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Manual Army Manual - Silencers: Principles and Evaluations

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Notably, the reduction of the oncosuppressor properties in the SCs, resulting in an altered differentiation program, may be, at least in theory, pathologically relevant for the schwannoma transformation. The identification of such altered mechanisms opens new questions on the exposure of SCs to the EMFs, although the fine identification of the intracellular signaling involved deserves further investigation.

By this way, SCs alter their differentiation program, becoming, in principle, pathologically relevant for schwannoma development. SC cultures were obtained by the method of Brockes 26 with minor modifications. Immunopanning for final purification was carried out with mouse anti-rat -Thy1. SaOS-2 is a mature osteoblastic cell line derived from a human osteosarcoma. SCs used as control were plated in same culture conditions, without EMF exposure. SaOS-2 cell proliferation was evaluated using the automated Luna cell counter as above and by the MTT 3- 4,5-dimethylthiazolyl -2,5-diphenyltetrazolium bromide assay.

Each experimental point was in quadruplicate and experiments replicated at least three times. Three random objective fields were counted for each well and the mean number of migrating cells was calculated. Controls for the specificity of antibodies included a lack of primary antibodies. Data analysis was performed using the CFX Manager 2.

The threshold cycle number Ct values of both the calibrator and the samples of interest were normalized to the Ct of the endogenous housekeeping genes. As calibrator we used the RNA obtained from control samples. Thermal cycling and fluorescence detection were performed using an ABI Prism Sequence Detection System Applied Biosystems, Monza, Italy , then the expression of Hippo signaling regulated transcripts was compared between groups.

Data were statistically evaluated by GraphPad Prism 4. Carroll SL. Molecular mechanisms promoting the pathogenesis of Schwann cell neoplasms. Acta Neuropathol ; : — Evans DG. Neurofibromatosis type 2 NF2 : a clinical and molecular review. Orphanet J Rare Dis ; 4 : Neurofibromatosis--review of the literature and case report.

Acta Dermatovenerol Croat ; 14 : — Rates of loss of heterozygosity and mitotic recombination in NF2 schwannomas, sporadic vestibular schwannomas and schwannomatosis schwannomas. Oncogene ; 29 : — Hergovich A , Hemmings BA. Biofactors ; 35 : — Zhou L , Hanemann CO. Merlin, a multi-suppressor from cell membrane to the nucleus. FEBS Lett ; : — The Hippo-YAP pathway in organ size control and tumorigenesis: an updated version. Genes Dev ; 24 : — Trends Mol Med ; 21 : — Dev Cell ; 19 : 27— Effects of electromagnetic fields on cells: physiological and therapeutical approaches and molecular mechanisms of interaction.

A review. Cells Tissues Organs ; : 59— Cell phone radiation exposure on brain and associated biological systems. Indian J Exp Biol ; 51 : — Extracellular electrical fields direct wound healing and regeneration. Biol Bull ; : 79— Int J Radiat Biol ; 90 : — Carpenter DO. Human disease resulting from exposure to electromagnetic fields. Rev Environ Health ; 28 : — Pooled analysis of two case-control studies on the use of cellular and cordless telephones and the risk of benign brain tumours diagnosed during Int J Oncol ; 28 : — Use of mobile phones and cordless phones is associated with increased risk for glioma and acoustic neuroma.

Pathophysiology ; 20 : 85— Association between vestibular schwannomas and mobile phone use. Tumour Biol ; 35 : — Cancer Epidemiol ; 35 : — Stimulation of rat sciatic nerve regeneration with pulsed electromagnetic fields. Brain Res ; : — Pretreatment of rats with pulsed electromagnetic fields enhances regeneration of the sciatic nerve. Bioelectromagnetics ; 14 : — Electromagnetic field stimulation potentiates endogenous myelin repair by recruiting subventricular neural stem cells in an experimental model of white matter demyelination.

J Mol Neurosci ; 48 : — Zidovetzki R , Levitan I. Use of cyclodextrins to manipulate plasma membrane cholesterol content: evidence, misconceptions and control strategies. Biochim Biophys Acta ; : — Mol Cell Neurosci ; 47 : 1—9. Lysophosphatidic acid promotes survival and differentiation of rat Schwann cells. J Biol Chem ; : — S7, S8 ; Supplemental Table S1. PosCORs were thus used as controls in further analysis. TF occupancy and histone modifications along DNA sequences constitute two layers of transcriptional regulation machinery.

