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|Year : 2005
: 7 | Issue : 29 | Page
|Sleep quality in noise exposed Brazilian workers
Ana Lucia Rios1, Geruza Alves da Silva2
1 Department of General Medicine, University of São Paulo School of Medicine at Ribeirão Preto - São Paulo, Brazil
2 Division of Pulmonology, Department of General Medicine, University of São Paulo School of Medicine at Ribeirão Preto - São Paulo, Brazil
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This study investigated the effect of chronic workplace exposure to excessive noise on sleep quality. It involved 40 male workers aged 33 to 50 years, 20 of whom had been exposed to environmental workplace noise levels of 85 dB or more on 40-hour-a-week jobs. Another 20 workers who were not exposed to excessive noise were used as controls. All subjects were interviewed and submitted to physical examination, pure tone and speech audiometry, immittance testing and nocturnal polysomnography. Comparative analysis demonstrated that the two groups were similar, except for the exposure to noise. Fisher's test comparison of pure tone and speech audiometry and immittance testing revealed mild to moderate noise-induced hearing loss ( P <0.001) in the ≥ 85-dB group. Indicators of sleep continuity were abnormal in both groups, demonstrating poor sleep quality; however, sleep quantity was normal. Of the 40 individuals, 13 (32.5%) presented respiratory sleep disorders. Of those 13, 10 presented daytime somnolence according to the Epworth Scale. The Mann-Whitney test showed that sleep was identical in the two groups. Fisher's exact test revealed no association between altered sleep and hearing status in either group. Our results show that active men working 40-hour-a-week in the presence of excessive noise without adequate protection for more than eight years presented with noise-induced hearing loss but their quality or quantity of night sleep was unaffected. Sensori-neural deafness may represent an element of adaptation against noise during sleep.
Keywords: Audiometry, hearing, (NIHL) noise, noise-induced hearing loss, sleep
|How to cite this article:|
Rios AL, da Silva GA. Sleep quality in noise exposed Brazilian workers. Noise Health 2005;7:1-6
| Introduction|| |
In Brazil, like in many other places it is still usual to practice labour activities under excessive noise without appropriate hearing protection.
Since the 1940s, noise pollution has been considered the leading cause of hearing disorders and deafness in adults. In 1980, the World Health Organization set 80 dB as the maximum noise level that the human ear can safely tolerate and recommended obligatory use of personal protective equipment (PPE) when the noise level exceeds 80 dB.
Because of abrupt and harmful effects on quality of life, in recent decades, noise induced hearing loss (NIHL) has become one of the most extensively studied occupational diseases. Its importance and objectivity have guided the regulation of acceptable noise levels on the job and have informed decisions regarding preventative measures. This condition occurs in workers exposed to elevated noise indices and is related to the type and intensity of noise, duration of individual exposure events, overall duration of exposure and individual sensitivity. The use of PPE and mechanisms for noise reduction at the source can prevent NIHL.
Noise induced hearing loss is usually bilateral, symmetric and sensori-neural.
The results of previous studies by Fournier and Seligman demonstrated that, , in addition to causing mechanical and functional damage to the hearing apparatus, environmental noise (urban or industrial) has damaging effects that are unrelated to auditory function. Most studies have focused on the acute manifestations of exposure to excessive noise since the evaluation of such effects allows researchers to define the profile of individuals able to withstand the adverse effects of noise, whereas chronic manifestations have received less attention.
Noise levels between 80 and 90 dB are typically associated with NIHL since the ill-defined threshold of hearing protection falls within this range. Noise levels below 80 dB do not present a risk for hearing loss as pointed out by Godlee.  Regarding nighttime noise, a mean noise level of even 55 dB is harmful to the emotional health of human beings and a maximum noise level of 35 dB for maintaining undisturbed sleep is recommended. 
It is generally accepted that the sleep-wakefulness cycle is altered by both endogenous and exogenous influences.  Excess noise causes deep sleep to become superficial and can even cause waking from superficial or paradoxical sleep according to Alexandre.  Although it seems clear that noise interferes with sleep, this interference becomes difficult to evaluate in terms other than those of the direct effect that noise during sleep has on sleep structure. The present study was designed to determine whether continuous and intermittent noise from machinery has a delayed effect on the night sleep of workers and, if so, to what extents that sleep is affected.
