Noise is a common occupational health hazard in most industrial settings. An assessment of noise and its adverse health effects based on noise intensity is inadequate. For an efficient evaluation of noise effects, frequency spectrum analysis should also be included. This paper aims to substantiate the importance of studying the contribution of noise frequencies in evaluating health effects and their association with physiological behavior within human body. Additionally, a review of studies published between 1988 and 2009 that investigate the impact of industrial/occupational noise on auditory and non-auditory effects and the probable association and contribution of noise frequency components to these effects is presented. The relevant studies in English were identified in Medknow, Medline, Wiley, Elsevier, and Springer publications. Data were extracted from the studies that fulfilled the following criteria: title and/or abstract of the given study that involved industrial/occupational noise exposure in relation to auditory and non-auditory effects or health effects. Significant data on the study characteristics, including noise frequency characteristics, for assessment were considered in the study. It is demonstrated that only a few studies have considered the frequency contributions in their investigations to study auditory effects and not non-auditory effects. The data suggest that significant adverse health effects due to industrial noise include auditory and heart-related problems. The study provides a strong evidence for the claims that noise with a major frequency characteristic of around 4 kHz has auditory effects and being deficient in data fails to show any influence of noise frequency components on non-auditory effects. Furthermore, specific noise levels and frequencies predicting the corresponding health impacts have not yet been validated. There is a need for advance research to clarify the importance of the dominant noise frequency contribution in evaluating health effects.
Keywords: Frequency spectrum, octave band center frequency, physiological effects, industrial noise, resonant frequency, adverse health effects, pathophysiological effects
|How to cite this article:|
Mahendra Prashanth K V, Venugopalachar S. The possible influence of noise frequency components on the health of exposed industrial workers - A review. Noise Health 2011;13:16-25
|How to cite this URL:|
Mahendra Prashanth K V, Venugopalachar S. The possible influence of noise frequency components on the health of exposed industrial workers - A review. Noise Health [serial online] 2011 [cited 2013 Jun 19];13:16-25. Available from: http://www.noiseandhealth.org/text.asp?2011/13/50/16/73996
| Introduction|| |
Sound is very essential for all human activities, e.g. communication and day-to-day functioning in any environment. Continuous exposure to the unwanted sound called noise is reported to cause many adverse effects, both physiological and psychological, and it is a very serious problem among people working in an industrial environment. According to WHO 1994, "Adverse effect of noise is a change in morphology and physiology of an organism which results in impairment of functional capacity or impairment of capacity to compensate for additional stress or increase in susceptibility to the harmful effects of other environmental influences." This definition includes any temporary or long-term reduction of physical, psychological, or social functioning of humans or human organs. 
Noise from machinery of industrial plants significantly impacts the working community. In most of the industries, dominant noise frequencies from the machinery are observed to be in the low (>22-500 Hz) and mid (>500-2 kHz) octave band center frequency (OBF) characteristics.  It is apparent that different organs of the human body are associated with different resonance frequencies, and they resonate considerably at low and infrasound frequencies. As stated in the report,  the chest wall can resonate at frequencies of about 50-100 Hz and the head at 20-30 Hz. In an occupational environment, resonance of the thorax was noticed in the 63-160 Hz frequency range.  It is evident that interference of noise is a serious health issue whose effects are widespread and not just restricted to auditory effects. Due to acoustic interference, physiological changes take place in the body and these changes are related to the frequency of the sound and its intensity level.
Apart from loudness, other characteristics of noise, such as the frequency, duration of exposure, frequency of interruption, and duration of the interruption, become relevant in evaluating the effects of noise. It is observed that frequency-related characteristics of noise, for instance intermittent, irregular, tonal, pulse, etc. generate more annoyance than steady noise of the same intensity. Evaluation of exposed noise and its adverse health effects based on intensity is inadequate, because pathophysiological effects have also been observed at low levels, which impairs auditory functions. Accordingly, inclusion of noise frequency spectrum analysis will yield more insights from evaluation. One of the review studies  observes that, with very few exceptions, environmental noise assessments rarely include a frequency spectra analysis. Furthermore, the study indicates that from the past decades, scientific investigation into the extra-aural, whole-body, noise-induced pathology issue has been infrequent, and existing data are often regarded as inconclusive.
