Abstract
Noise and organic solvents are common in many industries and both of them affect hearing. In this study, we estimated the concurrent effect of them on hearing by evaluating the existence of notch in audiograms of workers. The number of 540 persons were enrolled in this study after eliminating workers who had the exclusion criteria. We divided them into 4 groups based on their exposure status; no exposure, exposure to noise, exposure to solvent, exposure to both of them. The presence of notch in left, right, or both ears were assessed through Coles model. The rates of notch presence in both ears in the groups of noise and organic solvents exposure, noise exposure only, solvents exposure only were 11.72, 4.49, 1.86 times higher than the control group and sole solvent exposure didn't affect hearing significantly. The same pattern was seen for notch presence in left or right ear and the solvent-noise exposure group had the highest rate of notch presence. This study aims to show the synergic effect of noise and organic solvents exposure on hearing loss. Hence, we recommend implementing a hearing protection program and a higher frequency of audiological assessments in the industries involved with concurrent exposure to noise and organic solvents.
Keywords: Noise induced hearing loss, Solvent, Tire industry, Organic solvents, Notch
Introduction
Hearing impairment is a widespread occupational impairment in various industries [10]. Several factors are strongly associated with hearing loss in workers such as family history of hearing impairment, aging, ototoxic drugs, smoking, and exposure to ototoxic substances in the workplace [3]. Noise is the most common cause of hearing impairment in the workplace. Several recent studies showed the effect of concurrent exposure to noise and ototoxic chemicals including organic solvents [10–12]. Nevertheless, some evaluations suggested that the workplaces in which the exposure to noise and chemicals exist simultaneously, the effect of noise is notable on hearing loss and it doesn't worsen in the future after exposure to chemicals like organic solvents [9].
These substances have ototoxic effects in animals [13]. Barregard et al. suggested that there is a synergistic effect in the existence of both noise and organic solvents including toluene, benzene, xylene, and tetrachloroethylene on the hearing loss in the occupational setting [2]. A study suggested that hearing impairment is correlated with exposure to organic solvents in a standard limit of noise at work place [11].
Organic solvents are commonly used in many manufacturing industries such as the production of plastic, rubber, dyes, tire, electronic, and printing. Nearly 1.7 million people have exposure to these substances in their workplace [14]. There are many industries with simultaneous exposure to organic solvents and noise in Iran, but to the best of our knowledge, there is little proof about their concurrent effect. In addition, most of the studies have evaluated the effect of the high concentrations of organic solvents (more than Threshold Limit Value (TLV)).
The aim of this cross-sectional study is to evaluate the synchronous effect of combined exposure to noise and organic solvents within standard concentration levels according to standard threshold of American Conference of Governmental Industrial Hygienists (ACGIH) TLV in a large manufactory of tire industry workers in Iran.
Material and Methods
This cross-sectional study is conducted in a large manufactory of the tire industry in the west of Tehran in 2017. All of the workers who had been in the manufactory at least 6 months were recruited in this study. The number of 700 workers participated in our survey. Initially, the participants were provided with informed consent and the Ethics Committee of Tehran University of Medical Sciences approved our study. The data of 659 workers (94.1%) was completely gathered. The data included demographic characteristics (age, weight, and height), occupational characteristics (second job, work experience, shift work, income), educational level, history of smoking, thyroid dysfunction, and metabolic disorder (Diabetes mellitus, hypertension, hyperlipidemia). Participants with a positive history of otitis media, congenital ear disease, history of ototoxic drug usage (Aminoglycosides, salicylates, chemotropic agents, loop diuretics), severe head trauma and previous head surgery and thyroid dysfunction were excluded from our study. Besides, all of the samples of this study had a normal audiogram at the beginning of employment. Finally, 542 workers were registered in our study. All of them divided into four groups according to their status of exposure to organic solvents and noise: (1) noise exposure (2) organic solvents exposure (3) concurrent exposure to organic solvents and noise (4) no exposure.
Organic solvents that are used in this factory include Benzene, Ethylbenzene Toluene, Xylene. These were measured through the sampling of the worker’s ventilation with activated charcoal, then the analysis were done according to the mentioned equation for calculating the admissible limit for the mixture of organic solvents. This admissible limit was determined based on the American Conference of Governmental Industrial Hygienists Threshold Limit Values (ACGIH-TLV): EM = C1/L1 + C2/L2 + ⋯ + Cn/Ln.
