Noise exposure and hearing loss among tractor drivers in India

Abstract

BACKGROUND:

Tractors emit high intensity noise and prolonged exposure to high intensity noise causes hearing loss to the drivers.

OBJECTIVE:

The aim of this study was to measure noise intensity at the tractor drivers’ ear level and hearing loss among tractor drivers.

METHODS:

Noise intensity was measured on 6 models of tractors which were operated with 5 different tillage implements. In order to assess hearing loss, audiometric test was performed at 7 frequencies, i.e. 250, 500, 1000, 2000, 4000, 6000, and 8000 Hz, among 30 tractor drivers and 30 control group subjects. All the selected tractor drivers and control group subjects were also interviewed to obtain personal information and noise exposure details.

RESULTS:

The tractor drivers were exposed to noise intensity in the range of 91.7–97.5 dB(A). Audiogram analysis shows that the hearing threshold levels were significantly (p < 0.05) higher among the tractor drivers as compared with the control group subjects. The effect was significantly (p < 0.05) more on the left ear as compared with the right ear among the tractor drivers. Increase in age has significant (p < 0.05) effect on the hearing threshold levels on the left ear. Prevalence of high frequency hearing loss was 50% among tractor drivers as compared with 10% among control group subjects. Hearing loss was significantly (p < 0.05) more in the low frequency as compared with the high frequency among the tractor drivers.

CONCLUSION:

Indian tractor operators are exposed to high noise levels which may result in hearing loss. It is recommended that hearing conservation programs should be initiated to prevent noise hazards and hearing loss among the tractor drivers.

Keywords

Sound pressure level noise-induced hearing loss hearing impairment hearing threshold level tillage implements

1 Introduction

The engine speed, engine load and interaction between ground and tyre affect the noise emission of a vehicle. The load on the tractor due to various operations slightly affects the noise level. Primary tillage implements such as mould board plough and disc plough emit more noise as compared with secondary tillage implements such as harrows and cultivators require lower draft [1 –3].

The use of tractors causes noise exposure to tractor drivers. Dewangan et al. [3] reported noise intensity (L_eq) in the range of 80.3–92.3 dB(A) at the driver’s ear level during tillage operations for tractors without cabs, while another study reported noise intensity at the drivers’ ear level between 91.7 and 97.5 dB(A) [2]. Kumar et al. [4] reported a considerably high level of noise in the range of 90–110 dB(A) on tractors for different operations. A cab is installed on a tractor for drivers’ safety which also reduces noise exposure [5 –8]. The noise level at the drivers’ ear level was also very high for the tractors equipped with cabs [9 –11].

Noise-induced hearing loss is one of the common health hazards in the workplace [12–13]. Hearing loss is a serious health problem in Asia because most Asian countries are economically developing and lack of awareness is a barrier to the prevention of hearing loss [14]. Studies have established two characteristics of noise-induced hearing loss. First, the hearing loss increases with exposure to high intensity noise for a longer duration. Second, susceptibility to hearing loss greatly differs among individuals [15]. It is established that acoustic properties of the noise affect the parts of the ear differently, therefore there is variation in noise-induced hearing loss across occupations throughout the globe.

Studies have shown that the prevalence of hearing loss is relatively more among farmers as compared with the control group or other occupational groups [4 , 16–22]. Hearing loss is also relatively more among males living in a rural setting [23], students actively involved in farm work [24] and farm residents [25]. The left ear has been more severely affected as compared with the right ear because the tractor operators have to monitor the operation of equipment and thus expose the left ear to the source of noise [4, 17].

Studies have reported a significant association between hearing loss in the farmers and various factors such as age [17 , 25–30]; gender [21 , 31]; years’ work [17 , 27], chemical spraying and use of grain dryer [25]; owner/operator and livestock [26]; driving tractors without cabs [28]; and living in a farm, and all agricultural tasks combined among men [32]. Self-reported hearing loss was 22% among 1622 farmers [26]. Plakke and Dare [19] reported that 10%, 30% and 50% of the farmers in the age groups of 25–34, 35–44, and 45–54 years, respectively have hearing loss due to farm noise while none of the white collar workers of the same age group have significant hearing loss. Another study shows that hearing loss was prevalent among 50% of farm youth [33]. Brackbill et al. [34] reported that the prevalence of hearing loss was significantly higher only for farmers older than 65 years.

A few studies have been done on the assessment of hearing threshold shift and hearing loss among farming communities in India. Kumar et al. [4] found 24 tractor drivers out of 50 have high frequency hearing loss. Khadatkar et al. [35] estimated a mean excess risk of hearing loss of 7.1% among tractor drivers. Khadatkar and Mehta [36] reported that the mean hearing threshold values exceeded 25 dB(A) for tractor drivers with more than 31 years of driving experience.

