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|Year : 2010 | Volume
| Issue : 46 | Page : 49--55
Noise induced hearing loss risk assessment in truck drivers
Ali Karimi1, Saleh Nasiri2, Farshid Khodaparast Kazerooni2, Mohammad Oliaei3,
1 Occupational Health Department, Faculty of Health, Tehran University of Medical Sciences, Tehran, Iran
2 Occupational and Environmental Health Department, Public Health Center of Shiraz, Shiraz, Iran
3 Occupational Health Department, Faculty of Health, Shiraz University of Medical Sciences, Shiraz, Iran
Occupational Health Department, Faculty of Health, Tehran University of Medical Scinces, Tehran
Hearing sense is one of the key elements which may have impact on the driver's task quality. This cross-sectional study investigates the hearing status of 500 truck drivers by pure tone audiometry (AC) in one of the cities in Fars province, Iran. Hearing threshold levels of the subjects were measured in frequencies of 500Hz-8000Hz. Screening and determination of permanent threshold shift (PTS) was the first aim of this study. Hence tests were done at least 16 hours after any exposure to noticeable sound. The effect of age as a confounding factor was considered using ISO equation and subtracted from whole hearing threshold. The threshold of 25 dB HL and above was considered abnormal but the calculation of hearing was also carried out using 0 dB HL as reference. Subjects were categorized into two groups on the basis of working experience and the hearing threshold of 25 dB was considered a boundary of normal hearing sense. The results of Pearson Chi-Square test showed that working experience as an independent variable has significant contributing effect on hearing thresholds of truck drivers in frequencies of 500, 1000, 2000 and 4000 Hz (p greater than 0.05). Also, it was shown that currently nine and 12.6 % of truck drivers suffer from impaired hearing sense in left and right respectively (hearing threshold level greater than 25 dB) in mid frequencies (500, 1000, 2000 Hz) and 45% in high frequencies of both ears (4000 and 8000 Hz). The results indicated that hearing damage of professional drivers was expected to occur sooner at 4000 and 8000 Hz than lower frequencies. Finally it was deduced that the occupational conditions of truck drivers may have bilateral, symmetrical harmful effect on hearing threshold sense in all frequencies mainly in frequency of 4000 Hz, so health surveillance programs such as education and periodic medical examinations are emphasized for pre-diagnosing and prevention of any possible impairment and an urgent need to take up some interventions such as better maintenance of roads, automobile industry efforts to reduce the noise level emission of vehicles and reducing number of working hours per day of drivers are highlighted to improve the harmful working conditions of truck drivers.
|How to cite this article:|
Karimi A, Nasiri S, Kazerooni FK, Oliaei M. Noise induced hearing loss risk assessment in truck drivers.Noise Health 2010;12:49-55
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Karimi A, Nasiri S, Kazerooni FK, Oliaei M. Noise induced hearing loss risk assessment in truck drivers. Noise Health [serial online] 2010 [cited 2023 Oct 1 ];12:49-55
Available from: https://www.noiseandhealth.org/text.asp?2010/12/46/49/59999
The two most common causes of hearing loss in adults are generally accepted as being:
Effects of ageingNoise-induced hearing loss (NIHL, the effects of excessive noise exposure).  Noise is a cause of poor health. It can damage hearing and is associated with a number of psychological problems that can contribute to stress. As noise is usually experienced at the same time as vibration there is a combined assault on the lorry and bus driver from both sources. This is particularly worrying for bus and lorry drivers who already experience elevated stress levels from the traffic environment and time pressures of the work. 
For those who drive trucks and buses there has been deterioration in work conditions over the last 20 years. This deterioration is largely the result of traffic congestion and its associated air and noise pollution along with the pressure of maintaining a demanding schedule in circumstances which make the task almost impossible. A truck or bus driver is at the sharp end of the failure of our transport system. Buses find it almost impossible to maintain schedules and the pressure on truck drivers to meet demanding schedules is intense. The driver, in both circumstances, has to absorb the failures of the transport system in the form of increased stress levels, conflict with customers and the intensification of a wide range of work pressures in a hostile environment. 