EM is the equivalent exposure for the mixture of organic solvents; Cn is the mean concentration of organic solvent in environment air, and Ln is the exposure limit for organic solvents [7]. Em > 1 indicates that the organic solvents mixture concentration in the workplace ambient is above the threshold. Em should be less than 1 in all workstations of this industry. Sound pressure and organic solvents levels were measured by a team of occupational hygiene of the Tire factory in 2017. Sound pressure level was measured by a sound level meter (CEL450, UK).
The National Institute method for Occupational Safety and Health was used to measure the environmental air concentration of organic solvents at the breathing level [1]. Air samples were collected on a charcoal tube (SKC 226-01). The exposure was monitored for 8 h. Analysis of Exhaled air was done by gas chromatography using a flame ionization detector. We did not detect any other neurotoxic agent in ambient air or their concentrations were neglectable.
All of the participants' hearing threshold measurement was done using pure tone audiometry with a standard audiometer (AD 229b Interacoustics, Denmark) in an acoustic room accomplished by an audiologist after 16 h of not having any exposure to occupational noise. The hearing threshold of workers was measured at these frequencies: 0.5, 2, 3, 6, and 8 kHz. Air and bone conduction in both ears were investigated.
Coles criteria were used to determine the notch of both ears (the hearing threshold at 3, 4, or 6 kHz 10 dB or more compared with at 1 or 2 kHz) [8].
Statistical Analysis
Chi-square and ANOVA test were used to compare the association between notch and exposure to noise and organic solvent in four groups. Logistic regression analysis was performed to eliminate the confounding factors. p Values of less than 0.05 were considered statistically significant. SPSS v.20 software was used for statistical analysis.
Results
A total of 540 men were recruited in this study. The mean and standard deviation age and work experience of workers were 32.65 ± 7.01 and 7.95 ± 3.56 years, respectively. More than two-third of the participants had upper diploma degree and less than 10% were smokers. Table 1 shows more details about characteristics of control, noise-only, solvent-only, and noise-solvent exposure groups.
Table 1.
Characteristics of study groups
| Total (n = 540) |
Noise exposure only (n = 209) |
Solvent exposure only (n = 51) |
Noise and Solvent exposure (n = 48) |
No exposure (n = 232) |
p value | |
|---|---|---|---|---|---|---|
| Age, years mean ± SD | 32.65 ± 7.01 | 31.67 ± 6.89 | 32.53 ± 7.15 | 29.23 ± 5.71 | 34.27 ± 6.96 | < 0.0001 |
| BMI, kg/m2 mean ± SD | 26.03 ± 3.91 | 26.09 ± 3.56 | 25.68 ± 4.28 | 26.05 ± 3.70 | 26.05 ± 4.16 | 0.941 |
| Working experience, years mean ± SD | 7.95 ± 3.56 | 8.25 ± 3.54 | 7.10 ± 3.39 | 7.72 ± 3.54 | 7.91 ± 3.61 | 0.207 |
|
Smoke pack year* (n = 42), mean ± SD |
5.29 ± 5.53 | 3.68 ± 2.69 | 2.80 ± 4.08 | 2.90 ± 3.67 | 7.65 ± 7.14 | 0.269 |
| Sex (male), frequency (percent) | 534 (98.9) | 208 (99.5) | 50 (98) | 48 (100) | 228 (98.3) | 0.488 |
| Education level | ||||||
| Upper diploma | 383 (70.9) | 167 (79.9) | 38 (74.5) | 38 (79.2) | 140 (60.3) | < 0.0001 |
| Under diploma | 157 (29.1) | 42 (20.1) | 13 (25.5) | 10 (20.8) | 92 (39.7) | |
| Smoking (yes) | 42 (7.8) | 19 (9.1) | 3 (5.9) | 2 (4.2) | 18 (7.8) | 0.636 |
| Notch in right ear | 103 (19.1) | 54 (25.8) | 6 (11.8) | 23 (47.9) | 13 (5.6) | < 0.0001 |
| Notch in left ear | 101 (18.7) | 57 (27.3) | 5 (9.8) | 24 (50) | 20 (8.6) | < 0.0001 |
| Notch in two ears | 84 (15.6) | 47 (22.5) | 4 (7.8) | 20 (41.7) | 15 (6.5) | < 0.0001 |
*Smoke pack year has been calculated by multiplying the number of packs of cigarettes smoked per day by the number of years the person has smoked
According to the pure tone audiometry, 84 (15.5%) of workers had notch in both ears. Higher work experience was significantly related with presence of notch in both ears (p value < 0.0001). BMI, age, smoking, education level didn't have any correlation with notch presence in both ears. More characteristic details of the study population and their relationship with notch presence in both ears are available in Table 2. The characteristics of patients with notch in left or right ear are presented in supplements.