Hearing loss varies with sound frequencies [37]. Thelin et al. [16] reported that hearing loss was significantly more among the farmers as compared with the office workers at 2000 and 4000 Hz frequencies, while hearing loss was not different at 1000 Hz. May et al. [17] reported that 37% and 65% of dairy farmers had an abnormal hearing at the pure tone and high frequency, respectively. After adjusting for age and sex, Marvel et al. [18] obtained 65% and 37% hearing loss at higher and lower frequencies, respectively among dairy farmers as compared with 37% and 12% among non-farmers in New York. Gomez et al. [38] reported that hearing loss among farmers and farm residents was 9% in the low frequencies (500, 1000 and 2000 Hz), 29% in the medium frequencies (1000, 2000, 3000 and 4000 Hz) and 47% in the high frequencies (3000, 4000, 6000 and 8000 Hz). Hearing loss was assessed in Poland through several studies among farmers including tractor drivers with 20 dB shift in hearing threshold as a criterion of hearing loss. A study reported hearing loss of more than 56% among the tractor drivers within the high frequencies (3000, 4000 and 6000 Hz), and among 22% of drivers in the medium frequencies (500, 1000, 2000 and 3000 Hz) while no hearing loss was observed in the control group [20]. Another study reported that hearing loss among the farmers has been more than 78% in the high frequency range and 45% within the medium frequency range [27]. Solecki [20] predicted the average expected hearing loss, excluding association with age and after 30 years of employment was 15 dB for operators of medium-power farm tractors and 6 dB for the operators of high-power tractors. Furthermore, hearing loss over 27 dB was found among 37.9% of tractor drivers of medium-power tractors and 13.0% of tractor drivers of high-power tractors. At 4000 Hz frequency, 67% of farmers exhibited hearing loss of more than 25 dB in either ear [39]. Renick et al. [32] reported more hearing loss in the high frequency range particularly at 6000 Hz.

Noise-induced hearing loss causes human suffering and thus an economic burden. Many adverse effects of hearing loss have been reported such as communication interference, work performance, self-esteem, and quality of life at work and outside work [15]. In India, a few studies have been performed on the measurement of noise exposure to tractor drivers [2 –4], hearing threshold levels among tractor drivers [34–35] and hearing loss among tractor drivers [4]. Thus, an investigation was conducted to assess noise exposure to tractor drivers and the prevalence of hearing loss among tractor drivers. Noise intensity at the drivers’ ear level was measured on 6 models of tractors operated with 5 different tillage implements. The hearing threshold level of the tractor drivers was measured through audiometric test to assess the prevalence of hearing loss. The hearing threshold level of the control group subjects residing in the same area but not exposed to noise was also measured for comparing the effect of tractor noise on hearing loss.

2 Materials and methods

2.1 Measurement of noise exposure

2.1.1 Selection of tractors

Six tractors of different models from 4 makes with a wide range of power were selected for the experiments. One tractor was 4-wheel drive and 5 tractors were 2-wheel drive. The specifications of the tractors are given in Table 1. The selected tractors are manufactured in India and are popular among farmers. The tractors had no cab and were operated without ballast. Standard size tyres were fitted to these tractors according to manufacturers’ recommendation and the depth of tread was more than 65% of the depth of the new tread. Inflation pressures of the front tyres and the rear tyres of the tractors were 172 and 79 kPa, respectively during experiments. Five tillage implements namely mould board plough, disc plough, tandem disc harrow, rotavator and duck foot cultivator were selected for operation of tractors.

Table 1
Brief specifications of the selected tractors for the study (adapted from Lalremruata et al. [2])

Items Tractor A Tractor B Tractor C Tractor D Tractor E Tractor F

Engine 3-cylinder, 4-stroke, water cooled 3-cylinder, 4-stroke, water cooled 4-cylinder, 4-stroke, water cooled 3-cylinder, 4-stroke, water cooled 2-cylinder, 4-stroke, water cooled 3-cylinder, 4-stroke, water cooled

Power drive 2-WD 2-WD 4-WD 2-WD 2-WD 2-WD

Power (kW) 37.3 41.0 55.9 41.0 18.7 26.1

Rated speed (rpm) 2200 2300 2300 2000 2000 2000

Weight (kg) 2340 2055 2270 2020 1735 1870

No. of gears 8 forward 2 reverse 8 forward 2 reverse 12 forward 3 reverse 8 forward 2 reverse 10 forward 2 reverse 8 forward 2 reverse

Items	Tractor A	Tractor B	Tractor C	Tractor D	Tractor E	Tractor F
Engine	3-cylinder, 4-stroke, water cooled	3-cylinder, 4-stroke, water cooled	4-cylinder, 4-stroke, water cooled	3-cylinder, 4-stroke, water cooled	2-cylinder, 4-stroke, water cooled	3-cylinder, 4-stroke, water cooled
Power drive	2-WD	2-WD	4-WD	2-WD	2-WD	2-WD
Power (kW)	37.3	41.0	55.9	41.0	18.7	26.1
Rated speed (rpm)	2200	2300	2300	2000	2000	2000
Weight (kg)	2340	2055	2270	2020	1735	1870
No. of gears	8 forward 2 reverse	8 forward 2 reverse	12 forward 3 reverse	8 forward 2 reverse	10 forward 2 reverse	8 forward 2 reverse

