Evaluation of the effects of occupational noise exposure on serum aldosterone and potassium among industrial workers.

The existing literature indicates that occupational exposure to noise may have adverse effects on workers' health. The aim of this study was to evaluate the possible effects of exposure to different sound pressure levels (SPLs) on serum aldosterone and potassium concentration among Iranian blue collar workers in Golgohar Mining and Industrial Company in Sirjan, Kerman Province, Iran. This case-control study was performed on 45 workers of Golgohar Mining and Industrial Company. The subjects consisted of 30 workers from manufacturing departments and 15 office employees of the mining company. The controls, mainly with administrative jobs were exposed to 72 dBA SPL. Cases, in two separate groups, were exposed to noise levels of 88 dBA and 103 dBA, respectively. Noise intensity was measured at the desired locations. Noise measurements were performed according to the International Organization for Standardization (ISO) 9612. To measure the serum aldosterone and potassium concentrations, a 5 mL blood sample was taken from each worker at the specified time intervals and aldosterone concentration was determined using enzyme-linked immunosorbent assay (ELISA) test in the laboratory. Repeated measurement and Spearman's correlation coefficient analysis were used with α = 0.05. Exposure to the different levels of sound pressure resulted in different aldosterone concentrations and meanwhile an increase in the SPL did not affect the concentration of potassium. From 10:00 AM to 10:30 AM, as SPL increased, aldosterone concentrations did not increase significantly but from 13:30 PM to 14:00 PM, raised SPL led to a significant increase in aldosterone concentration. However, there was no correlation between the concentration of potassium and different factors. This study indicated that increases in SPLs affect aldosterone concentration but at the same time do not have significant effects on serum potassium level.

Introduction

The existing literature indicates that occupational exposure to noise may have adverse effects on workers’ health. Exposure to elevated noise levels can impair the auditory system and cause hypertension, ischemic heart disease, annoyance, stress, and sleep disturbance. Progress in all areas of the industry and widespread application of machinery and industrial equipment have led to a remarkable increase in the noise levels at workplaces. This has made many workers, particularly workers in the industrial sectors, exposed to excessive amounts of noise levels.[1]

It is estimated that over six million people in the world are exposed to excessive noise levels (>85 dBA) in their working environments, out of whom, 50-60 million live in Europe and North America.[2] When noise levels exceed the occupational limit values, they can leave debilitation effects on the performance of different systems in the human body such as hearing and cause hypertension, ischemic heart disease,[34] annoyance, stress,[56] and hormonal disturbances.[78]

Noise-induced hearing loss (NIHL) is among the top frequent work-related illnesses worldwide. The damage caused by exposure to higher levels of the noise has been reported to be among the first 10 work-related impairments worldwide. The World Health Organization (WHO) estimates that more than 12% of the world population is at risk of developing NIHL.[9] According to previous studies, in the United States alone 7.4-10.2 million industrial workers are at risk of developing NIHL.WHO estimates that a daily NIHL-related cost is about US$4. Annually, in Sweden, over US$100 is paid as disability payments to those who have developed hearing loss.[10] Vascular strip is part of the inner ear that is responsible for maintaining the bioelectric and biochemical characteristics of endolymph.[11]

Sodium and potassium play an important role in the inner ear and are involved in cellular functions to convert the mechanical energy into nerve impulses.[12] Such potential is the consequence of higher levels of potassium and lower levels of sodium in the endolymph and the opposite trend in the perilymph.[13] In order to produce and maintain the potential in the cochlea, the vascular strip makes use of Na +/K + – ATPase enzyme, which is saturated in the cochlear outer wall.[14] To maintain the cochlear potential, various factors such as hormonal functions and most notably aldosterone control and regulate vascular strip functions. Serum levels of sodium and potassium are regulated by aldosterone, a mineralocorticoid that is secreted by the adrenal cortex.[15]