We formulated the following hypotheses: 1) the noise level studied may produce hearing loss as well as other conditions unrelated to hearing and 2) exposure to excessive daytime noise may produce night sleep disorder. Very few studies have explored this topic in an objective manner. Auditory damage has been used as an indicator of subject sensitivity to noise.
| Methods|| |
In a comparative study, we evaluated 40 active male subjects aged 33 to 50, belonging to the same socioeconomic and cultural class. Subjects were divided into Noise group and Control group. The Noise group consisted of 20 workers exposed for at least 8 years to a noise level of 85 dB or more for 40 hours a week, with no night shift. Control group consisted of 20 workers exposed to a noise level of 72.7 dB. The all-male subject groups generated a homogeneous sample in terms of physiological, psychological and social variables.
The Research Ethics Committee of the University Hospital approved the study (process nº 2746/01) and all subjects gave written informed consent.
Noise in the workplace
The level of environmental noise was determined using an internally calibrated Lutron SL 4001 decibel meter equipped with a microphone, an amplifier and a noise-level indicator (Lutron Electronic Enterprise, Taipei, Taiwan). The meter was positioned at 20 centimeters from the ear of the subject and 100 centimeters from the noise source.
An interview was used to obtain occupational and health data. All subjects were submitted to otoscopy and audiometric evaluation under conditions of auditory rest (at least 14 hours after the most recent exposure to excessive noise). After determination of tonal thresholds, the speech recognition percent index was determined through logoaudiometry.
Prior to the polysomnographic exam, the subjects maintained their daily work routine but abstained from consuming alcohol, coffee, carbonated drinks and cigarettes. The exam was conducted over a period of 6 to 8 hours, from the habitual bedtime of the individuals until 5:00 or 6:00 a.m., depending upon the habitual wake time related to the work routine of each individual.
A battery of audiometric tests was applied using a Midimate 622 audiometer (Madsen Electronics, Copenhagen, Denmark). Acoustic immittance was determined through tympanometry and through analysis of the stapedial reflex with a Zodiac 901 middle-ear analyzer (Madsen Electronics). The audiograms were interpreted according to the classification criteria proposed by Merluzzi. 
Polysomnographic exams were performed using a computerised "Alice 3" system (Respironics, Murrysville, PA, USA), calibrated at a velocity of 10 mm/s and a sensitivity of 75 mV/cm. The system provided 14 channels for the recording of classical: electroencephalographic and respiratory variables. The same physician with the aid of a computer analyzed the tracings.
All electrodes used for electroencephalogram recordings during sleep were referenced to the contralateral retroauricular points (C3/A2 - C4/A1; O1/A2 - O2/A1; ROC/A1; LOC/A2), according to the standard manual written by Rechtschaffen and Kales and adhering to the international 10-20 system of electrode placement.
The following polysomnographic parameters were evaluated:
Total recording time (TRT); Total sleeping period (TSP); Total sleeping time (TST); Sleep efficiency (SE); Latency to NREM sleep; Latency to REM sleep; Sleep stages; Time awake during TSP; Number of awakenings; Number of changes in stage; Incidence of oxyhemoglobin desaturation; Incidence of respiratory sleep disorders.
Data regarding these parameters were statistically analyzed by means of frequency distribution analysis, measurement of central tendency and variability and Mann-Whitney and Fisher's test comparisons using the biostatistics software GraphPad InStat. The level of significance was set at 5%.
| Results|| |
Characteristics of the study subjects
The 40 individuals in the two groups (noise group and control group) showed statistically identical biometric characteristics, as shown in [Table - 1].
Analysis of demographic data by the Mann-Whitney test revealed two homogeneous groups [Table - 2]. The subjects had been working at their current jobs 40 hours a week for an average of 16 years. Failure to use PPE for ear protection, when warranted, was due to either lack of concern regarding hearing loss or the equipment was not provided by the employer. Analysis of subject responses regarding respiratory disorders during sleep and daytime sleepness showed that the distribution of somnolent individuals, as classified by the Epworth Sleepiness Scale according to Johns was similar between the two groups  [Table - 2].
Analysis of noise levels
The mean noise level in the work environment of subjects in the Noise group was 86 ± 1 dB, compared to 72.7 ± 3.3 dB in that of those in the control group.
Results of the audiologic test battery
Analysis of the audiometric data revealed that 65% of noise group individuals presented hearing loss. Among those, the hearing loss was bilateral in 45%, right unilateral in 10% and left unilateral in 10%. In control group individuals, 20% presented hearing loss. In both groups the degree of loss ranged from mild to moderate and involved high frequencies and was sensorineural. In this respect, the noise group differed significantly from the control group in the Fisher's exact test ( p0 <0.01).