2.1.2 Experiments

According to IS-12180-2 [40] and ISO 7216 [41], the sound pressure levels of the tractors were measured at the drivers’ ear level to characterize noise exposure. The experiments were performed at the agricultural farm of North Eastern Region Farm Machinery Training and Testing Institute, Biswanath Chariali, Assam. There was no obstruction to reflect or absorb sound within a radius of 100 m. A plot of about 250×100 m was selected for the experiments. The soil at the experimental plot was sandy, with 92.74% sand, 0.013% silt and 7.24% clay. Background noise in the farm varied from 45 to 50 dB(A). The experiments were performed during the period from August to January in the year 2015. The details of the experiments are presented in Lalremruata et al. [2]. The tractors were operated in the field with selected tillage implements at 1.42 m/s. The noise was measured at the drivers’ ear level by placing the microphone at 50 mm from the ear and 100 mm ahead. Figure 1 shows the measurement of noise during tillage operations. The equivalent sound pressure level (L_eq), weighted ‘A’ was recorded in a sound level meter (Model-824, Larson and Davis, USA) for a travel distance of 25 m. Time taken to travel 25 m was also recorded. Three trials were conducted for each experiment and the experiments were randomized.

Fig. 1

Noise measurement at the drivers’ ear level in different tillage operations (adapted from Lalremruata et al. [2]).

2.1.3 Data analysis

The data recorded in the instrument were downloaded at the end of the day on the personal computer using Utility 824 software (Larson and Davis, USA) for data analysis. Noise intensity was measured for all the tractors with tillage implements and speeds were checked to determine normality by the Shapiro-Wilk test. The mean (L_eq) and standard deviation of noise intensity for the tractors with the tillage implements were calculated. Two-way analysis of variance (ANOVA) for noise intensity was performed with the tractors and implements as main factors with a 95% confidence interval (CI).

2.2 Assessment of hearing loss

2.2.1 Selection of subjects

A total of 50 tractor drivers were initially identified from two provinces of northeast India, namely Arunachal Pradesh and Assam, for assessment of hearing loss. A minimum of 5 years’ experience in tractor driving, age between 25 and 45 years and not exposed to intense noise other than tractors were criteria for their recruitment. The drivers were contacted and the objectives of the experiments, procedure of audiometric test and test location were explained to them. Tractor drivers with prior ear abnormalities or suffering from any major disease were excluded from the experiments as the medication could affect hearing. In order to identify ear abnormality, otoscopic examination by a physician specialized in ear, nose and throat was performed in the Civil Hospital, North Lakhimpur, Assam. A total of 30 tractor drivers without any abnormalities and who were ready to participate in the study were recruited for the audiometric test. In order to compare the effect of tractor noise on hearing loss, a total of 30 non-tractor drivers were recruited and were considered as a control group. The same selection criteria were adopted for the recruitment of subjects under the control group except that they did not drive a tractor. The ethnic group and economic condition of the subjects of both groups were similar and were residing in the same area. Written consent approved by the ethical committee of NERIST was obtained from the subjects to participate in the study. The subjects from both groups were male.

Personal information and noise exposure details of each subject from both groups were obtained through face-to-face interviews. A form was designed in a local language and information was recorded. Personal information included: age; education; work experience; years of tractor driving; smoking; chewing tobacco; and alcohol intake. Details of work routine, mode of travel and leisure noise exposure were also obtained. Stature and body mass of each subject were measured with anthropometric scale and personal weighing balance, respectively and were recorded. Characteristics of the tractor drivers and control group are shown in Table 2.

Table 2
Characteristics of the tractor drivers and control group

Characteristics Tractor drivers Control group p-value

Mean (SD) Number (%) Mean (SD) Number (%)

Age (years) 33.2 (5.6) – 33.2 (5.8) – 0.964

Age group (25–34 years) 29.2 (3.5) 17 (56.7%) 29.4 (3.6) 18 (60%) 0.861

Age group (35–45 years) 38.3 (3.2) 13 (43.4%) 38.9 (2.9) 12 (40%) 0.622

Stature (cm) 166.0 (7.0) – 166.5 (8.8) – 0.805

Body mass (kg) 59.2 (6.2) – 61.3 (11.9) – 0.394

BMI (kg/m²) 21.5 (2.0) – 22.1 (3.9) – 0.443

Total work experience (years) 10.6 (3.1) – 11.1 (3.5) – 0.356

Tractor driving experience (years) 10.1 (3.4) – – – –

Tractor driving experience less than 10 years – 12 (40.0%) – – –

Tractor driving experience more than 10 years – 18 (60.0%) – – –

Personal habits

Smoking – 17 – 16 0.906

Chewing tobacco – 20 – 21 0.781

Drinking alcohol – 26 – 24 0.488

Characteristics	Tractor drivers	Control group	p-value
Age (years)	33.2 (5.6)	–	33.2 (5.8)	–	0.964
Age group (25–34 years)	29.2 (3.5)	17 (56.7%)	29.4 (3.6)	18 (60%)	0.861
Age group (35–45 years)	38.3 (3.2)	13 (43.4%)	38.9 (2.9)	12 (40%)	0.622
Stature (cm)	166.0 (7.0)	–	166.5 (8.8)	–	0.805
Body mass (kg)	59.2 (6.2)	–	61.3 (11.9)	–	0.394
BMI (kg/m²)	21.5 (2.0)	–	22.1 (3.9)	–	0.443
Total work experience (years)	10.6 (3.1)	–	11.1 (3.5)	–	0.356
Tractor driving experience (years)	10.1 (3.4)	–	–	–	–
Tractor driving experience less than 10 years	–	12 (40.0%)	–	–	–
Tractor driving experience more than 10 years	–	18 (60.0%)	–	–	–
Personal habits
Smoking	–	17	–	16	0.906
Chewing tobacco	–	20	–	21	0.781
Drinking alcohol	–	26	–	24	0.488