This paper focuses on reviewing studies published between 1988 and 2009 on the effects of industrial noise, investigating auditory and non-auditory effects, and the probable association and contribution of noise frequency components to these effects.
The first section describes the materials and methods used for locating, selecting, extracting, and synthesizing data. In the second section, the paper aims to substantiate the understanding of the importance of studying the contribution of frequencies of noise (frequency spectrum analysis) in evaluating health effects. In the third section, an attempt is made to find out the association of frequencies of noise with physiological changes in various human organs. The fourth section ("Discussion" section ) presents a summary of studies determining whether noise frequency component contributions have been considered in evaluating health effects along with intensity levels and other parameters. This section is followed by conclusions and scope for future work.
| Methods|| |
Examination of studies involving the association between industrial noise exposure and auditory, non-auditory effects, published between 1988 and 2009 in English, were identified in Medknow, Medline, Wiley, Elsevier, and Springer. Journal papers and reports were searched using citation database SCOPUS and scanned manually for industrial/occupational noise and health effects. Data were extracted from the studies that fulfilled the following criteria: title and/or abstract of the given study that had included industrial noise/occupational noise exposure in relation to auditory and non-auditory effects or health effects. Data on the study characteristics such as authors, year of publication, noise exposure level, noise characteristics, number of respondents, type of industry, noise frequency characteristics, auditory and non-auditory effects, and factors ascertained for assessment were considered in the study.
Frequency spectrum analysis in evaluating health effects
Different organ systems of the human body are designed to respond, locate, and analyze different characteristics, levels, and frequencies of noise. It is observed that the auditory system analyses sound by frequency. Since almost all sounds are structured by frequency, their characteristics can be detected by spectrum analysis. Loud sounds can cause an arousal response in which a series of reactions occur in the body. The human autonomic nervous system attempts to adapt the body functions to physiological stress like noise. However, if the stress due to noise exposure exceeds tolerable limits, different functions of the body may get disturbed and this may lead to severe health effects. As stated by Suter,  noise influences perceptual, motor, and cognitive behavior, and also triggers glandular, cardiovascular, and gastrointestinal changes, by means of the autonomic nervous system.
Emmerich et al.  state that exposure to industrial noise can cause morphological changes in the middle turns of the cochlea and electrophysiological changes in the middle frequency range. Continuous exposure to industrial noises damages the outer hair cells. Due to various direct and indirect pathways that exist between the inner ear and brain centers, control of physiological, emotional, and behavioral responses of the body takes place. An acute reaction to noise causes a central nervous system (CNS) -mediated stress reaction in the body by upsetting the normal equilibrium of several physiological functions. These reactions manifest for a short period, and for some, habituation can occur depending on exposure frequency and intensity.  The psychophysical studies ,, and paper  points out that the spatial position of a sound source helps: (i) the capacity to perceive the spatial location of a sound and to indicate its location and (ii) segregation of concurrent acoustic signals belonging to different sound objects. The position of a sound source is inferred from differences in interaural time (or phase) and interaural intensity. The former differences were shown to play a role while facing low frequencies and the latter when facing high frequencies.
The findings of Shtyrov et al.  suggest that continuous noise exposure facilitates distraction by additional auditory events outside the current focus of attention and they also demonstrate that negative long-term effects on cerebral processing of speech sounds and auditory distractibility also increase. Another study by Kujala  on continuous effects of occupational noise on central sound discrimination ability, distractibility evaluated by measuring event-related brain potentials (ERP) and task performance explains that long-term noise may cause prolonged disturbances at different levels of central auditory processing.
The concept of critical bandwidths within the cochlea is very significant. As per study by Fletcher  they help in understanding auditory behavior with respect to speech perception, auditory fatigue, loudness, pitch perception, and masking. Their bandwidths change as a function of frequency and they become broad when the signal intensity is increased signifying the importance of frequency association.
