significant difference in exposure durations (an average of 6 years for one group and 15 years for the other) complicates the comparison.
Pourabdian, et al. [13] reported hearing loss in 26.8% of 65,533 heavy vehicle drivers, including bus and truck drivers. The participants, aged 20 and above (mean age: 38.2 years), lacked data on their exposure duration. Golbabaei Pasandi, et al. [11] found that 64.8% of the 1,461 drivers studied had hearing impairment (>25 dB) in the right ear and 54.9% in the left ear. However, 19.6% of participants had prior exposure to loud noise and medical conditions like diabetes and hypertension, which could have influenced the results.
Although numerous studies have investigated the effect of noise on hearing in bus and truck drivers [3,7-11,13], many participants in these studies were over 40 years old [8,9,11,13] or had comorbidities such as diabetes and hypertension [11]. Given that age-related hearing loss tends to begin after the age of 40 [18], the age of participants should be taken into account when assessing the effect of noise on hearing sensitivity. Additionally, none of these studies have considered the duration of noise exposure as a critical variable.
Duration of noise exposure and NIHL
Several studies have shown a significant association between the duration of noise exposure and hearing thresholds in professional drivers [9,10,12,19,20]. For example, Cordeiro, et al. [19] found a positive correlation between hearing loss and cumulative working time, although age influenced this relationship. Similarly, Malar and Kavitha [20] reported that hearing thresholds at 4,000, 6,000, and 8,000 Hz worsened as noise exposure duration increased (with a minimum exposure of 8 years) in both ears.
Patwardhan, et al. [12] studied 200 bus drivers and found that all drivers exposed to noise for more than 16 years had hearing loss, compared to 72% of those with less than 5 years of exposure. Meanwhile, Pai [10] found that fewer than 20% of drivers exposed to noise for 10 years or more had a hearing loss greater than 25 dB (averaged across 500 Hz, 1 kHz, and 2 kHz), and drivers with less than 5 years of exposure did not exhibit hearing loss.
These studies broadly indicate that the incidence of hearing loss increases with longer noise exposure. However, the observed relationship is not consistent. For instance, Patwardhan, et al. [12] found hearing loss even in drivers exposed to noise for less than 5 years, while Pai [10] did not observe hearing loss in this group. Patwardhan, et al. [12] also did not account for the confounding effects of age, with participants ranging from 20 to 58 years old. Furthermore, Majumder, et al. [9] grouped drivers into broad categories of noise exposure duration (less than 10 years and more than 10 years), making it difficult to pinpoint the exact relationship between noise exposure and hearing loss.
Although there is substantial evidence that prolonged exposure to noise damages hearing, data on the timing of hearing loss onset—specifically how early hearing loss begins following noise exposure—remains limited. Some insights into the relationship between the duration of noise exposure and hearing loss can be inferred from studies like Patwardhan, et al. [12] and Pai [10], though they do not directly address the question of how soon hearing loss sets in after exposure. Given that NIHL is irreversible [21], early detection is crucial to implement preventive measures. Additionally, early NIHL accelerates presbycusis, worsening age-related cochlear dysfunction by damaging the sensory-neural epithelium, causing synaptopathy, and affecting vascular function in the cochlea [22]. Information on when sensitivity to pure tones begins to decline due to noise exposure is critical for early detection and prevention of hearing impairment.
Study focus
This study focused on bus drivers in Kolar, a district headquarters, who are regularly exposed to high levels of noise in their work environment. In addition to noise, these drivers experience heavy vibrations and other environmental hazards. Long-term exposure to these conditions is likely to result in NIHL. Given that many drivers are unaware of the impact of noise on hearing, studies like this are essential for promoting preventive and early intervention measures. As part of a broader study on “Early Indicators of Auditory Deviations Following Exposure to Noise,” this research examined the time of onset of hearing loss among professional bus drivers exposed to varying durations of noise (ranging from 6 months to 15 years).