2.2.2 Audiometric test

A pure tone audiometric test was performed on the selected subjects to measure hearing threshold level. A portable audiometer (Aryan 5000A, Shree Electronics, India) was used in the study. The audiometer was calibrated according to the manufacturer’s recommendation before the experiments. An audiologist performed the experiments in a soundproof audiometry room in the Civil Hospital, North Lakhimpur, Assam. Hearing threshold levels were measured at 7 frequencies, i.e. 250, 500, 1000, 2000, 4000, 6000, and 8000 Hz, for each ear which generated a total of 14 pieces of data from each audiogram.

The subject was called for an audiometric test in the morning. It was ensured that the subject has not been exposed to noise on the day. The subject was instructed on various signals required during measurement by the audiologist. The subject was asked to put on headphones connected to the audiometer. Initially, few trials were made to ensure that the subject understood the procedure of audiometry test. Subsequently, hearing threshold level was measured for both ears separately. The ascending threshold technique was used to determine hearing levels at the audiometric frequencies. Thus the experiment started with 250 Hz and a pure tone of 60 dB(A) was presented to the subject for a duration of 1 s. If the subject could hear the tone, then the sound intensity was presented by 10 dB(A) decrement until the subject could no longer hear the tone. The intensity of the tone to which the subject could not hear for the first time was presented to the subject three more times. If the subject could hear the tone correctly two or more times out of the four, then that sound intensity was recorded as his hearing threshold. However, if the subject could not hear it three or more times, then the sound intensity was raised by 5 dB(A) increments until the subject heard the tone. If the subject could hear the tone, this tone was presented three more times. If the subject could hear the tone correctly two or more times out of the four, the sound intensity was recorded as his hearing threshold. This test procedure was repeated at another ear and the remaining frequencies. A similar test procedure was used for the audiometry test of other selected subjects.

2.2.3 Data analysis

Analysis of data was performed using SPSS and Microsoft Excel software. Audiometric tests data of all the subjects for both ears were checked to determine normality by the Shapiro-Wilk test. Mean and standard deviation of the age, stature, body mass, body mass index (BMI) and work experience of the tractor drivers and control group subjects were calculated. The t-test on the different characteristics between the tractor drivers and the control group was performed with 95% CI. Odd ratio (OR) with 95% CI was also calculated to determine the variation between the tractor drivers and the control group on personal habits. The effect of age on hearing threshold levels was studied for the two age groups 25–34 and 35–45 years. The effect of tractor driving experience on hearing threshold levels was also studied for the drivers with less than 10 years’ experience and more than 10 years’ experience.

From audiometric data, mean hearing threshold levels for the left ear and the right ear were calculated at each frequency among tractor drivers and the control group. Mean hearing threshold levels at each frequency were also calculated for the left ear and the right ear under the two age groups of the tractor drivers and control group. Mean hearing threshold levels of the tractor drivers under the two groups of tractor driving experiences were calculated at each frequency. The t-tests were performed to determine significant effects on different groups for the hearing threshold levels for each frequency.

Different studies have used different criteria for hearing loss [42]. In the present study, the mean hearing threshold levels of the selected frequencies were grouped into two. The first group included frequencies 250, 500, 1000 and 2000 Hz, and was called low-frequency. The second group included frequencies 4000, 6000 and 8000 Hz, and was referred to as high frequency. Pure tone hearing threshold level of more than 25 dB(A) in either low-frequency average (LFA) or high-frequency average (HFA) was regarded as hearing loss. Hearing loss among the tractor drivers and control group on the left and right ears under the LFA and HFA were determined. Hearing loss among the tractor drivers and control group for the left ear and the right ear under the two age groups was determined under the LFA and HFA. Hearing loss among the tractor drivers for the two groups of tractor drivers was determined under the LFA and HFA. OR and p-values were calculated to study the effect of tractor driving, body mass, BMI, age, work experience and personal habits on hearing loss.

3 Results

3.1 Noise intensity

Mean noise intensity (L_eq) at the drivers’ ear for different tractors with implements for travel distance of 25 m is presented in Fig. 2. The results show that L_eq for different tractors varied from 92–97.5 dB(A). Noise intensity is relatively more for tractor B as compared with other tractors while relatively less noise intensity is obtained for tractor E. ANOVA shows that the noise emission of tractors is significantly (p < 0.001) different (Table 3). Noise intensity is considerably higher when the tractor is operated with the rotavator; however, noise intensity is comparable when the tractor is operated with other tillage implements such as mould board plough, disc plough, tandem disc harrow and duck foot cultivator (Fig. 2). ANOVA shows that the noise intensity is significantly (p < 0.001) different with implements (Table 3). Interaction between tractors and implements is also significantly (p < 0.001) different. Noise spectra of different tractors at the drivers’ ear level in tillage operation with the mould board plough are shown in Fig. 3. The results show that the noise intensity of the tractors varied considerably in the lower frequencies up to 100 Hz. The peak noise intensity of the tractors was in the range of 50–125 Hz in the centre frequency of the 1/3^rd octave band.