When indicators of sleep continuity and percentage of time spent in each sleep stage were analyzed, the sleep of the subjects in the two groups was found to be of poor quality. However, when the groups were compared using the Mann-Whitney test, the quality of sleep was found to be statistically identical between the two. Although subjects in both groups suffered no respiratory sleep disorders, their sleep was mainly characterized by an excessive number of awakenings and changes in stages. In the Noise group subjects, there were a median of 152 awakenings and 109 changes in stage, compared to 159 and 128, respectively, in control group subjects [Table - 3].
Respiration during sleep
Thirteen individuals experienced a great number of sleep apnea/hypopnea episodes with an RDI > 5. Of these, 6 had an RDI > 13, 5 had an RDI ¢13 and ¢ 30 and 2 were severe cases with an RDI > 30. All subjects with sleep apnea/hypopnea presented nocturnal oxyhemoglobin desaturation of variable degree and duration. The number of individuals with a high RDI was statistically comparable in the two groups, as was the overall mean oxyhemoglobin saturation during sleep.
Comparison Between Quality of Sleep and Hearing
Disturbances that affected sleep quality included excess sleep time spent in superficial stages (S1 and S2) at the expense of deep stages (S3 and S4), excessive number of awakenings, small quantity of REM sleep and excessive changes between stages. When Fisher's exact test was used to compare all individual occurrences of these disturbances with hearing loss and tinnitus, no significant correlations were found.
| Discussion|| |
Many studies have investigated the effect of environmental noise on sleep quality.
According to Kryter and Jerger and Jerger, , NIHL occurs mainly in adult males. This is supposedly due to the fact that men are more frequently exposed to excessive noise, in the workplace and during leisure activity. This gender-related difference may also be attributable to males being more sensitive to noise than are females, as stated by Neves-Pinto. 
The sensitivity of the human ear for pure tones decreases progressively with age and hearing loss in the high-frequency range occurs more rapidly, although the magnitude of the effect varies considerably among individuals. Since NIHL primarily impairs hearing at high frequencies, we studied subjects in the age range of 33 to 50 years. Individuals in this age bracket are typically not yet affected by presbyacusis, a condition with a greater hearing impairment at high frequencies, especially at 8000 Hz. However, all of the audiograms showed impairment at this frequency.
The general clinical health history of our subjects, explored in its relevant aspects, showed that none had suffered a hearing or cranial trauma that might interfere with their quality of life. The quantitative and qualitative variance in consumption of alcoholic beverages, cigarettes and recreational drugs was relevant and statistically identical in the two groups, as were complaints of nervousness and financial problems. Although smoking may hasten hearing loss, which is related to carbon monoxide and nicotine, Kwitko  reported in systematic survey, no differences in hearing loss between smokers and non-smokers.
The perception of sound in the absence of an external source, a condition known as tinnitus, is a frequent and unpleasant factor accompanying, to varying degrees, occupational hearing losses. It appears to be related to the magnitude of hearing damage rather than to the patient's age. Phoon et al  studied a group of 647 workers with noise-induced hearing loss and observed that those with tinnitus also had higher degrees of hearing loss. In our study among the hearing complaints of the subjects, tinnitus was a relevant finding in both groups.
However, among all subjects with hearing loss, only in the Noise group there was a majority (7 of 13) reporting tinnitus. Pruritus in the outer auditory meatus (10%) and irritability (20%) were also observed in the noise group.
Only 30% of the noise group and 45% of the control group had used PPE. This was justified as either resulting from their personal lack of concern regarding the risk of hearing loss or because their employer did not provide such equipment. When it is not possible to remove the source of excess noise, the use of PPE is a prophylactic measure aiming at attenuating the sound energy that reaches the cochlea.
The duration of exposure required to induce damage to the human ear varies in relation to the noise intensity. The studied subjects had been working in the same environment for 40 hours a week, for at least 8 years, with a maximum of 27 years for noise group subjects and 29 years for control group subjects. Although the intrinsic changes provoked by noise in the human auditory system are unknown, based on experimental animal studies, histologically significant lesions may be assumed to occur. Portmann et al  provoked NIHL in guinea pigs and detected responses that were similar, in terms of the intensity of the hearing loss and the frequencies affected, to those obtained in workers exposed to excess noise. The authors found histological damage to the hair cells of the organ of Corti after the second or third exposure.