Reduced aldosterone affects the inner ear and impairs speech perception in the cortical area. Some researchers believe that the reduction in the serum aldosterone level might contribute to hearing loss, and conversely, its increased levels can lead to hearing protection and prevent possible damage. It is obvious that the incidence of pathological conditions and misuse of medications during childhood and adolescence can cause aldosterone deficiency.[16] Aldosterone has been shown to influence the cochlear ion cycle in vascular strip.[17] Alterations in the serum potassium level in pathological conditions can cause more harmful effects on the outer hair cells rather than the inner hair cells and auditory nerve fibers.[18]

The relationship between age-related hearing loss (presbycusis) and aldosterone levels have been investigated in several studies.[19] However, only a few studies have investigated the possible effects of noise exposure on the secretion of aldosterone and levels of serum potassium. To the best of our knowledge, there is also little understanding about the possible relationship between the exposure to different sound pressure levels (SPLs) and aldosterone and potassium concentrations worldwide. As a result, high levels of industrial noise, that is known to be as a main stressor, can be assessed via monitoring serum aldosterone in occupationally exposed individuals.[20]

The present study was designed to:

Assess and compare the serum aldosterone and potassium concentrations at three intervals while distinctively exposed to three levels of SPLs among blue collar industrial workers,

Investigate the possibility of using serum aldosterone as a feasible biomarker of noise-induced stress in blue collar industrial workers, and

Introduce a statistical model related to aldosterone and potassium concentration among noise exposed workers.

Methods

Participants

This cross-sectional study was performed on 45 workers of Golgohar Mining and Industrial Company, Sirjan situated in Kerman Province in the southeast of Iran during the autumn of 2014. Right before conducting the study, subjects’ health status with regard to the hearing status, heart and vascular conditions along with mental condition were monitored by reviewing their medical records and health individuals were recruited. In order to control the potential confounding effects of shift work, only day shift workers were selected for this study.

Industry selection

Golgohar Mining and Industrial Company was selected as it provided the appropriate conditions for the study. Workers were not exposed to thermal stress. Also, there were no main vibration sources. Due to SPL variability in the measurement domains, ±5 dBA was thought to be suitable as standard deviation for this variable.

Study design

Before conducting the experiment, the purpose of the study was fully explained to the subjects. They were also asked to read and sign the written informed consent form indicating their willingness to participate in the study. On the experiment day, the demographic data form was completed by participants and body mass index (BMI) values were computed. The subjects consisted of 30 workers from manufacturing departments and 15 office employees of the mining company. Controls were selected from those with administrative jobs. Selected individuals from the exposed group (cases) did not use earmuffs and based on International Organization for Standardization (ISO) 8996 their working practices were defined as light work.[21] Noise intensity values at the desired locations were also measured as environmental variable. Serum aldosterone and potassium levels were measured for controls and cases at three different time intervals: At the beginning of the shift and before exposure to noise (7:30-8:00 AM), during exposure to noise (10:00-10:30 AM), and continuous exposure (13:30 PM-14:00 PM). Considering the aldosterone circadian rhythm in the previous study,[22] since the shift work of studied subjects was 3-3-3-3 (three morning shifts, three afternoon shifts, three night shifts, three days off), the sample taken in each studied group was performed at the first day after the rest days.

Measurements

Noise

Noise measurements were performed according to ISO 9612 for each workstation by a sound level meter (CEL-440, Casella, USA) (CEL-440). Right before performing the measurements, CEL-282 calibrator (Casella, USA) was used to calibrate the sound level meter.[23]

Aldosterone and potassium

In order to measure the concentrations of serum aldosterone and potassium, 5 mL of the blood sample was taken from each worker at the three specified time intervals during shift work hours. It should be noted that during sampling, all the subjects were in a sitting position. Blood samples were collected in numbered tubes containing anticoagulant ethylenediaminetetraacetic acid (EDTA) and immediately under controlled conditions (ice box) were transferred to an authentic medical diagnostic laboratory. Aldosterone concentration was determined using the enzyme-linked immunosorbent assay (ELISA) test. Serum potassium concentration was also measured by a kit (DBC-Diagnostic Biochem Canada Inc., Ontario, Canada) and a flame photometer (Jenway PFP7, Essex, UK).[2425]

Statistical analysis

Collected data were analyzed by Statistical Package for the Social Sciences (SPSS) 18 (SPSS Inc., Chicago, IL, USA) using statistical tests such as Spearman's rho and repeated measures analysis of variance. The results were significant at the P = 0.05 level.