Fig. 2

Mean noise intensity (L_eq) emission of different tractors with tillage implements.

Table 3

p-values from two factor analysis of variance (ANOVA) of noise emission of tractors and tillage implements

Source	p-value
Tractors	<0.001
Implements	<0.001
Tractors * Implements	<0.001

Fig. 3

Noise spectra of different tractors at the drivers’ ear level in tillage operation with mould board plough.

3.2 Hearing impairment

3.2.1 Characteristics of subjects

Characteristics of the tractor drivers and control group subjects are shown in Table 2. Age, stature, body mass, BMI and work experience of the tractor drivers and control group subjects were comparable (p > 0.05). The mean driving experience of the tractor drivers was 12.1 years. Driving experience was less than 10 years among 12 (40%) tractor drivers while the driving experience was more than 10 years among 18 (60%) tractor drivers. Personal habits between the two groups were also similar. Seventeen (56.7%) tractor drivers and 16 (53.3%) control group subjects were smokers (p < 0.906). Twenty (66.7%) tractor drivers and 21 (70%) control group subjects reported chewing tobacco (p < 0.781) while 26 (86.7%) tractor drivers and 24 (80%) control group subjects admitted drinking alcohol (p < 0.488).

3.2.2 Hearing threshold level

Figure 4 shows the mean hearing threshold level at different audiometric frequencies among the tractor drivers and control groups for the left and right ears. The mean hearing threshold levels of the tractor drivers were considerably higher than the control group for both ears irrespective of the frequencies. The mean hearing threshold levels of the left ear of the tractor drivers were higher than the right ear for all frequencies; however, the hearing threshold levels were comparable for the left and right ears among the control group. The mean hearing threshold levels are higher at 4000 Hz among both the groups for both ears. Lower values of the hearing threshold levels were obtained at 500 Hz among both the groups for both ears. Table 4 shows p-values of the hearing threshold level between the tractor drivers and the control group and also between the left ear and right ear. The hearing threshold levels between the tractor drivers and the control group were significantly different for the left ear (p < 0.001) as well as the right ear (p < 0.05). The hearing threshold levels between the left and right ears were also significantly (p < 0.05) different among the tractor drivers; however, they were not significantly different among the control group.

Fig. 4

Mean hearing threshold levels among the tractor drivers and control group in the left and right ears at different frequencies.

Table 4

p-values obtained from t-test of the hearing threshold levels between tractor drivers and control\\ group and between left and right ears

Frequencies (Hz)	Tractor drivers vs control group		Left ear vs right ear
	Left ear	Right ear	Tractor drivers	Control group
250	0.000	0.000	0.026	0.366
500	0.001	0.062	0.037	0.860
1000	0.000	0.009	0.011	0.417
2000	0.000	0.000	0.011	0.374
4000	0.000	0.000	0.043	0.365
6000	0.000	0.000	0.053	0.490
8000	0.000	0.001	0.216	0.864
LFA (250–2000)	0.000	0.000	0.011	0.342
HFA (4000–8000)	0.000	0.000	0.075	0.726

The mean hearing threshold levels in the left and right ears of the tractor drivers and control group for the 2 age groups (25–34 and 35–45 years) at different frequencies are presented in Fig. 5. The figure also shows the mean binaural threshold levels among the tractor drivers and control group at different frequencies. The results show that the hearing threshold levels in the age group 35–45 years were higher than the age group 25–34 years among the tractor drivers and control group. The effect of age is significantly (p < 0.05) more among the tractor drivers as compared with the control group on the left ear at all frequencies, while the effect is significant (p < 0.05) at 250 and 500 Hz on the right ear. The effect of age on the binaural hearing threshold levels was also generally significant (p < 0.05) among the tractor drivers. An increase in age also significantly increased the hearing threshold levels at the higher frequencies among the control group.

Fig. 5

Mean hearing threshold levels for two age groups: (a) tractor drivers and (b) control group (*: p < 0.05).

Figure 6 shows the mean hearing threshold levels on the left and right ears at different frequencies for the two groups of tractor drivers (driving experience less than 10 years and driving experience more than 10 years). This figure also shows the binaural threshold levels for the two groups of tractor drivers. The results show an increase in the hearing threshold levels with an increase in driving experience but statistical analysis shows that the hearing threshold levels were significantly (p < 0.05) different between the two tractor driver groups and the effect is relatively more at the higher frequencies as compared with the lower frequencies.

Fig. 6

Mean hearing threshold levels of the tractor drivers based on their work experience: (a) left ear; (b) right ear; and (c) binaural.

3.2.3 Hearing loss

The prevalence of hearing loss among the tractor drivers and control group for any one ear or both ears is presented in Table 5. The results show that the prevalence of hearing loss was significantly more among the tractor drivers as compared with the control group (LFA: p < 0.05; HFA: p < 0.01). High frequency hearing loss was 50% (15 cases) among the tractor drivers while it was 10% (3 cases) among the control group. Hearing loss was spread among the tractor drivers at the lower frequencies and was 23.3% (7 cases) as compared with 3.3% (1 case) among the control group. Hearing loss in the HFA and LFA was more severely affected in the left ear (LFA: 23.3%, 7 cases; HFA: 43.3%, 13 cases) as compared with the right ear (LFA: 3.3%, 1 case; HFA: 26.7%, 8 cases) among the tractor drivers.