The binding sites of well-known ubiquitous repressors were enriched in silencers across most cell lines, as highlighted by the blue bar in Figure 2 D. S10, S Other well-known tissue-specific activators also displayed notable binding depletion in the silencers, suggesting that negCORs perform a function potentially opposite to activators.

The TFBS analysis of silencers identifies putative regulatory mechanisms at the foundation of silencer activity across different cell lines and provides directions for follow-up biochemical analyses of these sequences. To expand the reach of our silencer detection framework, we decided to capitalize on specific TFBS patterns of silencers Fig.

The classification performance of our SVM models varied across different cell lines, with the area under the curve AUC ranging from 0. SVM classification performance. To evaluate SVM silencers, we studied their transcriptional impact. Distal SVM genes provided an independent validation of SVM silencers as the expression of these genes has no contribution to silencer prediction. These findings, reminiscent of those of negCOR silencers, confirm the reliability of the newly identified silencers, which significantly expand the landscape of silencer elements.

In support of our model's ability to predict silencers, a large fraction of the predictions features the classical polycomb signature of repressive activity. In addition, histone modifications H3K9me3 and H4K20me1 are well-known to be associated with the repression of gene regulation Mozzetta et al. S13 , further advocating for the repressive function of SVM silencers. S13 , suggesting that the well-known repressive histone marks including H3K27me3, H3K9me3, and H4K20me1 are incapable of accurately identifying silencers on their own due to a complicated interaction among transcriptional regulators including histone modifications and TFs.

Additional analysis such as the proposed model is necessary to distinguish potential silencers from general DHSs carrying repressive histone marks. Thus, negCOR silencers represent an essential step toward building a larger map of silencers in the human genome. All 14 tested sequences S1…S10 and H1…H4 showed a significant decrease in the reporter gene activity Fig. We observed a decrease in the reporter gene activity for the H1…H4 sequences, which were not classified as silencers.

Although this decrease is significant as compared to the reporter gene activity driven by the naked enhancer, it is at the level of other previously reported controls Petrykowska et al. While our method is not rejecting a possible silencer function of these sequences and some of them might be acting as silencers, we used H1…H4 sequences as a strict control while screening for predictions, which would result in reporter gene activity lower than the activity associated with H1…H4 sequences.

As a group, the S1…S10 predicted silencers drove the reporter gene expression 3. Predicted silencer activity in K cells. The gray rectangle indicates the luciferase activity range of H1…H4. Also, the results of massively parallel report assays MPRAs , in which the impact of REs was measured using the expression of reporter genes Melnikov et al. Also, the predicted silencers display a stronger decrease in the level of gene expression than the H3K27me3-DHSs regions not predicted as silencers Supplemental Fig.

This independent test further supports the predicted silencers having a repressive function on the activity of nearby genes. To counterbalance the sparsity of chromatin interaction data, we assumed that H3K27me3-DHSs act on their neighboring genes, rather than on distal genes, as a part of our model. This assumption resulted in an underprediction of silencers and can partially explain why some of the H1…H4 sequences might have been acting as silencers and thus resulting in a lower reporter gene expression Fig.

Those H3K27me3-DHSs that only target distal genes would not be picked up by our model, while using additional information such as chromatin interaction data across cell lines in future studies could reduce the number of false negative predictions and result in a larger set of predicted silencers. We next examined the genomic distribution of silencers. Following the widely used proximity rule, we assigned silencers and enhancers marked by the DHSs overlapping both H3K27ac and H3K4me1 to their most proximal genes see Methods.

This suggests that silencers determine the identity of a cell through suppressing genes highly expressed in other cell lines. Also, we examined the tissue specificity of the genes targeted by enhancers and silencers via Hi-C loops Rao et al. As compared with enhancers, silencers more frequently link to tissue-specific genes All these observations advocate for the essential role of silencers in establishing a unique transcriptional profile of a cell.

Distribution of silencers. C Clustering of silencers. Background clustering was established using randomly selected H3K27me3-DHSs peaks matching the number of silencers. D Association of silencers and silencer clusters with tissue-specific genes. E Enrichment of clustered silencers in the loci of superenhancers SEs. Enhancers were used as a background reference. Since regulatory cluster formation is well-known for enhancers Parker et al.