Table 2.
Characteristics of workers with and without notch in both ears
| Total (n = 540) |
With notch in two ears (n = 84) |
Without notch in two ears (n = 456) |
p value | |
|---|---|---|---|---|
| Mean ± SD | Mean ± SD | Mean ± SD | ||
| Age, years mean ± SD | 32.65 ± 7.01 | 32.55 ± 6.67 | 32.67 ± 7.08 | 0.884 |
| BMI, kg/m2 mean ± SD | 26.03 ± 3.91 | 26.17 ± 3.85 | 26.01 ± 3.92 | 0.749 |
| Working experience, years mean ± SD | 7.95 ± 3.56 | 9.90 ± 3.68 | 7.59 ± 3.43 | < 0.0001 |
| Smoke pack year (n = 42), mean ± SD | 5.29 ± 5.53 | 3.57 ± 2.27 | 5.59 ± 5.90 | 0.512 |
| Sex (male), Frequency (percent) | 534 (98.9) | 84 (100) | 450 (98.7) | 0.290 |
| Education level | ||||
| Upper diploma | 383 (70.9) | 62 (73.8) | 321 (70.4) | 0.311 |
| Under diploma | 157 (29.1) | 22 (26.2) | 135 (29.6) | |
| Smoking (yes) | 42 (7.8) | 8 (9.5) | 34 (7.5) | 0.469 |
| Exposure status | ||||
| No exposure | 232 (43) | 13 (15.5) | 219 (48) | < 0.0001 |
| Noise expose only | 209 (38.7) | 47 (56) | 28 (6.1) | |
| Solvent expose only | 51 (9.4) | 4 (4.8) | 47 (10.3) | |
| Noise and solvent (mix) | 48 (8.9) | 20 (23.8) | 28 (6.1) | |
The odds ratios for notch presence in both ears in the noise-only group, solvent-only group, noise-solvent group were respectively 4.49, 1.86, 11.72 times higher than control group after adjustment of age, working years, and education level (Table 3).
Table 3.
Notch presence in the study groups
| Outcomes | Notch in two ears | Notch in right ear | Notch in left ear | |||
|---|---|---|---|---|---|---|
| OR (95% CI) | p value | OR (95% CI) | p value | OR (95% CI) | p value | |
| Model 11 | ||||||
| No exposure | 1 | 1 | 1 | |||
| Noise exposure only | 4.88 (2.55–9.33) | < 0.0001 | 3.69 (2.12–6.42) | < 0.0001 | 5.42 (2.96–9.93) | < 0.0001 |
| Solvent exposure only | 1.43 (0.44–4.59) | 0.544 | 1.41(0.53–3.71) | 0.483 | 1.57 (0.54–4.54) | 0.403 |
| Noise and solvent exposure | 12.03 (5.39–18.81) | < 0.0001 | 9.75 (4.71–16.21( | < 0.0001 | 14.46 (6.69–25.26) | < 0.0001 |
| Model 22 | ||||||
| No exposure | 1 | 1 | 1 | |||
| Noise exposure only | 4.49 (2.23–9.01) | < 0.0001 | 3.41 (1.87–6.24) | < 0.0001 | 4.61 (2.39–8.89) | < 0.0001 |
| Solvent exposure only | 1.86 (0.55–6.22) | 0.312 | 1.81 (0.66–4.99) | 0.247 | 1.94 (0.64–3.91) | 0.239 |
| Noise and solvent exposure | 11.72 (4.81–20.58) | < 0.0001 | 9.91 (4.37–17.44) | < 0.0001 | 12.83 (5.46–20.14) | < 0.0001 |
Model 1: without adjustment
Model 2: age, working years, education level adjusted
Exclusive exposure to noise and noise-solvent exposure had strong correlations with notch presence in both ears, left ear, and also right ear (p value < 0.0001), but exclusive exposure to solvent didn't have any correlation with notch presence. There are more details of the analysis available in Table 3.