Table 5
Prevalence of hearing loss among tractor drivers and control group

Parameter Tractor drivers Control group Odd ratio p-value

(N = 30) (N = 30) (95% CI)

High Frequency PTA

Both or any one ear 15 3 9.000 0.001

Left ear 13 2 10.706 0.001

Right ear 8 3 3.273 0.095

Low Frequency PTA

Both or any one ear 7 1 8.826 0.023

Left ear 7 0 - 0.005

Right ear 1 1 1 1.0

Parameter	Tractor drivers	Control group	Odd ratio	p-value
High Frequency PTA
Both or any one ear	15	3	9.000	0.001
Left ear	13	2	10.706	0.001
Right ear	8	3	3.273	0.095
Low Frequency PTA
Both or any one ear	7	1	8.826	0.023
Left ear	7	0	-	0.005
Right ear	1	1	1	1.0

The effect of age on hearing loss in one ear or both ears in the LFA and HFA among the tractor drivers and control group are shown in Fig. 7. The results show that hearing loss was more in the age group 35–45 years as compared with the age group 25–34 years. Hearing loss was about 46% and 69% in the LFA and HFA, respectively among the tractor drivers in the age group 35–45 years while hearing loss was nearly 35% and 6% in the LFA and HFA, respectively in the age group 25–34 years. The effect of age is also evident in hearing loss among the control group. Hearing loss in the HFA was 6% in the age group 25–34 years which increased to 17% in the age group 35–45 years. The hearing loss increased to 8% with an increase in age in the LFA.

Fig. 7

Effect of the age on hearing loss among the tractor drivers and control group in the (a) LFA and (b) HFA.

Figure 8 shows the effect of work experience on hearing loss for one ear or both ears among the tractor drivers in the LFA and HFA. The results show that hearing loss was comparable among the two groups of tractor drivers in the HFA; however, significantly (p < 0.05) higher hearing loss was observed among the tractor drivers with more than 10 years of driving experience as compared with the drivers with less than 10 years of experience.

Fig. 8

Effect of work experience on hearing loss among the tractor drivers in the LFA and HFA (*: p < 0.001).

OR with 95% CI of the tractor drivers’ characteristics on hearing loss is shown in Table 6. The results show that tractor driving was 9 times (95% CI: 2.24–36.17, p = 0.001) more likely to cause hearing loss. A decade increase in age from 25–34 years to 35–45 years has a significant effect on hearing loss (OR 4.13, CI: 0.88–19.27, p = 0.05) among tractor drivers. Personal habits such as smoking cigarettes (OR: 3.21, CI: 0.70–14.74), drinking alcohol (OR: 3, CI: 0.28–32.75) and chewing tobacco (OR: 2.85, CI: 1.57–14.33) were more likely to have hearing loss.

Table 6

The odd ratio for hearing loss among tractor drivers

Parameters	Odd ratio	95% CI	p-value
Occupation status (drivers compared to control group)	9.0	2.239–36.171	0.001
Driver’s age group (25–34 years compared to 35–45 years)	4.125	0.883–19.273	0.065
Work experience > 10 years compared to < 10 years	0.80	0.185–3.460	0.765
Smoking cigarettes (smoking compared to not smoking)	3.214	0.701 –14.743	0.127
Alcohol consumption (drinking compared to not drinking)	3	0.275 –32.746	0.351
Chewing tobacco (chewing compared to not chewing)	2.852	1.568 –14.326	0.196

4 Discussion

4.1 Noise intensity

Noise intensity at the drivers’ ear level in the operation of the tractors without cab with tillage implements were in the range of 92–97.5 dB(A) (Fig. 2). Noise level on different tractors exceeded 90 dB(A) [2–4 , 17]. Tractor B was the oldest and exceeded its useful life among all the tractors. Owing to more wear and tear, noise emitted from tractor B may be higher as compared with other tractors. Depczynski et al. [43] reported that considerably more noise occurs on old generation tractors. Franklin et al. [6] obtained 6 dB louder noise in older tractors than in newer tractors. In India, tractors are tested according to IS-5994 [44]. This test code does not specify noise emission limit which may be one of the reasons that tractor noise could not be reduced and new generation tractors in India are still noisy. A very high level of noise emitted by the tractors may be also due to the absence of a cab. Noise emitted by tractors without cabs was higher than those obtained by tractors with cab [6 , 24]. The results also showed that the peak noise occurred in the range of 50–125 Hz and noise intensity in these frequency ranges was more than 90 dB(A). The human ear is relatively more sensitive to noise in the frequency range 2000–5000 Hz [37]. Mean noise intensity within this frequency range was around 75–84 dB(A). Noise spectra obtained in the present study are different than that reported in Aybek et al. [10]. Variation in the two studies may be due to variations in the characteristics of the tractors and operating conditions.