Traffic noise is a major source of environmental pollution in developed and developing nations. Professional drivers are most susceptible to high noise levels for long duration. The most common goal for protecting drivers from auditory effects of occupational noise is the preservation of hearing for speech discrimination.  One study in India has identified noise levels in bus cabs of 89-106 dB(A) and observed that 89% of the bus drivers had abnormal audiograms i.e. they had impaired hearing.  Mukherjee et al. investigated some occupational harmful agents (noise, heat, dust and volatile organic compounds) of bus drivers in Kolkata and indicated that drivers undertaking three consecutive trips within Kolkata city traffic routes in a special bus have higher noise exposure than the recommended standard.  Investigations of Krishan Kumar and Jain on noise in various modes of transport in Delhi showed that among the various modes of transport, noise levels are greatest in auto-rickshaws (81-96 dBA) followed by trucks (83-90 dBA) and buses (77-92 dBA). Noise levels in cars were appreciably lower (72-80 dBA) when compared to other modes of  transport. 65 dB(A) is normally taken as an acceptable level so levels above 80 dB(A) give serious cause for concern. A loss of hearing is typically the result of continuous exposure to high noise levels of 85 dB(A) and above for a number of years. Noise at Work Regulations (2005) require personal hearing protection to be provided where noise levels reach 85 dB(A) or more. 
Noise protection is not an easy matter for drivers. Drivers need to be in aural contact with the outside world and need to be protected from the health damaging consequences of noise. Resolving this dilemma through cab design and more fundamental solutions for the general traffic environment will be a high priority.  Hearing sensitivity at 0 dB(HL) is normalized to represent average normal hearing for young, otologically normal adults (male and female) between the ages of 20 to 29.  The range of normal hearing is considered to be between 0 to 25 dB(HL).  The four frequencies pure tone average of hearing levels at 500; 1,000; 2,000; and 3,000 Hz were used for mid-frequency hearing loss. High frequency range of 3,000; 4,000; 6,000; and 8,000 Hz was used for hearing impairment. Threshold of 25-dB(HL) and above was considered abnormal but the calculation of hearing was also carried out using 0 dB(HL) as reference. 
The American Academy of Otolaryngology has established referral criteria. Briefly, if the initial audiogram shows a significant loss (i.e. worse than 25 dB) in either ear or a significant difference between ears (i.e. 15 and 30 dB for the low and high frequencies, respectively), a referral should be made to an audiologist or otolaryngologist. Also, if a confirmation audiogram reveals a 15 or 20 dB shift in the low or high frequencies, respectively, a referral should be considered.  Noise induced hearing loss usually commences around the frequency of 4000 Hz and gradually progresses within this frequency region and spreads into adjacent frequencies.  This dip in hearing is known as a 'notch' in the audiogram. A 'four kHz notch' is a common characteristic of noise induced hearing loss.  In this context, the current study attempts to investigate working conditions of truck drivers and possible effect on their auditory systems. Additionally, the current perspective of hearing status of truck drivers would be highlighted to emphasize any necessary interventions to make recommendations for a significant improvement in the working environment of this neglected group of professionals.
Materials and Methods
There are several methods to test the hearing sensitivity of an individual. It is possible, for example, to examine hearing levels with an audiometer, tuning fork, or a free field voice test. Audiometric testing is the accepted standard for measuring hearing levels, but other procedures are often used by medical personnel to identify persons with hearing impairments or hearing disorders. Testing for hearing ability may take the following forms; testing to determine if a person can hear at a specified hearing level (screening), or testing for a threshold of hearing sensitivity. Screening involves testing for the ability to detect a specified level of sound at various frequencies. An individual either responds or does not respond to the signal. Thus, they pass or fail depending upon their ability to "hear" at this one level. The pure-tone screening test usually performed by audiometers, and this test have become the standard method for measuring hearing ability.  If hearing screenings are to be part of a medical examination, an audiologist can set-up the screening protocol and train the subject on how to response to the signals in a physician's office to carry out the test. Screening at discrete frequencies can be a fast procedure when administered by a trained individual to a cooperative patient. Numerous drivers usually refer to occupational medial examination centers to receive health identification cards. Among them 500 drivers (male) were considered by random sampling method (in Iran it is not common for a female to be truck driver so all of the drivers were male). Pure tone audiometery (AC: stands for air conduction, means measuring auditory sense of examinee about sounds that conducted through the external and middle ears to the cochlea) is a simple screening method for hearing loss diagnosis in occupational health. The effect of temporary threshold shift (TTS) was controlled by considering 16 hours interval between any exposure to noticeable noise and pure tone audiometery. Before test, the subjects were clearly instructed about the test procedure and necessary information [Table 1] was logged by audiometric technician.
The working hours per day were varying but the declared; they usually work more than 10 hours per day. All drivers in this study were selected according to the following criteria: not had an accidental hearing loss, not had hearing loss due to medical surgeries, disease, not exposed to accidental explosions, explosions or heavy weaponry noise during war/army activities; not used hearing protective equipment.