Emmerich et al.  show that exposure to industrial noise produces frequency-specific changes in distortion product otoacoustic emission (DPOAE) and they are reflected in morphological changes of the cochlear hair cells. It was observed that different kinds of industrial noises tend to produce typically different changes in DPOAE levels, indicating specific damage to outer hair cells in animals (e.g., guinea pig). Though, these data are partially related to human subjects, adopting these animal data completely in studies on hearing hazards in man is difficult because factors like recreational noise, etc. may produce additional reductions in similar frequency ranges. Another study by Maschke et al.  demonstrated that the psychophysiological stressor "noise" can permanently influence stress hormones. The endocrine reaction is affected by many factors, e.g., individual adaptation, individual resistance, possibility of influencing the noise, and also gender.
In the light of the above, it is evident that contribution of frequencies of noise is also important in evaluating adverse health effects; further auditory damage risk and physiological system response of human system are related to noise frequency. Therefore, frequency spectrum analysis in evaluating health effects due to noise exposure becomes essential.
It is observed that the central auditory system of human body is built on frequency-specific processing channels (tonotopic organization) and thus assessing and characterizing an acoustic environment requires both the dB level and the frequency distribution considerations.  This was also supported by Pereira, Branco in the study,  which states that assessment of noise effects should consist data of both intensity and frequency spectrum analysis because the frequency range to which whole-body organ systems respond is not the same as that for the auditory system. In other words, different organ systems are susceptible to different acoustic frequencies.
Physiological response to noise and its associated frequencies
Different physiological responses have been identified as the impacts of noise such as a circulatory response dominated by vasoconstriction of peripheral blood vessels affecting blood pressure, heart rate and blood pressure variations, a reduced rate of breathing, galvanic skin response, a reduction in the electrical resistance of the skin, a brief change in the skeletal-muscle tension, hormonal changes, etc.  Most of these physiological behaviors within the body are observed to be associated with different frequency characteristics.
The actual pressure transformation in the human ear depends on the frequency of the acoustic stimulus.  The pressure increase between eardrum and inner ear is ≥30 dB in the region of 2.5 kHz. However, the ratio was observed to decrease at frequencies >2.5 kHz. Hearing impairment depends on the characteristics of the noise one is exposed to, frequency, in particular. An increase in auditory threshold due to noise exposure, temporary threshold shift (TTS) is different for different frequencies, even though noise level is the same. In general, a more TTS is produced at higher frequencies at least up to 3-6 kHz. Thus, when the exposure is to a broadband noise, a maximum TTS is found at frequencies between 3 and 6 kHz. When the exposure is to a pure tone, the TTS is found to increase as frequency of the exposure tone increases. 
Laboratory studies, conducted by Landstrom et.al as reported,  evaluated the physiological effects of low-frequency sound with reference to sleep reveals the reduction in wakefulness occurred during a repeating 42 Hz signal at 70 dB, while an increase in wakefulness occurred for a repeating frequency of 1 kHz signal at 30 dB.
Some studies have attempted to describe physiological and psychological effects of noise. These studies discuss the probable symptoms on exposure to various low-frequency noise characteristics. The study  elucidates that low-frequency noise (0-500 Hz) acoustic phenomena can affect several organs and tissues, but the effect depends on the frequency of the acoustic event because every organ and tissue has its own acoustic properties.
Experimental studies ,,, demonstrate that biological tissue is very sensitive to lower frequencies, below 100 Hz. Specific frequencies have deleterious impacts on specific biological tissues. Hanlon and Errede  observe mild nausea, giddiness, and respiration-related effects for the frequency range of 50-100 Hz at 150 dB.