Noise emitted by the tractors with the rotavator was significantly higher than those obtained with other tillage implements (Fig. 2). A rotavator is an active tillage implement and is driven by the power take-off (PTO) shaft of a tractor. The rotary motion of the PTO shaft and tilling blade impact the tilling blade on soil, and tilled soil on the guard of the rotavator might have attributed for higher noise. Soil of northeast India contains small stones. The impact of tilling blade on stones and the impact of stones on the guard of the rotavator might also be reasons for relatively more noise. The other tillage implements such as mould board plough, disc plough, disc harrow and duck foot cultivator are passive tillage implements and do not have rotary motion which may be the reason for comparable noise on passive tillage implements.

4.2 Hearing impairment

The tractor drivers and control group subjects were selected carefully from similar living conditions. The tractor drivers were exposed to noise from the tractor; however, subjects in the control group were not exposed to noise in the workplace. Thus, the hearing threshold levels and hearing loss among tractor drivers may primarily be due to noise exposure from tractors.

4.2.1 Hearing threshold

Exposure to excessive noise is a major cause of annoyance [45, 46] and hearing loss [18]. Studies have shown that the shift in hearing threshold is influenced by the type of noise such as continuous intermittent and repetitive to a moderate degree [47]. Singh et al. [48] reported that impulsive and impact noise is more hazardous to hearing loss than that continuous noise. Singh [47] observed that impulsive noise is more than the other types of noise. Short-duration, high-level impulse noises cause different types of injuries in the ears as compared with long-duration, low-variance noise exposures [49]. Higher hearing threshold levels among the tractor drivers as compared with the control group (Fig. 4) show that tractor noise affected the hearing ability of the drivers. Continuous noise emitted by tractors without cabs could be an important factor for higher hearing threshold levels among the tractor drivers in the present study. McBride et al. [28] reported a higher hearing threshold among tractor drivers due to tractors without a cab. The tractor drivers in northeast India do not use protective devices which may partly be the reason for higher hearing threshold levels among the tractor drivers. Studies have also shown considerably higher hearing threshold levels among farmers including tractor drivers [16 , 24] as compared with office workers. Higher hearing threshold levels have also been reported among farmers [20 , 31].

Studies have shown that noise-induced hearing loss starts at around 4000 Hz [37]. As the number of years of noise exposure increases, the hearing loss around 4000 Hz becomes more pronounced but is generally restricted to a frequency range of 3000 –6000 Hz creating a V-shape notch [15]. A notch was also obtained in the present study in hearing threshold levels at 4000 Hz for the tractor drivers as well as the control group (Fig. 4). With further noise exposure, the hearing loss at 4000 Hz continues and spreads over a wider frequency range [50]. Noise-induced hearing loss does not become apparent until the speech frequencies (300 –700 Hz) are affected [15]. Thelin et al. [16] and Karlovich et al. [23] also reported a higher risk for hearing at 4000 Hz, however, McBride et al. [28] and Stewart et al. [31] obtained the greatest hearing threshold at 6000 Hz in the aggregate data. In the present study, significantly higher (p < 0.001) hearing threshold levels were obtained for the HFA (4000–8000 Hz) and the LFA (250–2000 Hz) among the tractor drivers (Table 4). Previous studies have also reported higher hearing threshold levels at higher frequencies as compared with medium and low frequencies for farmers [19 , 51]. The effect of noise is significantly (p < 0.05) more on the left ear as compared with the right ear among the tractor drivers (Table 4). This trend of hearing threshold levels is in line with the previous studies [4 , 52]. Significantly (p < 0.05) higher hearing threshold levels in the left ear as compared with the right ear (Fig. 4) may be due to the requirement of the tractor operation. Owing to the advantages of the 3-point hitch and drawbar of a tractor, rear mounted implements and machinery are most commonly used. In order to monitor the performance of implements and machinery, tractor drivers are required to turn their head to look backward. Most of the persons are right handed and thus turn their head towards the right. Turning of the head exposes the left ear to higher intensity noise from the engine while the right ear is shielded by the head.

Age and years of noise exposure are two important confounders to hearing threshold shift [18]. The results show that the hearing threshold levels were significantly (p < 0.05) higher with an increase in age from 25–34 years to 35–45 years, particularly on the left ear as compared with the control group (Fig. 5). Higher hearing threshold levels with an increase in age have been reported in Thelin et al. [16] and the effect is more at the higher frequencies as compared with the lower frequencies [19 , 31]. Higher hearing threshold levels were obtained at 4000 Hz for both the age groups in the present study (Fig. 5); however, the hearing threshold levels were higher at 6000 Hz for the 40- and 50-year age groups while at 8000 Hz for the 30-year age group [19]. Hearing threshold levels were higher for the experienced tractor drivers on the left ear and binaural (Fig. 6). McBride et al. [28] also estimated higher hearing threshold levels for experienced tractor drivers.