Exceeding noise usually causes enhancement in hearing threshold levels.  If we suppose that truck drivers were exposed to noticeable exceeding noise therefore the hearing threshold must have affected and increased, it means that there may be a direct correlation between work experience and hearing threshold level changes. On the other hand, it is reasonable to assume that aging makes some contribution to loss of hearing in people exposed to excessive noise, so that the hearing threshold would increase with age. The effect of aging on hearing thresholds has been reported long ago.  Adjustment is needed about aging to clearly determine if work duration as a contributing variable may have any effect on hearing threshold level. The international standard organization equation (ISO-7029) was used to carry out these adjustments. 
The most important question that would be posed with any job would be - Will the working experience (as duration of occupational noise exposure) as an independent variable (apart from other confounding variables such as age, disease, etc) have any noticeable effect on hearing sense?
The aim of this study was to answer to this question for truck drivers. To examine the recent declaration corrected hearing threshold levels in different frequencies were categorized on the basis of working experience (year); so we have two groups in each frequency: working experience less than 10 years, working experience more than 10 years This pattern of stratification is similar to research of Majumder et al.  Hearing handicap is usually denoted as an average hearing threshold level of greater than 25 dB(HL) for both ears at selected frequencies. 
The hearing threshold level of 25dB is considered the cut point for normal hearing sense in each frequency. Hence the subjects were stratified into two groups on the basis of their corrected with age hearing threshold level in each frequency: group 1: hearing threshold level less than or equal to 25 dB, group 2: hearing threshold level greater than 25 dB. Several methods of analysis were examined to reach a meaningful conclusion. The independent T-test was performed initially to compare average of hearing threshold levels vs. working experience categories, in each frequencies, but the results of the aforementioned test indicated that in this method of analysis the hearing threshold level of impaired subjects were masked by hearing threshold levels of normal subjects and it was not possible to discover impaired drivers, so other methods were applied as discussed in next sections.
Hearing threshold level in the frequencies in question can be adjusted using the recent ISO equation, to control for the effect of age on hearing threshold level. For this, the threshold shift value due to aging calculated from the ISO equation is subtracted from the whole hearing threshold level for each frequency. After adjusting the aging effects on hearing threshold shifts, Pearson Chi-Square test performed to examine if working condition of truck drivers may cause noticeable hearing loss in subjects. [Table 2] and [Table 3] show results of Pearson Chi-Square tests on the basis of working experience and hearing threshold levels for left and right ears respectively.
The results in [Table 2] exhibit a significant relationship between working experience and hearing threshold ascending in frequencies of 500, 1000, 2000 and 4000 Hz (p less than 0.05). These results indicate that hearing threshold of 19.8% of subject with working experience more than 10 years, were exceeded 25dB in frequency of 500 Hz (left ear), while just 11.9% of those with working experience less than 10 years have shown this threshold shift, and this difference is statistically significant. A similar conclusion can be achieved generally, except for frequencies of 8000 Hz in left ear and 500 and 8000 Hz in right ear. There are however, some differences in the number of cases in each aforementioned category [Table 3]. Results indicate that the longer the time employed as a truck driver, greater is the hearing loss. The finding is consistent with conclusions  of other investigators. This indicated that loud noise exposure due to driving might have significantly affected the hearing threshold levels of professional drivers. This may be due to being in a noisy environment at different locations for several years as well as continuous exposure to noise driving all day. [Figure 1] and [Figure 2] exhibit overall features of current hearing threshold of truck drivers in this study. These figures show that hearing threshold pattern is analogues in both ears. Also, these figures illustrate that hearing frequency of 2000Hz contains the least hearing impaired cases (about 10%) and frequency of 4000Hz contains the most hearing impaired cases (42%).
Also, hearing thresholds for left and right ears were compared at each test frequency using Student's t-test. No significant difference was determined (P greater than 0.05). It was deduced that the occupational conditions of truck drivers have bilateral, symmetrical effect on hearing threshold sense in all frequencies. In [Figure 3], two group of frequencies were considered, mid frequencies includes 500, 1000 and 2000 Hz that usually contribute to speech and routine conversation and high frequencies consists of 4000 and 8000 Hz that usually founded in sounds produced by manmade and mechanical equipments and have little contributions in normal conversations (in fact mid frequencies include 500, 1000, 2000 and 3000 Hz and high frequencies includes 4000, 6000 and 8000 Hz, however, in this study some of these frequencies haven't been measured).
Average hearing threshold level was calculated for both mid and high frequencies separately to have single value of hearing threshold level for each group of frequencies. [Figure 3] exhibits per cent of impaired truck drivers in mid and high frequencies hearing thresholds (greater than 25 dB) vs. those how have normal hearing threshold level in mid and high frequencies (less than or equal to 25 dB) in both ears. The recent results showed that about 9-12.6% of truck drivers suffer from hearing impairment in mid frequencies, meanwhile the percent of truck drivers with hearing impairment increased to about 45 in high frequencies for both ears.