Furthermore, Vassilatos and Gavreau  identified symptoms like the lack of visual acuity, fall of IQ (Intelligent Quotient) scores below normal, distortion of spatial orientation, poor muscular coordination, loss of equilibrium, and slurred speech in the frequency range of 43-73 Hz. Further, symptoms like coughing, severe sternal pressure, choking, excessive salivation, extreme swallowing pains, inability to breathe, headache, and abdominal pain were found to occur in the frequency range of 60-73 Hz. Additionally, at 100 Hz frequency, irritation, mild nausea, giddiness, skin flushing, and body tingling were observed.
The above-stated observations reveal the association of frequencies of noise related to physiological changes in various human organs. However, specific noise levels and frequencies predicting corresponding health impacts have not yet been validated.
| Discussion|| |
[Table 1] (for graphical representation see [Figure 1]) provides summary of the reviewed studies on industrial noise effects. Data on the study characteristics as mentioned in the materials and methods have been considered in the study.
|Table 1: Summary of the reviewed studies on industrial/occupational noise effects|
Click here to view
|Figure 1: Graphical details of Table 1, depicting references, noise exposure levels, number of subjects (samples) and frequency contributions (in brackets) in their studies. (Note: 7901* = 790.1 × 10; 47388** = 473.88 × 100)|
Click here to view
Thiery and Meyer  discuss the risk of hearing loss due to exposure to 87-90 dB (A) noise in workshop. Those who suffered hearing loss were found to have a threshold of 25 dB in the 1-3 kHz frequency range. Examination of the eardrums and measurements of arterial pressure and heart rate at rest were performed before audiometric testing. In the study by Bauer et al.  combined actions of different parameters such as personal, noise related and medical data of sampled workers on hearing ability are determined. The results reveal that hearing threshold at any frequency is dominated by age factor. The relative effects of other factors such as sex, noise imission level (NIL), ear diseases, tinnitus and hearing protector usage are related to audiometric frequency. The NIL affected the hearing threshold at 4 k Hz.
The results of Zhao et al.  indicate that exposure to industrial noise is an important determinant of risk of hypertension in working populations. Furthermore, the logistic model suggested by them estimates that a 30 dB (A) increase in SPL approximately doubles the risk of hypertension, independent of the other risk factors measured. An occupational study by Lang et al.  suggests that individuals exposed to continuous noise levels >85 dB have high blood pressure.
The results from Jovanovic et al.  indicate that communal and industrial noises are possible contributing factors to the development of arterial hypertension, myocardial infarction, angina pectoris non-stabilis and vasospastic, coronary risk factors. In their studies, exposed and control groups are similar except for exposure to noise. Significantly high values of plasma low-density lipoprotein cholesterol and triglycerides were also observed in the noise-exposed group.
The study by Celik et al.  shows that subjects exposed to high levels of noise (95-110 dB (A)) suffer symmetric hearing losses. Although these were detected at a noise frequency range 2-8 kHz, they were prominent at 4 and 6 kHz. It is also observed from the study that hearing loss (with an average of 40 dB H L) develops within the first 10 years of exposure to noise and it progresses in the following years. In the audiometric tests among the study group, 4 kHz dip was observed in the audiograms. The authors of that works conclude that the dip might reflect differences in individual susceptibilities of the active intra-cochlear mechanism to noise.
Results of Soleo et al.  reveal a positive influence of age, body mass index (BMI), and use of antihypertensive drugs for both systolic blood pressure (SBP) and diastolic blood pressure (DBP). However noise exposure has not influenced SBP but have a negative influence on DBP. From the study by Ahmed et al.  significant hearing loss (bilateral and symmetrical) was observed in noise-exposed subjects with a characteristic dip at 4 kHz. Around 38% of the exposed subjects had hearing impairment. In this study, on a group basis, maximum hearing loss (dip) was localized at 4 kHz, followed by recovery at 8 kHz
Strength of the study of Melamed et al.  is in its longitudinal design focusing on workers having a stable working environment and following them up for 2-4 years. Complexity of jobs performed was also considered while studying the effects of chronic noise exposure. The findings of this study depict that exposure to noise has a stronger effect on blood pressure over time among those performing complex jobs. Eleftheriou  observes that the high-frequency threshold shifts among workers < 35 years old. However, a quarter of the surveyed population experienced a noise-induced permanent threshold shift (NIPTS) greater than 30 dB.
