Study Highlights Impact of Smoking on Hearing Loss Among Workers Exposed to Noise

Some 432 million adults worldwide suffer from hearing loss, and this number is estimated to double by 2025. ¹ About 16% of global cases of hearing loss is caused by exposure to noises that are higher than recommended levels for a workplace. ² Likewise, nearly 600 million workers are exposed to noises with levels higher than threshold limit value (TLV) (85 dBA) recommended by the American Conference of Governmental Industrial Hygienists (ACGIH). ³ Noise-induced hearing loss (NIHL) has been introduced as the most important reason for hearing impairment in 7%–21% of workers. ⁴ Statistics of Bureau of Labor reveal that NIHL is now recognized as the most commonly recorded occupational disease in manufacturing plants ⁵ that increases hearing loss disabilities in more than 500 million laborers worldwide. ⁶ They also reported NIHL is among the nine recorded diseases. ⁵ No statistics have been done on NIHL in Iran; however, this growing health issue is probably more widespread than developed countries.

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Table 1: Demographic data, hearing loss status, and noise exposure level of the participants.

Table 2: Results of the Ordinal logistic regression test for the effect of smoking status on hearing loss.

According to the World Health Organization (WHO), smoking is a serious public health concern. ⁷ There are more than 1.3 billion smokers worldwide, about 880 million of whom currently live in developing countries. ⁸ Iran ranks 73rd out of 181 countries in cigarette smoking based on the Tobacco Atlas report. Each Iranian smoker older than 15 consume an average of 936. ⁵ cigarettes per year. ⁹ Iran is among the list of countries that have not addressed smoking well. ¹⁰ According to WHO, about 20.2% of Iranian men over the age of 15 smoke on a regular basis, ¹¹ and this recently has been increasing. ¹² Smoking has several hazards to the health—-increased risk of cancer, ^13,14 premature death, ¹⁵ respiratory tract infections, and increased amount of pollutant in the environment. ^16,17

However, results from three out of eight cohort studies ^18–25 showed a relationship between smoking and hearing loss, ^23–26 whereas others did not find any effect of smoking on hearing loss. ²⁰ Because noise exposure might be a significant factor that interacts with (or affects) the effect of smoking on hearing loss, our study aimed to assess the impact of smoking on hearing loss among workers exposed to industrial noise 85 dBA or higher at a metalwork plant in Arak, Iran.

STUDY METHODS

A cross-sectional study was conducted among all metalworkers in a manufacturing plant in Arak. All men who worked eight hours a day for five days a week while exposed to 85 dBA or higher were included in the study (n=354). A number of 81 workers with a long working experience in a noisy environment, long time use of hands-free devices (such as hands-free mobile phones and headphones), otitis media, heart and neurological disorders, thyroid gland disorders, hyperlipidemia as well as diabetes, head injury, and consumption of drugs affecting hearing system according previous studies^27,28 were excluded.

These criteria summed up to 273 workers for final analysis. The participants performed different tasks such as driving a forklift, monitoring control panels, operating overhead cranes, and material handling. Hearing protection devices were not used by the study population due to safety concerns in the workplace. The subjects were divided into two groups of ever-cigarette smokers (n=132) and non-smokers (n=141). According to the Centers for Disease Control and Prevention (CDC) definition, non-smokers consist of those who had never smoked, or had smoked 100 cigarettes in their lifetime.

In our study, ever-smokers included those who smoked at least 100 cigarettes in their life and now consume at least six cigarettes per day. ^29,30 Smokers usually continue this habit while resting or at lunchtime. The background information of all workers was obtained from demographic questionnaires and by referring to their medical records. They were allowed to stop participating at any time. All participants were required to sign an informed consent form approved by the ethics committee of the Shahid Beheahti University of Medical Sciences (ethics code: 1398.142).

MEASUREMENT OF NOISE EXPOSURE LEVEL

The exposure level to occupational noise was measured by a calibrated noise dosimeter (Casella, Cell-320; USA). The dosimeter microphone was placed 10 to 30 cm away from the workers’ ear and attached to their collars. ³¹ According to the standard ISO 9612, the equivalent level (Leq) of received dose was calculated using Equation 1 and the eight-hour equivalent level was calculated using Equation 2. ³²

Where, the daily shift was 8 hours, the duration of exposure was 4 hours and the permissible SPL (sound pressure level) was 85 dBA. Lpi and ti are the measured equivalent level and exposure duration, respectively.

Hearing Loss Measurement

The audiometric test was conducted based on ANSI S3.1 standards (1999) ³³ and when workers were at least 14 hours out of the noisy workplaces. Before doing the test, the subjects were given necessary training. The audiometer (Madsen ORBITER 922; Denmark) was located in an acoustic room. Hearing thresholds were assessed at the frequencies of 500 Hz, 1000 Hz, 2000 Hz, 3000 Hz, 4000 Hz, and 6000 Hz. Hearing loss was classified into five ordinal categories according to WHO guidelines. Hearing loss in the range of 0-25 dB, 26-40 dB, 41-60 dB, 61-80 dB, and > 80 dB categorized as normal, mild, moderate, severe, and occupational profound impairment, respectively. ³⁴ The highest hearing loss of each ear was considered as a person’s hearing loss status. Moreover, the age-correction table recommended by Occupational Safety and Health Administration (OSHA) ³⁵ for male was used to eliminate the interfering factor of age on the incidence of hearing loss.