Subjects and Methods
This cross-sectional study focused on measuring pure tone hearing thresholds among a group of bus drivers. Ethical clearance for the study was obtained from the Ethics Committee of Sri Devaraj Urs Academy of Higher Education and Research (SDUMC/KLR/IEC/219/2018-19).
Participants
The study group (Group A) consisted of 102 bus drivers, all males (operating front-engine buses) aged 25 to 40 years. A key inclusion criterion was that participants must have at least 1,000 hours of noise exposure, equivalent to approximately 6 months of service (6 days a week, with 8 hours of driving per day). This information was verified using the official records from the Transport Corporation that employed these drivers. It turned out that the driver participants had exposure to noise between 6 and 183 months.
The control group (Group B) comprised 102 age and gender matched (all males) with normal hearing and no history of noise exposure. Both groups exhibited “A” type tympanograms with normal acoustic reflexes on immittance audiometry. Individuals with other tympanogram types were excluded from the study. Additional exclusion criteria included a history of middle ear pathology, ototoxic drug use, infections (like mumps/ measles), comorbid conditions such as diabetes mellitus and Bell’s palsy, traumatic ear injuries, or regular use of personal music systems for more than one hour per day. This information was gathered through a detailed interview before participants were recruited for the study and testing.
Comprehensive personal, medical, work environment and noise exposure histories were gathered from participants using a standard questionnaire. An otoscopic examination (Welch Allyn) was performed to check for impacted wax, foreign bodies, growths, infections, and to inspect the tympanic membrane for a visible cone of light. All participants in both groups had intact eardrums with visible cones of light. Information on the hearing status of bus drivers before their exposure to noise was not available, but no participant complained of any hearing loss at the time recruitment into the present study. This perhaps is an indication that the participants did not have any hearing impairment before they were exposed to noise. Impedance audiometry (Inventis Clarinet Plus) was used to assess middle ear function. These results, along with demographic data, were used to select participants. Both groups were similar in characteristics except for noise exposure, with only the drivers (Group A) being exposed to occupational noise.
Noise level measurement
The average noise level (Leq) and maximum noise level (Lmax) near the driver’s seat in the buses were measured using an Android-based app (Decibel X). Four one-minute recordings were taken at 10-minute intervals in five different buses traveling on busy streets in the city of Kolar during peak traffic hours (9:00–10:00 AM and 4:30–5:30 PM). Morning and evening recordings were carried out on different days, and noise measurements were completed over 6 to 8 days. Similarly, noise levels on the university campus, where the control group participants were recruited, were measured on three different days, at three different times each day.
Pure tone audiometry
Pure tone audiometry was conducted using air conduction headphones (RadioEar DD45) and bone conduction transducers (RadioEar B-71) with a calibrated diagnostic audiometer (GSI Audiostar Pro). Audiometric thresholds were obtained at octave frequencies ranging from 0.25 kHz to 8 kHz using the modified Hughson–Westlake method [23]. All testing was done in a sound-treated room built to meet ANSI S3.1 (1999) standards. The pure tone average (PTA) was calculated as the average threshold at octave frequencies between 0.5 kHz and 4 kHz. Hearing loss was classified as follows based on the Modified Goodman’s Classification [24]: normal hearing, 0–15 dB HL; slight hearing loss, 16–25 dB HL; mild hearing loss, 26–40 dB HL; moderate hearing loss, 41–55 dB HL; moderately severe hearing loss, 56–70 dB HL; and severe hearing loss, 71–90 dB HL.
Statistical analysis
Data were analyzed using SPSS Statistics Version 23 (IBM Corp.). The Shapiro–Wilk test indicated that the PTA data were not normally distributed (p<0.001), so nonparametric tests were used for statistical analysis.
Results
Participants in Group A (bus drivers) ranged in age from 25 to 40 years, with a mean age of 35.37 years. Participants in Group B (control group) were also aged 25 to 40 years, with a mean age of 35.14 years. The two groups were matched for age to eliminate its potential confounding effect on hearing. The duration of noise exposure for each participant in Group A was calculated from their start date with the organization up to the date of audiometric testing. At the time of recruitment, participants in Group A had experienced noise exposure ranging from 6 to 183 months. It was assumed that the drivers typically worked 6 days a week, for 8 hours each day.