Ethical considerations

Ethical approval was obtained from the Ethics Committee of Tehran University of Medical Sciences (ID: 1394.51). All participants signed a consent form.

Results

Demographic characteristics

The average age and BMI of workers in the control group were 28.8 ± 2.05 years and 25.1 ± 2.28 kg/m2, respectively [Table 1].

Table 1

Demographic characteristics of subjects

Variables	Control group exposed to low noise level (72 dBA)	Case group exposed to medium noise level (88 dBA)	Case group exposed to high noise level (103 dBA)
	Mean ± SD†	Mean ± SD	Mean ± SD
Age (years)	28.8±2.05	30.1±2.37	29.4±2.63
BMI (kg/m2)	25.1±2.28	25.5±3.25	25.5±2.97

Noise intensity measurements

Having measured the SPLs in the identified locations, the control group was exposed to a maximum level of 72 dBA SPL. Cases were formed by two distinct groups who were exposed to noise levels of 88 dBA SPL and 103 dBA SPL.

Variations in serum aldosterone and potassium concentration

Figures 1 and 2 illustrate the variations of the means of serum aldosterone and potassium concentrations with respect to exposure to three different SPLs and at three different time intervals.

Figure 1

Comparisons between the levels of serum aldosterone in different noise exposed groups

Figure 2

Comparisons between the levels of potassium in different noise exposed groups

Toward the end of the shift work, Figure 1 represents a declining trend in the means of serum aldosterone concentrations in three different groups. As the SPLs increase, the means of serum aldosterone concentrations increase as well.

As shown in Figure 2, there was no significant difference in the means of serum potassium concentration with respect to different noise levels and at three different time intervals and increase in the noise levels did not have a significant effect on the concentration of potassium.

The effects of various factors on aldosterone and potassium concentrations

Repeated measures analysis of variance was employed to investigate the effects of various factors on aldosterone and potassium concentrations. The data from Table 2 indicate that the relationships between the time intervals in which the aldosterone and potassium concentrations were measured and the factors such as SPL, BMI, and age were not significant. This rejects any possible interaction between the mentioned factors and the time intervals. Likewise, the independent impact of time, BMI, and age on potassium and aldosterone concentrations are not significant. However, SPL has a significant effect on aldosterone concentration (P = 0.04), which indicates that the exposure to the different levels of SPL results in different levels of aldosterone concentration. Conversely to the trend associated with aldosterone concentration, the increase SPLs did not increase serum potassium [Table 2].

Table 2

Results of the effects of different factors on serum aldosterone and potassium levels

Source	Aldosterone		Potassium
	F	P value	F	P value
Time	2.59	0.09	1.07	0.34
Time by SPL	1.09	0.32	0.32	0.72
Time by BMI	0.56	0.52	2.41	0.09
Time by Age	0.07	0.87	0.13	0.87
SPL	3.62	0.04	0.81	0.37
BMI	0.65	0.42	1.28	0.26
Age	0.000	0.99	0.05	0.81
Intercept	21.03	<0.001	43.72	<0.001

†SD: Standard deviation

Spearman's correlation coefficients showed that in the time interval 10:00-10:30 AM, there was no significant correlation between sound pressure level and aldosterone concentration (r = 0.22, P = 0.13). But in the time interval 13:30-14:00 PM, there was a statistically significant correlation between SPL and aldosterone concentration (r = 0.54, P < 0.001); in other words, with an increase in SPL in this time period, there was a significant increase in aldosterone concentration.