Silencer cluster formation might be beneficial for functional backup or battering Spitz and Furlong This cluster formation might also provide evolutionary stability to the repression of gene expression in a specific cell where the activity of particular genes might be detrimental to the fitness of the species. This effect greatly increased only when clustered silencers were considered This trend is significant in almost all differentiated tissues. In HepG2, for instance, The enrichment of silencer clusters in the loci of tissue-specific genes is lower in the case of stem and progenitor cells.

To address a possible bias caused by the locus length discrepancy on average, the loci of tissue-specific genes are longer than those of housekeeping genes Supplemental Fig. S16 , we compared silencers with the background, which was generated through randomly selecting H3K27me3-DHSs matching the corresponding silencers, and consistently observed a significant association of silencers from silencer clusters with tissue-specific genes, especially in differentiated cells Supplemental Fig.

Superenhancers, i. The strong preference of clustered silencers toward tissue-specific genes, as reported above Fig. We observed a depletion of silencers in the loci hosting active superenhancers. At first, this antagonistic presence of clustered silencers and superenhancers appears counterintuitive given the individual enrichment of each of these two groups of REs in the loci of tissue-specific genes.

This suggests a regulatory model for the genes controlled by superenhancers, which requires strong transcriptional deactivation in tissues or cell types where superenhancers are absent. We can hypothesize that this deactivation might be taking place by silencer clusters acting directly on superenhancers to deactivate key gene regulators instead of acting directly on promoters of these genes, as a direct competition with superenhancers might not be feasible.

However, we would like to note that either a validation or rejection of this hypothesis would have to be performed by a follow-up biochemical study. Silencers in TF gene loci. Gene loci are stratified into three groups: silencer-rich loci; enhancer-rich loci; and silencer-enhancer loci see Methods. Green asterisks depict significant differences between silencer-enhancer loci and silencer-rich loci, while red asterisks indicate significant differences of TF fraction among silencer-enhancer loci across gene groups.

To gain further insight into the cooperation between silencers and enhancers, we focused on gene loci hosting a mixture of enhancers and silencers named silencer-enhancer loci see Methods. As gene expression increases, the fraction of silencer-enhancer loci among all the loci hosting at least one silencer or enhancer decreases, while the enhancer-rich loci gain prominence Fig. Nevertheless, there is a notable fraction of highly expressed genes that require silencers for their proper regulation. Highly expressed silencer-enhancer genes were significantly enriched for TFs, including canonical TFs and transcriptional cofactors Schmeier et al.

Collectively, highly expressed TFs, which are the backbone of the transcriptional regulation system Vaquerizas et al. The opposite effect of enhancers and silencers enables a fine-tuning of these TFs and provides great sensitivity and robustness to the entire regulatory circuitry Daniel et al. Finally, we explored Hi-C connections to examine silencer-enhancer genes, although Hi-C connections are available only for a part of the enhancers and silencers. S18 , which is in line with the results presented in Figure 6 B. We superimposed silencers onto the SNPs associated with phenotypic effects.

Approximately 6. The enrichment of GTEx and GWAS SNPs in silencers, which is comparable to that of enhancers, suggests a role of silencers in human diseases and disorders that should not be overshadowed by enhancer studies. Histone modifications have been an extremely powerful approach for investigating the cell-type—specific regulatory landscape of the human and other genomes. Hundreds of thousands of tissue-specific enhancers have been successfully identified Roadmap Epigenomics Consortium et al.

However, it appears to be much more difficult to predict silencers. One of the challenges is the dual nature of the repressive H3K27me3 mark i. To address this issue, we correlated H3K27me3 activity with gene expression across a set of distinct tissues to identify negCORs, i. NegCOR silencer sequences are enriched for the binding sites of repressors across different cell lines and therefore were exploited to build SVM models for identifying additional silencers. With these models, we expanded the silencer landscape of the human genome from negCOR silencers to SVM silencers per cell line.

Their enrichment levels are comparable to enhancers, highlighting the biological importance of silencers. The regulatory model suggesting an active interplay of different classes of REs is one of the dominant topics of the postsequencing era Kolovos et al. We demonstrated that silencers form clusters of activity and intimately collaborate with enhancers and superenhancers to accurately orchestrate the transcription of genes, especially tissue-specific genes and TF genes.