Powazka et al.  examine that exposure to noise affects SBP. Their hypothesis was that the effect of noise exposure on blood pressure becomes apparent within 5 years of exposure. The major drawback of this study is that blood pressure measurements were made on a single occasion. Another study by Boatang and Amedofu  measured and studied noise levels in saw mills, corn mills, and printing houses. The prevalence of high frequency sensorineural hearing loss was significantly higher among noise-exposed workers than those in the control group. Positive relationships were found between the two independent variables (age and duration of exposure) and the threshold was at 4 kHz for workers in saw mills and corn mills.
The study of Sueyoshi et al.  evaluates the effects of floor impact sound in a wooden house, using the physiological index. A temporary rise in SBP was observed due to prolonged light floor impact sound of 60 and 80 dB(A), generated by the tapping machine. Further, sensory evaluation was conducted using the semantic differential (SD) method. Results of Mahamood et al.  reveal that both SBP and DBP were significantly raised on exposure to noise for 10 min. However, they do not discuss the characteristics of the noise.
Atmaca et al.  studied the effects of noise on industrial workers in various industries, with respect to the level of noise they are exposed to. This is to determine the physical, physiological, and psychosocial impacts of the noise based on questionnaire analysis. Annoyance, nervousness, and hearing impairment symptom were significantly noticed among sampled workers. The results of the study by Uday et al.  reveal that hypertension and hearing impairment were common effects associated with duration of exposure among continuously noise exposed workers to high levels of occupational noise (98-110 dB [A]). Furthermore, it was found that hypertension was more common in workers with hearing impairment.
Results  of octave band analysis of noise in work areas revealed high levels of sound (>90 dB (A) for 8 h, 6 days/week) at 4 kHz. This work demonstrates that workers have a high risk of developing noise-induced hearing loss and other associated hearing impairment due to this excessive noise exposure. The study of Mokhthar et al.  is based on questionnaire analysis and they found that noise levels significantly affect physiology, hearing capability, auditory communication, and sleep of industrial workers. However, details about the noise spectrum and the type of questions used to predict physiological and psychological effects are unavailable.
Chen et al. studied on short-term, intense noise exposure during heavy work load in hot environment. They show that more TTS (at >95 dB (A)) is induced. Further they conclude that heat stress and heavy work load are the potential factors of increasing TTS due to exposure to noise.  The study of Patel and Ingle  was aimed to determine the prevalence of hearing loss among pulse processing workers, based on hearing threshold levels (HTLs) with a low fence of 25 dB. The binaural hearing impairment observed at high frequencies among these workers appears to be due to the higher noise exposure.
According to Prasanna Kumar and Dewangan,  around 63% of sampled workers stated that noise interferes with their conversation for level > 80 dB(A) having frequency range 300-700 Hz. Surprisingly, physiological effects were found to be non-significant even though the exposed noise was dominated by low-frequency characteristics.
The study  identified the major noise frequency components and sound pressure levels (SPLs) exposed by workers in different industrial sectors. The study results showed that the sampled industrial workers were repeatedly exposed to noise of low (> 22-500 Hz) and mid (>500-2 kHz) OBFs. It is found that symptoms such as "frequent ear vibration," "chronic fatigue," "repeated headache," "awakening from sleep," "pains in neck," "backache," "eye ball pressure," and "repeated ear pulsation" are observed to be highly associated with low- and mid-OBF noise exposure among the sampled workers. Further, around 36% of the sampled workers were found to be suffering from hearing problems. It was noted that 125, 250, and 500 Hz OBFs (88-707 Hz) were the noise frequency components that prominently affected hearing among these subjects. (A frequency band is said to be an octave in width when its upper band-edge frequency (f2 ) is twice the lower band-edge frequency (f1 ): f2 = 2f1 . Each octave band is named for the center frequency of the band, calculated as follows: fc = (f1 f2 ) 1/2 , where fc = center frequency).The possibility of the occurrence of cardiovascular disease linked to noise exposure revealed that around 29% of sampled workers, who had cardiovascular problems, were found to be associated with a noise frequency of 500 Hz OBFs (354-707 Hz).