Statistical Analysis

Statistical analysis was performed using the SPSS V22 software (Chicago Il, USA). The Kolmogorov–Smirnov Test was used to determine data distribution normality. Mean and standard deviation (SD) were calculated for continuous variables, frequency, and percent for categorical variables. Smokers and non-smokers were compared using Student’s t-test and chi-square test. Binary logistic regression were applied to explore the relationship between smoking and hearing loss. Binary logistic regression is a statistical analysis method that can be used to model the relationship between a dichotomous (binary) response variable (smoking status) and one or more explanatory variables (hearing loss). The odds ratio (as a statistic that quantifies the strength of association between two events) and 95% confidence interval (a range of values that is likely to include a population value with 95% of confidence) were reported. A probability value (P-value) of less than 0.05 were regarded as statistically significant.

Results

Table 1 presents the demographic data, hearing loss status, and noise exposure level of the participants. The mean age of participants was 36.17±1.20 years. There were 141 (51.65%) non-smokers and 132 (48.35%) ever-smokers among them. There was no significant difference between non-smokers and ever-smokers in terms of age, duration of employment, body mass index (BMI), and noise exposure (P>0.05). The occurrence of occupational hearing loss in severity of mild, moderate, and severe was reported as 34.75% and 54.55% among non-smokers and ever-smokers, respectively. No profound hearing loss was observed among participants. The result of noise measurement showed that the non-smokers were exposed to the eight-hour equivalent level of 89.20± 12.00 dBA and smokers of 89.60± 11.00 (P-value: 0.34).

Table 2 shows the results of the binary logistic regression test for the effect of smoking on hearing loss in both unadjusted and adjusted model for demographic data. There was a statistically significant relationship between smoking and hearing loss. The odds ratio of hearing loss in ever-smokers were higher than non-smokers based on an unadjusted model (OR=2.25, CI=1.50-3.41; and P-value: 0.01) and an adjusted model based on demographic data (OR =2.09, CI=1.32-3.07 and P-value: 0.025).

According to our results, the prevalence of different WHO categories of hearing loss is 44.65% for all participants who are exposed to noise higher than a threshold value of 85 dBA. ³ Likewise, most of the metal working industries had a hearing threshold more than normal ^36,37 that may increase medical costs and reduce productivity at workplace. ³⁸ The risk of hearing loss is also higher among metal workers compared to that of the other industries among non-smokers and ever-smokers. ³⁹ According to WHO, hearing loss was reported to occur 34.75% and 54.55% among non-smokers and ever-smokers, respectively. Logistic regression indicates there is a statistically significant relationship between smoking and hearing loss. According to Table 2, smoking multiplies the odds ratio of hearing loss by 2.25 and 2.09 when the model was adjusted and unadjusted for demographic data, respectively. The results indicated that the habit of smoking among participants might increase the chances of hearing loss.

There are controversial findings on the relationship between smoking and hearing loss in literature. A cross-sectional study in Yazd showed a threshold for hearing loss at frequencies of more than 1 kHz for workers exposed to noise level of 92.1 dBA was higher in smokers than non-smokers; however, there was no statistically significant differences between hearing loss and smoking in there. ⁴⁰ Some researchers concluded smoking can be a risk factor for serious hearing loss among workers exposed to noise. ^5,41

There was also a dose-response relationship for the number of cigarettes smoked per day and hearing loss. ⁴² The results of a cohort study of 50,195 Japanese workers showed that the risk of hearing loss was lower in workers who quit smoking than in those who were currently smokers. ⁴³ Also, the results of a meta-analysis study showed that smoking in exposure to noise increases the occurrence rate of hearing loss and current smokers have a higher risk of hearing loss than former smokers. ⁴⁴ Lin et al. (2020) showed people who quit smoking have a risk of hearing loss as well; however, as the duration of smoking gets longer, the risk of hearing loss increases, too. In other words, the risk of hearing loss because of noise exposure decreases as the duration of smoking cessation increases. ¹⁶

In addition, it has been shown that the rate of hearing loss in ever-smokers was five times higher among non-smokers, which could be due to the persistent damage to the hearing system. ⁴⁵ All participants in this study are exposed to high-risk noise levels (89.40±13dBA), and the prevalence of hearing loss among non-smokers is worthy of attention (34.75%). From the perspective of improving the health of employees and preventing occupational diseases, two aspects are worth studying in the future: noise control and hearing protection. ⁴⁶

Noise can increase mitochondrial oxidative phosphorylation and the reactive oxygen species (ROS) generation by increasing metabolic activity, which are effective factors in NIHL. ⁴⁷ Ischemic damage to the cochlea and the destruction of hair cells in exposure to noise are other factors that can cause hearing loss. Some studies show that genetic factors such as mitochondrial susceptibility to noise can affect the incidence of hearing loss, which may be one of the reasons for the conflicting results in the intensity of the effect of noise on hearing loss in different workers. ⁴⁸ A possible explanation for such a relationship is related to higher blood viscosity among smokers, ototoxic effect of nicotine and CO2 on hearing cells. Likewise, smoking can cause cochlear ischemia because of vasospasm and atherosclerosis as well as Corti damage due to reduced blood oxygen level. ^49–51 El Zir et al. (2016) showed that hearing loss among smokers are 1.73 times as nonsmokers. ⁵² In addition, research among workers of shipping industry exposed to noise level of 93 dBA revealed smoking leads to severe hearing loss.

The present study showed that smoking increased the odds ratio of occupational hearing loss (OR=2.25, 95% CI: 1.50 - 3.41) at a metalwork plant in Iran. Education and training are important for informing the workers and managers of smoking and noise hazards in inducing hearing loss.

Author notes: Ethical approval for this study was obtained from School of Public Health & Neurosience Research Center-Shahid Beheshti University of Medical Sciences (Approval ID. IR.SBMU.PHNS.REC.1398.142).

This study was financially supported by the Student Research Committee, Shahid Beheshti University of Medical Sciences, Tehran, Iran (Grant no. 8162).

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