The pure effect of working experience situation on hearing threshold shift have been calculated previously with the help of ISO equation, and the results of Chi-square test revealed significant relationships between adjusted with age hearing threshold shift and working experience (as duration of occupational noise exposure) in most of frequencies. Audiometric analysis also suggested that effect of duration of exposure was dependent on frequency. Additionally, it may be concluded that hearing damage at 4000 and 8000 Hz was expected to occur sooner than losses at lower frequencies (500, 1000 and 2000 Hz). The finding is consistent with conclusions of other investigators. Poor audiometric status of professional drivers may have resulted from damage to hair cells near the base of cochlea by high frequency or high pitch sound of automobiles.  Plus the fact that occupational conditions of truck drivers has significant relevant on auditory health of drivers, noticeable number of drivers currently suffer from hearing impairment (9-12.6% in mid frequencies and 45% in high frequencies).
Beyond the known consequences of bilateral high-frequency hearing loss and the likelihood of persistent tinnitus, there is growing evidence that workers exposed to noise in the workplace (daily dose greater than or equal to 80 dB-A) are also at higher risk of accident. Somehow, it would seem that noise exposure and hearing loss may interact to interfere with the safe discharge of occupational activities. , Stress-related disorders are also noteworthy  as is an increase of the risk of motor vehicle accidents.  When noise in the workplace induces temporary or permanent hearing loss,  it is reasonable to expect that this will also affect psychoacoustic performance  by compromising the perception and the localization of environmental signals, including the recognition of speech and of warning signals.  Hearing loss may contribute to traffic accident risk in several ways. Among children, it almost doubles the risk of pedestrian injuries,  indicating that audition does contribute to the safe management of road hazards. This is confirmed by Lundalv,  reporting that adult pedestrians and cyclists with moderate hearing loss are at a higher risk of being injured by a vehicle. The same applies to the risk of accidents by mature drivers who are moderately hearing impaired. 
Thus, in the presence of hearing impairment, it remains possible for the noise of an incoming vehicle and other vehicular warning signals not to be heard or localized for a timely and appropriate reaction.  Also, a comprehensive study was conducted by Michel Picaed et al.  to verify if there is an association between occupational noise exposure, noise-induced hearing loss and driving safety. Their investigations showed that daily occupational noise exposures greater than or equal to 100 dBA and noise-induced hearing losses, even when just barely noticeable (16 to 30 dB) may interfere with the safe operation of motor vehicles. Hence auditory health of truck drivers may have noteworthy influence on severity and frequency rates of traffic accidents. However, having a hearing loss does not necessarily mean that you are more "accident prone".
It may be concluded that the described risk of hearing impairment of truck drivers is high due to their occupational hazard. The study highlights the need to take up interventions to reduce potential harmful effect of noisy situations for the occupationally exposed truck drivers. Vehicle noise cannot be totally eliminated, but a lot could be done to reduce it.
Better maintenance of roads, steps may also be taken by automobile industry to reduce the noise level emission of vehicles, which will need introduction of improved automobile technology like masking of engine noise, sound proofing of vehicles etc. The year of manufacture and location of engine were two factors that contributed highly to the level of noise reaching the drivers. Potential harmful effects of noise exposure to drivers may also be reduced by reducing number of working hours per day of drivers or by use of hearing protection devices (HPDs). HPDs reduce sound in the driving environment, including noise and signals that may be of interest to drivers. HPDs may reduce the audibility of certain sounds. There is some concern that hearing protection devices may contribute to accidents by interfering with speech communications, the ability to detect horns or ability to localize incoming sounds.  Periodic check up of drivers' audiogram is extremely necessary to determine auditory threshold levels of the persons working in noisy area.
A truck driver can be exposed to intense noise over long periods of time. Continued audiometric evaluation is necessary among existing drivers to ensure not only that a driver meets the hearing criteria, but also to examine for any threshold shift due to noise exposure or any reason. However, the problem with identifying NIHL in truck drivers, though, is the lack of reliable baseline data (threshold measures before the person began the noise-related occupation), the influence of age on hearing, and individual susceptibility. Bi-annual medical examinations are required for existing truck driver's focus on hearing screening and provide data regarding the change that might be occurring in hearing thresholds. Training is a crucial component of an effective hearing loss prevention program. With education comes knowledge and motivation to protect hearing, but this is the most neglected aspect of hearing conservation programs about truck drivers in Iran.
Thus, this study shows that a noticeable number of truck drivers may have some deficiencies in hearing sense. Immediate measures such as training, reducing number of working hours per day of drivers, better maintenance of roads and reducing vehicle noise emission should be taken to control noise in truck cabins, as this will not only affects drivers health but also the accident frequency and severity rates may be influenced thereby.
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