The audiometric results from noise group subjects were significantly different than those from control group subjects, revealing that 65% of noise group subjects presented mild to moderate bilateral or unilateral hearing loss at high frequencies. All hearing loss was sensorineural, with similar characteristics to the standards established by the National Noise and Hearing Conservation Committee in 1993.
The results of the audiologic test battery revealed 100% type "A" tympanograms and the presence of the bilaterally contralateral stapedial reflex, showing that tympanic-ossicular mobility was within normal limits. Approximately 50% of the Noise group subjects complained of problems in communicating, even outside the workplace, what is in agreement with the observations of Fiorini et al  who stated that, in addition to causing hearing damage, exposure to excessive noise can provoke psychological and physiological reactions such as difficulty in communicating and insomnia, as well as nervousness, irritation, tinnitus and intolerance of loud sounds.
Alexandre observed that external noise during sleep can cause humans to wake,  especially when they are in superficial or REM sleep. Other authors, ,, describe noise-induced alterations including insomnia, frequent awakenings and a reduced amount of deep sleep, causing a person to wake feeling tired. Less importance has been given to the possible effect that excessive noise heard during the daytime has on night sleep.
The subjects under study did not spontaneously complain about sleep, but changes in sleepiness were detected by the Epworth Sleepiness Scale.  Among noise group subjects, 9 (45%) scored higher than 10, as did 13 (65%) of control group subjects.
Analysis of sleep studies revealed that both groups presented normal and statistically identical latency times of 5-30 minutes to NREM and 65-150 minutes to REM. Mean sleep time was 85% or more of the total recording time, although an excessive number of awakenings and stage exchanges occurred. The changes in these sleep quantity and quality indicators were identical in both groups.
Identification of respiratory sleep disorders is currently recognized as a determinant of poor sleep quality. We therefore monitored respiration during sleep. The proportions of habitual snorers were similar between the groups. A total of 13 individuals presented an apnea/hypopnea index > 5, allowing us to categorize them as subjects with sleep apnea/hypopnea syndrome, as defined by the American Sleep Disorder Association. 
Anedotical data suggest that exposure to excessive noise causes sleep disturbances that are unrelated to hearing damage and that occur even hours after exposure to the noise. Seligman  reported subject complaints of difficulty in falling asleep and of frequent awakenings.
The present study was innovative in that it demonstrated that there was no difference between the quality of night sleep of individuals who work in the presence of excessive noise levels during the day and that of those who work in the presence of acceptable noise levels. The hearing loss observed in the majority of Noise group subjects may have been responsible for this finding. In an animal study, Pedemonte et al  showed that hearing deprivation modified sleep, increasing the number of slow-wave sleep (S3 and S4) and paradoxical sleep (REM) episodes. However, such an effect has not been reported in humans. Stansfeld et al  have already pointed out the possible implications of human adaptation to excessive noise.
According to Almeida  exposure to the noise of card-perforating machines led to a large number of workers seeking treatment by neuropsychiatrists followed by a sharp decrease in these numbers after the environmental noise was reduced. At the other end of the spectrum are the individuals who reject the use of PPE when working in the presence of excessive noise levels, preferring to ignore the health risk.
The current wisdom leads us to believe that the hearing loss seen in our Noise group may represent a noise-adaptation factor, resulting in similarities between noise group subjects and control group subjects in terms of changes in night sleep patterns in response to sound stimuli encountered during daytime work.
Excessive environmental noise presents a great health concern and, from the point of view of some, the world will continue to be a hostage of environmental noise in the 21 st century.  In light of this, further studies involving larger samples are warranted in order to investigate the effects of this harmful element on the higher functions of the human organism.
We conclude that prolonged exposure to levels of environmental noise considered to be in even a little excess of safe limits, when appropriate precautions (in the form of PPE) are not taken, is enough to produce NIHL type hearing damage detectable through standard audiometric evaluation in the laboratory. No change on sleep quantity and quality was found as compared to control group. Our results lead us to believe that sensorineural deafness may represent an element of adaptation against noise during sleep.
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Geruza Alves da Silva
Division of Pulmonology, Department of General Medicine, University of São Paulo, School of Medicine at Ribeirão Preto - São Paulo
Source of Support: None, Conflict of Interest: None
[Table - 1], [Table - 2], [Table - 3]
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