Our results show that the former model mainly governs tissue-specific genes and is important for setting up a distinct identity of a cell. The latter greatly contributes to the regulation of highly expressed TFs and is essential for a stable and sensitive regulatory system.

Understanding the molecular mechanisms by which silencers function remains an outstanding topic in the field of transcriptional regulation Blackledge et al. Establishing reliable genome-wide silencer maps is the crucial step in promoting these studies. Through exploring DNA sequence signatures, epigenetic as well as transcriptional profiles, we identified a multi-tissue map of silencers in the human genome with improved accuracy and demonstrated that both TF occupancy and epigenetic modification contribute to the activation of silencers.

Using the DNase-seq peaks overlapping with H3K27me3 ChIP-seq peaks defined as H3K27me3-DHSs as input across different human cell lines, our method is composed of two sequential modules: a correlation-based module and a classification-based module Supplemental Fig. In the first module, we evaluated the correlation between the activity of H3K27me3-DHSs and the expression of their proximal genes by focusing on the H3K27me3-DHSs active in at least three tissues.

A low absolute value of corr G , h indicates a strong association between G and h. Similarly, an H3K27me3-DHS was regarded as significantly positive correlated posCOR only when at least one of its corr G , h values is significantly positive and none of its corr G , h values is significantly negative. In the second module, i. Z, and H3K36me3 , as well as the maximum and minimum of the expressions of proximal genes.

The values of each feature were linearly normalized to be in the range of [0, 1]. To test classification performance, a fivefold cross-validation scheme was used. In this scheme, a training data set was equally divided into five subsets. After using every subset for validating the SVM built on the other four subsets, we obtained validation results on all training samples and evaluated these results in terms of false positive rate, precision, and recall.

H3K27me3-DHSs having scores greater than a threshold were then marked as potential silencers. The threshold was determined in such a way that the false positive rate on the validation results was 0. As a principle of physics, the sound pressure level decreases 6 dB, on a Z-weighted i. This is a common way of expressing the inverse-square law in acoustics and is shown in Figure 4. If a point source in a free field produces a sound pressure level of 90 dB at a distance of 1 meter, the sound pressure level is 84 dB at 2 meters, 78 dB at 4 meters, and so forth.

This principle holds true regardless of the units used to measure distance. Free field conditions are necessary for certain tests, where outdoor measurements are often impractical. Some tests need to be performed in special rooms called free field or anechoic echo-free chambers, which have sound-absorbing walls, floors, and ceilings that reflect practically no sound. In spaces defined by walls, however, sound fields are more complex.

When sound-reflecting objects such as walls or machinery are introduced into the sound field, the wave picture changes completely. Sound reverberates, reflecting back into the room rather than continuing to spread away from the source. Most industrial operations and many construction tasks occur under these conditions. Figure 5 diagrams sound radiating from a sound source and shows how reflected sound dashed lines complicates the situation.

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The net result is a change in the intensity of the sound. The sound pressure does not decrease as rapidly as it would in a free field. In other words, it decreases by less than 6 dB each time the distance from the sound source doubles. Far from the noise source--unless the boundaries are very absorbing--the reflected sound dominates. This region is called the reverberant field. If the sound pressure levels in a reverberant field are uniform throughout the room, and the sound waves travel in all directions with equal probability, the sound is said to be diffuse.

In actual practice, however, perfectly free fields and reverberant fields rarely exist--most sound fields are something in between. Up to this point, this discussion has focused on sound pressure. Sound power, however, is an equally important concept. Sound power, usually measured in watts, is the amount of energy per unit of time that radiates from a source in the form of an acoustic wave.

Generally, sound power cannot be measured directly, but modern instruments make it possible to measure the output at a point that is a known distance from the source. Understanding the relationship between sound pressure and sound power is essential to predicting what noise problems will be created when particular sound sources are placed in working environments. An important consideration might be how close workers will be working to the source of sound. As a general rule, doubling the sound power increases the noise level by 3 dB. As sound power radiates from a point source in free space, it is distributed over a spherical surface so that at any given point, there exists a certain sound power per unit area.

This is designated as intensity, I, and is expressed in units of watts per square meter. Sound intensity is heard as loudness, which can be perceived differently depending on the individual and his or her distance from the source and the characteristics of the surrounding space. As the distance from the sound source increases, the sound intensity decreases. The sound power coming from the source remains constant, but the spherical surface over which the power is spread increases--so the power is less intense.