4.2.2 Hearing loss

In order to avoid the possibility of a temporary hearing threshold shift due to noise exposure, at least 16 h rest after the last exposure and 6–8 h rest from leisure activities are allowed to the subjects for obtaining a permanent hearing threshold shift [42]. The present study was performed in the early morning and the subjects have sufficient rest for obtaining a permanent hearing threshold level. Hearing loss was calculated from permanent hearing threshold shift. More than 25 dB hearing loss occurred among 50% of tractor drivers as compared with 10% in the control group (Table 5). About 23.3% and 50% hearing loss was prevalent among tractor drivers in the LFA and HFA, respectively and was significantly higher (LFA: p < 0.05; HFA: p < 0.001) in one or both ears as compared with the control group. Kumar et al. [4] reported 48% hearing loss in the HFA among tractor drivers in north India which is comparable to hearing loss in the present study. Gomez et al. [38] and Humann et al. [11] reported hearing loss in the HFA and LFA was 47.3% and 46.9%, respectively among farmers. Considerably higher hearing loss has been reported among 78% of farmers in either ear at 6000 Hz [39]. High frequency hearing loss has also been reported among 65% of dairy farmers [18] and 72% of farmers [25]. Hearing loss of more than 20 dB within the range of high frequencies was reported among 56% of tractor drivers [20] and 78% of farmers [27]. The prevalence of hearing loss is relatively low in the present study which may be attributed to the low level of mechanization and relatively lower age of the tractor drivers. Solecki [20, 27] reported that age was highly correlated with hearing loss. The present study shows that hearing loss increased from 35% –69% among tractor drivers with an increase at 10 years of age (Fig. 7). Beckett et al. [25] found that hearing loss increased by 5% with each passing decade. The effect of work experience on hearing loss is relatively small as compared with age (Fig. 8) and the increase in hearing loss with experience of the tractor drivers was not statistically significant. This trend is in line with hearing loss reported in Kumar et al. [4] and opposite to Hwang et al. [26]. Marvel et al. [18] and Solecki [20] reported high correlation between number of years worked in farming and hearing loss. Kumar et al. [4] opined that hearing loss might have occurred in first few years of noise exposure and thus hearing loss could not vary significantly over years of driving. Significantly higher (p < 0.001) prevalence of hearing loss in the LFA among the tractor drivers with more than 10 years of driving experience as compared with the tractor drivers with less than 10 years of driving experience (Fig. 8) shows that increase in driving experience has spread hearing loss from higher frequencies to lower frequencies.

Smoking affects the hearing ability of a person [53] and the present study also showed the effect of smoking cigarette on hearing loss. Smoking was 3.21 times more likely to cause hearing loss as compared with nonsmokers (95% CI: 0.70 –14.74) among tractor drivers (Table 6). Smoking may affect antioxidative mechanisms or the vasculature supplying the auditory system [54] and may damage the hair cells of the ear through an ischemic mechanism [55, 56]. Gaur et al. [57] reported a significant positive association between use of tobacco either by smoking or by chewing and the prevalence of disease in the ear among males in India. Effects of smoking and noise exposure increased the prevalence rate ratio for high frequency hearing loss by 2.56 [58]. Alcohol consumption and chewing tobacco were 3 and 2.85 times more likely for hearing loss, respectively in the present study (Table 6).

The tractor drivers were 9 times more likely to have hearing loss as compared to the control group (Table 6) which indicates that tractor driving is a hazardous occupation. Younger people were more sensitive to noise and susceptible for hearing loss [27]. Renick et al. [33] concluded that hearing loss which was common among adult farmers may be a problem for farm youth. Since hearing loss is preventable with proper precautions thus measures should be initiated to reduce noise exposure to tractor drivers. A tractor with a cab is effective to reduce noise exposure and thus it may prevent hearing loss [25]. Since the installation of a cab on a tractor is expensive and personal protective devices are cheaper thus personal protective equipment may also be used to reduce hearing loss. Kumar et al. [4] reported that Indian farmers wrap their head with a traditional piece of cloth known as safa. This traditional headgear may reduce noise exposure; however, covering the head with a piece of cloth is not practiced in northeast India. Furthermore, tractor drivers of northeast India are not aware of noise hazards; therefore, dissemination of information and training is required to help educate tractor drivers about noise hazards. Awareness of a personal protective device requires motivation and training thus hearing conservation programs in India should be initiated for the farmers including the tractor drivers.

5 Conclusions

Indian tractor drivers are exposed to high noise levels which may result in hearing loss. Tractor driving is a hazardous operation and the tractor drivers were nine times more likely to have hearing loss as compared with the control group subjects. Hearing threshold levels of the tractor drivers were significantly (left ear: p < 0.001, right ear: p < 0.05) higher as compared with the control group subjects. The prevalence of hearing loss is more in the left ear as compared with the right ear among tractor drivers. The higher frequency hearing loss was greater than the lower frequency hearing loss. The effect of age is significantly (p < 0.05) higher than the tractor driving experience. It is recommended that hearing conservation programs should be initiated to prevent noise hazards on hearing loss among tractor drivers.

Footnotes

Acknowledgments

The authors are grateful to the North Eastern Region Farm Machinery Training and Testing Institute, Biswanath Chariali, Assam for providing experimental and technical support. The authors are thankful to the Civil Hospital, North Lakhimpur, Assam for the otoscopic examination and audiometric test. The authors also thank Mr. Spondon Chelleng for assisting in the study. The tractor drivers and control group subjects are appreciated for their cooperation in this study.

Conflict of interest

None to report.

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