Noise levels
The equivalent continuous sound level (Leq) recorded in the buses ranged from 84.7 dB to 89.9 dB during the morning recordings and from 87.1 dB to 90.2 dB in the evening recordings. The maximum sound level (Lmax) reached 102.1 dB in the morning and 100.2 dB in the evening, with the highest levels often associated with honking. In comparison, at the university campus where the control group participants worked, the average noise level was recorded at 62.3 dB (Leq), with a maximum level of 65.6 dB (Lmax).
Pure tone audiometry
The mean PTAs of participants in both the driver and control groups were analyzed using the Mann–Whitney U test. The findings, as shown in Table 1, indicated that the hearing thresholds of bus drivers were significantly different from those of the normally-hearing control group for both the right ear (z=-7.474, p<0.001) and the left ear (z=-6.550, p<0.001).
PTA in control and driver groups for the right and left ears (n=102 in each group)
It was observed that a greater number of drivers exhibited decreased hearing thresholds compared to the control group. Specifically, 63 drivers (59 with “slight” and 4 with “mild” hearing loss) and 59 drivers (52 with “slight” and 7 with “mild” hearing loss) (see Modified Goodman’s classification [24]) showed decreased hearing sensitivity in the right and left ears, respectively. In contrast, only 8 participants in the control group exhibited “slight” hearing loss.
Pure tone thresholds by duration of noise exposure
Drivers were exposed to noise for varying durations, ranging from 6 months to 183 months. Participants in Group A were categorized based on the duration of exposure, with intervals of 2 years to ensure a sufficient number of participants in each category for statistical analysis. This interval also helped identify the earliest onset of auditory deviations due to noise exposure. Thus, drivers were divided into six subgroups based on their noise exposure duration: 6–24 months, 25–48 months, 49–72 months, 73–96 months, 97–120 months, and above 120 months. Table 2 outlines the sample size for each noise exposure duration category and the corresponding age ranges of the participants. For each noise exposure category, participants from the control group whose ages matched the observed age range of drivers in that noise exposure duration category were included for comparison. Table 2 shows that the age range of participants, both normal and drivers, is comparable in all the subgroups.
Sample size and age distribution by noise exposure duration and group
Hearing levels of drivers in each subgroup were compared to age-matched control participants (under each noise exposure duration) rather than the entire control group (n=102). The comparison of hearing levels between drivers in each “noise exposure” category and age-matched control subjects was performed using the Mann–Whitney U test. The results are presented in Table 3.
Comparison of hearing levels across noise exposure durations for the right and left ears
The results in Table 3 show that drivers exposed to noise for 25 months or more exhibited significantly different hearing thresholds compared to age-matched controls for both ears (p<0.01 for 25 to 48 months and 49 to 72 months categories; p<0.001 for 73 to 96 months, 97 to 120 months, and >120 months categories). No significant difference was found between drivers with less than 24 months of noise exposure and the control group for either ear.
Within-group comparison of PTA by duration of noise exposure
A Kruskal–Wallis test was used to compare mean PTAs within Group A based on noise exposure duration. Significant differences were found in the mean PTA for drivers with varying exposure durations (right ear: χ2=20.439, p<0.001; left ear: χ2=15.159, p<0.01). The data suggests that the mean PTA increased with longer noise exposure, though the relationship was not strictly linear.
Post-hoc analysis using multiple Mann–Whitney U tests revealed that drivers exposed to noise for over 120 months had significantly different mean PTA values compared to those with 6–24 months (right ear: z=-3.094, p<0.01; left ear: z=-2.621, p<0.01), 25–48 months (right ear: z=-2.703, p<0.01; left ear: z=-2.391, p<0.01), and 49–72 months (right ear: z=-3.099, p<0.01; left ear: z=-2.506, p<0.01) of exposure. Other pairwise comparisons did not show statistically significant differences.