Discussion
The effect of noise on high frequencies hearing loss and notch presence is a common problem in occupational impairments, but there are several risk factors that have additive effects. The results of our study showed the negative effect of concurrent noise and organic solvents on the hearing system. We used Cole's criteria for the definition of notch, which is a characteristic of noise and eliminates the age effect at high tone frequencies [8]. The pathogenesis of Noise-induced hearing loss (NIHL) involves the induction of a progressive, sensorineural hearing deficit, resulting from irreversible damage to sensory hair cells of the cochlea inside the inner ear [5, 7]. The ototoxic solvent may have negative effects on these hair cells [12].
Some studies suggested that noise exposure is the reason for approximately 37% of hearing loss [6, 16]. According to our results, 15.5% of audiograms showed notch presence which was similar to the findings of a study in the Brazilian metalworkers (15% presence of notch) [4]. We didn't find any association between notch presence and age. Some studies showed a strong correlation between age and development of NIHL in workers, particularly at noise levels below 98 dB [15]. This could be due to the low variety of age in our study population and also less than 15% of participants are more than 40 years old. High work experience was correlated with notch presence. It was evident that high exposure duration is a critical factor for severe outcomes. This was in agreement with the results of a study done by Metwally et al. [12].
There was not a significant association between exposure to organic solvents and hearing loss in our study. Previous studies report that there is a significant correlation between high frequencies hearing loss and combined exposure to noise and organic solvents and also noise [9, 10]. Similar to our analysis, Loukzadeh et al. found no association between organic solvents with hearing loss even at high concentrations (Em > 1) [8]. On the other hand, Mariola et al. showed that organic solvents exposure could cause moderate impairment of the hearing threshold [14].
Campo et al. found out that some organic solvents like toluene have an anticholinergic-like effect which could improve the cholinergic receptors and inhibit the middle ear reflex. Therefore, organic solvents allow higher acoustic energy penetration into the cochlea and it predisposes hearing loss due to noise [3].
Based on our results, if the solvent exposure was below the threshold limit value of American Conference of Governmental Industrial Hygienists (ACGIH), the additive effect of both noise and solvent exposure on notch presence was noticeable. Therefore, we suggested using hearing protection devices wherever there is simultaneous exposure to noise < 85 dB for 8 h and chemical solvents.
Strength and Limitation
We evaluated notch high-frequency hearing loss as our dependent variable, which was specific to noise-induced hearing loss, but other surveys assessed low and high-frequency hearing loss. Being Based on the National Institute for Occupational Safety and health guideline, a large sample size, and accuracy of the environmental monitoring results were the other strengths of our study. The insignificant OR for notch presence in only-solvent group may be due to the small sample size of only-solvent exposure group resulting in low power of the analysis for this group. The causal relationship and its generalizability can't be investigated due to the cross-sectional type of our survey. We couldn't find out the effect of gender through our analysis due to majority of male workers in our survey.
Conclusion
We found out that in the group with simultaneous exposure to noise and organic solvents the rates of notch presence in one or both ears were significantly higher than other groups. We recommend implementing a comprehensive hearing protection program for industries with combination exposure to noise and other auto toxic chemicals like organic solvents. Also, it would be better to assess the audiologic survey in shorter intervals even in the lower sound pressure, (< 85 dB recommended by The National Institute for Occupational Safety and Health (NIOSH)).
Acknowledgements
The authors would like to thank the Clinical Research Development Unit (CRDU) of Baharloo Hospital and Occupational Sleep Research Center, Tehran University of Medical Sciences, Tehran, Iran for their support, cooperation and assistance throughout the period of study.
Author Contributions
MS and RO wrote the manuscript. SA analyzed the data. NI collected the data. OA and reviewed the results. SEM reviewed the whole manuscript and provided guidelines for presentation and interpretation. All of the authors have read and approved the final manuscript.
Data Availability
All the relevant data and materials are presented in the paper.
Declarations
Conflict of interest
All of the authors state that there is no conflict of interest.
Ethical Approval
This study was approved by the Ethics Committee of Tehran University of Medical Sciences and performed in accordance with the ethical standards.
Informed Consent
All of the participants were provided with informed consent.
Footnotes
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Data Availability Statement
All the relevant data and materials are presented in the paper.