Lindgren et al.  studied the effects of noise on cabin crew, who are exposed to equivalent noise levels from gate-to-gate between 78 and 84 dB (A) by means of higher peak exposures. Though the hearing loss initially appears as a threshold shift in the frequency region 3-6 kHz, no significant relationship was observed between hearing loss and years of employment.
A cross-sectional study by Attarchi et al.  evaluated the simultaneous effect of smoking and exposure to noise on hearing level in an automobile manufacturing company. There were no other known occupational hazards affecting hearing, except for noise. All workers' bilateral pure-tone hearing thresholds were measured at 500, 1,000, 2,000, 3,000, 4,000 and 6,000 Hz for air and bone conduction applying an acoustic chamber meeting the ANSI S3.1-1991 standards with a diagnostic audiometer (model AD 229b, Inter acoustic Denmark Co. Ltd). The frequency of hearing loss in smokers was observed to be higher than in non-smokers. It was concluded in the study that smoking may accelerate noise-induced hearing loss.
The study by Lee et al.  tries to investigate how chronic noise exposure affects BP. The subjects were divided into four groups depending on their working environment and noise level category. No significant difference in DBP was observed in comparison with the control group; however, DBP showed a tendency to increase as the noise level increased. It was also revealed in the study that noise exposure is a predictive independent variable for SBP and as the noise exposure level increases BP tends to increase as well. However, an intermittent high level of noise to which workers older than 50 are exposed showed a close relationship with the incidence of hypertension.
It is demonstrated that not many studies elucidate the influence of noise frequency components on health effects (auditory and non-auditory) of industrial workers. Schust  also states that with a very few exceptions noise assessments rarely include frequency spectra analysis. Furthermore, in the past few decades, scientific investigations of extra-aural, whole-body, noise-induced pathology are a very few, and existing data are often inconclusive. He confirms that no scientific evidence is available for an association between frequency-weighted SPL and biological effects.
| Conclusions and Scope for Future Work|| |
It is evident from the study that contribution of noise frequencies is one of the important factors to be considered in evaluating adverse health effects due to exposure to noise; auditory damage risk and the physiological response of the human system are also related to noise frequency. The data suggest that significant adverse health effects due to industrial noise included auditory and heart-related problems. In most of the studies reviewed, the association of sound pressure level in dB with these effects has been identified but without any consideration of noise frequency contributions. Even in the few studies in which noise frequency is considered to study its adverse health effects, only auditory effects have been evaluated and not non-auditory effects. This study provides strong evidence for the claims that noise with a major frequency characteristic of around 4 kHz has auditory effects; and the deficiency in data fails to show any influence of noise frequency components on non-auditory effects. Furthermore, specific noise levels and frequencies predicting the corresponding health impacts have not yet been validated.
In view of the fact that physiological changes within the body take place when acoustic interference occurs and as these are mainly related to the frequency and intensity of sound, an extensive assessment of the perceived threat of different dominant noise frequencies should be undertaken. The influence of direct exposure of various noise frequency components on the whole-body may also be studied to get more insight onto the topic. Emphasis should also be given in the studies to consider the influence of other stressors together with medical monitoring to obtain good results. A comparative study of the effects of noise exposure on normal and auditory-disabled persons may be accomplished to obtain more support in understanding physiological effects. There is a need for advance research to clarify the importance of noise frequency contribution to evaluate health effects.
| Acknowledgement|| |
The authors of this article would like to thank Dr. K. M. Shivakumar, Head, Department of General Medicine, Mandya Institute of Medical Sciences, Mandya, Karnataka State, India, for his valuable discussions and suggestions.
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K V Mahendra Prashanth
Department of Electronics Engineering, Vivekananda Institute of Technology, Gudimavu, Kengeri Hobli, Bangalore - 560 074, Karnataka