Within-group (drivers) comparison of hearing thresholds by duration of noise exposure and audiometric frequencies
The Friedman test, a nonparametric alternative to repeated measures analysis of variance (ANOVA), was used to compare pure tone thresholds across different frequencies in the right and left ears (i.e., within the same noise exposure duration group). Additionally, hearing sensitivity at each frequency was analyzed across different categories of noise exposure duration using Kruskal–Wallis test. The results of these analyses are presented in Tables 4 and 5 for the right and left ears, respectively.
Within-group comparison of hearing thresholds among drivers categorized by noise exposure duration (Kruskal–Wallis test) and by pure-tone frequencies (Friedman test) for the right ear
Within-group comparison of hearing thresholds among drivers categorized by noise exposure duration (Kruskal–Wallis test) and by pure-tone frequencies (Friedman test) for the left ear
The findings revealed that hearing thresholds for a given duration of noise exposure differed significantly across frequencies ranging from 250 Hz to 8 kHz (see the last column of the table) for both ears. However, hearing thresholds varied significantly across different noise exposure duration categories only at 4 kHz (p<0.001) and 8 kHz (p<0.05).
Although the association is not entirely consistent, hearing thresholds generally increase from lower to higher frequencies within each noise exposure duration category. Similarly, thresholds tend to rise (though not perfectly linearly) with longer noise exposure durations, progressing from 6–24 months to over 120 months.
Discussion
Sensitivity to pure tones
Noise exposure in the workplace is widely recognized as an occupational hazard. Research involving workers such as millers, miners, and truck and bus drivers has consistently highlighted the detrimental effects of noise on hearing. In the present study, bus drivers exposed to noise were more likely to experience hearing loss compared to those who were not exposed. Hearing sensitivity decreased in 61.76% of drivers’ right ears (63/102) and in 57.84% of their left ears (59/102). However, most cases showed only “slight” hearing loss, even after prolonged exposure. Recording otoacoustic emissions would have been ideal to confirm that the observed hearing loss was caused by noise exposure. However, since all participants were carefully screened at the time of recruitment to ensure they had no ear canal or middle ear pathologies, comorbid conditions, or history of traumatic ear injury, it is reasonable to assume that the recorded hearing loss resulted from noise exposure.
Noise levels measured during peak traffic hours ranged from 84.7 dB(A) to 90.2 dB(A). It is important to note that drivers may not have been consistently exposed to these levels over the years. Some participants had been exposed to noise for up to 15 years, but conditions during earlier years might have differed due to advancements in vehicle technology and road infrastructure. Additionally, the data does not provide information on factors such as the frequency of workdays, holidays, or the rest periods between trips, making it difficult to establish a direct correlation between noise exposure and hearing loss.
A study by Golbabaei Pasandi, et al. [11] involving 1,461 professional drivers in Shahroud City found hearing loss in 64.8% and 54.9% of the drivers’ right and left ears, respectively. However, direct comparisons with the present study are not appropriate due to differences in methodology, such as variations in age groups, duration of noise exposure, and criteria for determining hearing loss.
The mean PTA for the drivers was significantly different from the control group in both ears. While drivers experienced “slight” hearing loss [24], the wide range of exposure durations (6 to 183 months) suggests that the mean PTA may not accurately reflect the hearing condition of each participant. Further analysis revealed that exposure to noise for less than 2 years had minimal impact, whereas exposure for 25 to 48 months or longer led to decreased hearing sensitivity, though the loss remained classified as “slight.” Importantly, age was not a confounding factor, as both groups were matched for age. Thus, it can be concluded that hearing loss, even if “slight,” can begin as early as 25 to 48 months of exposure to noise.
Within the driver group, hearing thresholds worsened significantly for those exposed to noise for more than 120 months compared to those with shorter exposure times. However, no significant differences were observed between drivers exposed to noise for 73–96 months, 97–120 months, and those exposed for more than 120 months. The most notable differences were seen between drivers with 6–24 months of exposure and those with over 120 months of exposure, underscoring the cumulative effect of noise on hearing. This observation suggests that although a greater degree of hearing loss was observed in drivers with more than 120 months of exposure, the hearing loss remained “slight.” This finding may indicate that the impact of noise on hearing has been overestimated in previous studies.