In other words, the sound power level of a source is independent of the environment. Most noise is not a pure tone, but rather consists of many frequencies simultaneously emitted from the source. To properly represent the total noise of a source, it is usually necessary to break it down into its frequency components. One reason for this is that people react differently to low-frequency and high-frequency sounds. Additionally, for the same sound pressure level, high-frequency noise is much more disturbing and more capable of producing hearing loss than low-frequency noise.

Engineering solutions to reduce or control noise are different for low-frequency and high-frequency noise. As a general guideline, low-frequency noise is more difficult to control. Certain instruments that measure sound level can determine the frequency distribution of a sound by passing that sound successively through several different electronic filters that separate the sound into nine octaves on a frequency scale.

Two of the most common reasons for filtering a sound include 1 determining its most prevalent frequencies or octaves to help engineers better know how to control the sound and 2 adjusting the sound level reading using one of several available weighting methods. These weighting methods e. The following paragraphs provide more detailed information. Octave bands, a type of frequency band, are a convenient way to measure and describe the various frequencies that are part of a sound.

The center, lower, and upper frequencies for the commonly used octave bands are listed in Table II The width of a full octave band its bandwidth is equal to the upper band limit minus the lower band limit. For more detailed frequency analysis, the octaves can be divided into one-third octave bands; however, this level of detail is not typically required for evaluation and control of workplace noise. Electronic instruments called octave band analyzers filter sound to measure the sound pressure as dB contributed by each octave band.

These analyzers either attach to a type 1 sound level meter or are integral to the meter. Both the analyzers and sound level meters are discussed further in Section III. Loudness is the subjective human response to sound. It depends primarily on sound pressure but is also influenced by frequency. Three different internationally standardized characteristics are used for sound measurement: weighting networks A, C, and Z or "zero" weighting.

The A and C weighting networks are the sound level meter's means of responding to some frequencies more than others. The very low frequencies are discriminated against attenuated quite severely by the A-network and hardly attenuated at all by the C-network. Sound levels dB measured using these weighting scales are designated by the appropriate letter i.

In contrast, the Z-weighted measurement is an unweighted scale introduced as an international standard in , which provides a flat response across the entire frequency spectrum from 10 Hz to 20, Hz. The C-weighted scale is used as an alternative to the Z-weighted measurement on older sound level meters on which Z-weighting is not an option , particularly for characterizing low-frequency sounds capable of inducing vibrations in buildings or other structures. A previous B-weighted scale is no longer used.

The networks evolved from experiments designed to determine the response of the human ear to sound, reported in by a pair of investigators named Fletcher and Munson. Their study presented a 1,Hz reference tone and a test tone alternately to the test subjects young men , who were asked to adjust the level of the test tone until it sounded as loud as the reference tone. The results of these experiments yielded the frequently cited Fletcher-Munson, or "equal-loudness," contours, which are displayed in Figure 6.

These contours represent the sound pressure level necessary at each frequency to produce the same loudness response in the average listener. The nonlinearity of the ear's response is represented by the changing contour shapes as the sound pressure level is increased a phenomenon that is particularly noticeable at low frequencies. The lower, dashed curve indicates the threshold of hearing and represents the sound-pressure level necessary to trigger the sensation of hearing in the average listener.

Among healthy individuals, the actual threshold may vary by as much as 10 decibels in either direction. Ultrasound is not listed in Figure 6 because it has a frequency that is too high to be audible to the human ear. See Appendix C for more information about ultrasound and its potential health effects and threshold limit values. The ear is the organ that makes hearing possible. It can be divided into three sections: the external or outer ear, the middle ear, and the inner ear.

Figure 7 shows the parts of the ear. The function of the ear is to gather, transmit, and perceive sounds from the environment. This involves three stages:. To categorize different types of hearing loss, the impairment is often described as either conductive or sensorineural, or a combination of the two. Conductive hearing loss results from any condition in the outer or middle ear that interferes with sound passing to the inner ear. Excessive wax in the auditory canal, a ruptured eardrum, and other conditions of the outer or middle ear can produce conductive hearing loss.