Statistical model of aldosterone and potassium concentration

Considering the effects of SPLs on aldosterone hormone at different time intervals and lack of any effects from factors such as time, BMI, and age, the statistical model associated with aldosterone and potassium concentrations at three different time intervals are shown in Table 3.

Table 3

The statistical model of aldosterone and potassium concentrations at three different time intervals

Time (h) Statistical models	7:30-8:00 AM	10:00-10:30 AM	13:30-14:00 PM
Serum aldosterone level (pg/mL)	Y=185.51+0.04SPL	Y=115.04+0.257SPL	Y=74.23+0.392SPL
Serum potassium level (mmol/L)	Y=5.14+0.002SPL	Y=4.80+0.001SPL	Y=3.39+0.006SPL

Discussion

This study set out to determine the effects of different levels of sound pressure on serum aldosterone and potassium concentration at different time intervals. Regulation of aldosterone secretion is directly related to various aspects of renal functions such as adjusting the concentration of electrolytes in the extracellular fluid, extracellular fluid volume, blood volume, and blood pressure. Therefore, any discussion with regard to regulating the secretion of aldosterone that excludes such aspects is difficult and almost impossible.[26] The regulation of aldosterone secretion is done by the glomerular area and is independent of regulation of cortisol secretion by the fascicule area.[27]

There are two formidable factors that regulate aldosterone secretion:

Increased concentration of potassium in the extracellular fluid, increase in the aldosterone secretion to a large extent and

Increased activity of the renin-angiotensin system (increased levels of angiotensin (II) also greatly increases aldosterone secretion).[28]

Thus, based on the abovementioned facts, in order to determine the possible effects of potassium concentration on aldosterone secretion, measurement of serum potassium was also considered as a priority.

The results from the measurements of aldosterone and potassium concentration in the three groups were within normal limits. Normal levels of aldosterone and potassium concentration in the blood are 6-22 ng 100 mL and 3.5-5 mmol/L, respectively.[29] It should be noted that the results from the present study are consistent with these values. The results from our study also showed that in all the three groups, compared to the beginning of the shift work, the mean concentration of aldosterone at the end of the work shift increases. Charloux et al. also demonstrated a significant reduction in aldosterone concentration at the beginning of the work shift, which was consistent with the results that were observed in our study.[22] In this study, we also examined the impact of age, BMI, and time on aldosterone concentration and potassium concentration.

The results showed that age, BMI, and time have no interactional impacts on aldosterone and potassium concentration; so after adjusting for these three factors, the effect of SPL on aldosterone and potassium concentrations was examined. The results showed that the SPL independently influences the aldosterone concentration and aldosterone concentration is raised if SPL increases. Conversely, increased noise levels did not affect the potassium concentration level.

Nawaz et al. (2011) showed that different SPLs would affect the aldosterone concentration. They studied the effect of three noise levels (SPL <80 dBA) and (80-94 dBA) and (SPL >94 dBA) on healthy subjects and concluded that the mean concentration of aldosterone in the three groups is statistically significant; as the SPL increases mean concentration of aldosterone is raised.[30]

The results of our study suggest that increase in SPL significantly increase the mean concentration of aldosterone (P < 0.05). The results also indicated that during 10:00-10:30 AM, the rise in SPL did not lead to an increase in aldosterone concentration while during 13:30-14:00 PM increased SPL significantly increased aldosterone concentration.

There is also a significant positive correlation between potassium concentration and aldosterone concentration during 13:30-14:00 PM in a way that increase in potassium concentration resulted in the increased level of aldosterone. This is comparable to the results of previous studies over the interaction of aldosterone and potassium.[31] Several studies have shown that a small percentage increase in potassium concentration would result in several fold increase in aldosterone secretion.[32] Therefore, it is concluded that the prolonged and excessive exposure to SPL seems to have more noticeable effects.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.