Within-group comparison of hearing sensitivity by duration of noise exposure and audiometric frequencies
Previous research has shown that higher frequencies are more significantly affected than lower frequencies in bus drivers exposed to occupational noise [25,26]. Corrêa Filho, et al. [25] found that the most affected frequencies were 6 kHz and 4 kHz, while Izadi, et al. [26] reported that hearing loss rates increased with higher audiometric frequencies, particularly at 4 kHz.
The findings of the present study suggest that in bus drivers, high frequencies (4 kHz and 8 kHz) exhibit greater hearing loss as the duration of noise exposure increases. Similarly, high frequencies are generally more affected than low frequencies in each noise exposure condition, though the results are somewhat inconsistent. Regardless, the pronounced impact on high frequencies aligns with the typical characteristics of NIHL. Additionally, the current results indicate that hearing loss can occur after as little as 2 to 4 years of noise exposure and that it is high frequencies which are likely to be affected most and earlier compared to the low frequencies.
Age plays a critical role in determining the effect of noise on hearing. Previous research has shown that individuals over 40 years of age exposed to noise suffer greater hearing loss [11]. Hearing sensitivity naturally declines with age, and the combined effects of noise and aging are cumulative [27]. A cautious conclusion from the present study is that in individuals under 40 years old, hearing loss due to noise exposure typically takes over 10 years to manifest.
Limitations of the study
A longitudinal design with baseline hearing assessments conducted prior to noise exposure would have been a more robust approach to documenting the effects of noise on hearing. However, practical constraints necessitated the use of a cross-sectional design in this study. As a result, the findings are limited by the absence of data on participants’ hearing status before noise exposure. In other words, the possibility of undiagnosed pre-existing hearing impairment prior to exposure to noise, though improbable considering the age of the participants, cannot be entirely ruled out.
Additionally, dividing participants into groups based on a 2-year duration of noise exposure led to smaller sample sizes within each category. This limitation may have reduced the statistical power of the analysis and affected the generalizability of the findings. Future research in this area may address these limitations by employing longitudinal designs and ensuring adequate sample sizes for more conclusive results.
Conclusions
The findings of this study confirm that prolonged exposure to noise results in “slight” hearing loss, which can occur as early as 25 to 48 months of exposure. The results also indicate that the high frequencies are affected to a greater extent than the low frequencies. The differences observed in hearing loss between 25–48 months and over 120 months of exposure suggest that the impact of noise on hearing may have been overestimated in previous studies. These findings are specific to bus drivers under 40 years old, and the effects of aging on noise.
Notes
Conflicts of Interest
The authors have no financial conflicts of interest.
Author Contributions
Conceptualization: Jayaram Mannarukrishnaiah. Data curation: H.M. Apoorva. Formal analysis: H.M. Apoorva. Investigation: H.M. Apoorva, Jayaram Mannarukrishnaiah. Methodology: Jayaram Mannarukrishnaiah, H.M. Apoorva. Project administration: Jayaram Mannarukrishnaiah, H.M. Apoorva. Software: H.M. Apoorva, Jayaram Mannarukrishnaiah. Supervision: Jayaram Mannarukrishnaiah. Writing—original draft: H.M. Apoorva. Writing—review & editing: Jayaram Mannarukrishnaiah. Approval of final manuscript: H.M. Apoorva, Jayaram Mannarukrishnaiah.
Funding Statement
None
Acknowledgments
The authors express their sincere gratitude to SDUAHER, Kolar, for its support, and to Dr. Vasanthalakshmi, AIISH, Mysore, and Mr. S. Ravishankar, Community Medicine, SDUAHER, for their guidance in statistical analysis. Special thanks are due to all the participants of the study for their time and cooperation.