Although work-related conductive hearing loss is not common, it can occur when an accident results in a head injury or penetration of the eardrum by a sharp object, or by any event that ruptures the eardrum or breaks the ossicular chain formed by the small bones in the middle ear e. Conductive hearing loss may be reversible through medical or surgical treatment. It is characterized by relatively uniformly reduced hearing across all frequencies in tests of the ear, with no reduction during hearing tests that transmit sound through bone conduction. Sensorineural hearing loss is a permanent condition that usually cannot be treated medically or surgically and is associated with irreversible damage to the inner ear.

The normal aging process and excessive noise exposure are both notable causes of sensorineural hearing loss. Studies show that exposure to noise damages the sensory hair cells that line the cochlea. Even moderate noise can cause twisting and swelling of hair cells and biochemical changes that reduce the hair cell sensitivity to mechanical motion, resulting in auditory fatigue. As the severity of the noise exposure increases, hair cells and supporting cells disintegrate and the associated nerve fibers eventually disappear. Occupational noise exposure is a significant cause of sensorineural hearing loss, which appears on sequential audiograms as declining sensitivity to sound, typically first at high frequencies above 2, Hz , and then lower frequencies as damage continues.

Often the audiogram of a person with sensorineural hearing loss will show a "Notch" at 4, Hz. This is a dip in the person's hearing level at 4, Hz and is an early indicator of sensorineural hearing loss. Results are the same for hearing tests of the ear and bone conduction testing. Sensorineural hearing loss can also result from other causes, such as viruses e. Figure 8 shows the typical audiogram patterns for people with conductive and sensorineural hearing loss. It is important to note that some hearing loss occurs over time as a normal condition of aging.

Termed presbycusis, this gradual sensorineural loss decreases a person's ability to hear high frequencies. Presbycusis can make it difficult to diagnose noise-related hearing loss in older people because both affect the upper range of an audiogram. An 8,Hz "Notch" in an audiogram often indicates that the hearing loss is aged-related as opposed to noise-induced. As humans begin losing their hearing, they often first lose the ability to detect quiet sounds in this pitch range. Workplace noise affects the human body in various ways.

The most well-known is hearing loss, but work in a noisy environment also can have other effects. Although noise-induced hearing loss is one of the most common occupational illnesses, it is often ignored because there are no visible effects. It usually develops over a long period of time, and, except in very rare cases, there is no pain. What does occur is a progressive loss of communication, socialization, and responsiveness to the environment. In its early stages when hearing loss is above 2, Hz , it affects the ability to understand or discriminate speech.

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As it progresses to the lower frequencies, it begins to affect the ability to hear sounds in general. The primary effects of workplace noise exposure include noise-induced temporary threshold shift, noise-induced permanent threshold shift, acoustic trauma, and tinnitus. A noise-induced temporary threshold shift is a short-term decrease in hearing sensitivity that displays as a downward shift in the audiogram output. It returns to the pre-exposed level in a matter of hours or days, assuming there is not continued exposure to excessive noise. If noise exposure continues, the shift can become a noise-induced permanent threshold shift, which is a decrease in hearing sensitivity that is not expected to improve over time.

A standard threshold shift is a change in hearing thresholds of an average of 10 dB or more at 2,, 3,, and 4, Hz in either ear when compared to a baseline audiogram. Employers can conduct a follow-up audiogram within 30 days to confirm whether the standard threshold shift is permanent. Under 29 CFR Recording criteria for cases involving occupational hearing loss can be found in 29 CFR The effects of excessive noise exposure are made worse when workers have extended shifts longer than 8 hours.

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With extended shifts, the duration of the noise exposure is longer and the amount of time between shifts is shorter. This means that the ears have less time to recover between noisy shifts. As a result, short-term effects, such as temporary threshold shifts, can become permanent more quickly than would occur with standard 8-hour workdays. Tinnitus, or "ringing in the ears," can occur after long-term exposure to high sound levels, or sometimes from short-term exposure to very high sound levels, such as gunshots.

Many other physical and physiological conditions also cause tinnitus. Regardless of the cause, this condition is actually a disturbance produced by the inner ear and interpreted by the brain as sound. Individuals with tinnitus describe it as a hum, buzz, roar, ring, or whistle, which can be short term or permanent. Acoustic trauma refers to a temporary or permanent hearing loss due to a sudden, intense acoustic or noise event, such as an explosion. The U. Bureau of Labor Statistics BLS publishes annual statistics for occupational injuries including hearing loss reported by employers as part of required recordkeeping.

This represents more than 18, workers who experienced significant loss of hearing due to workplace noise exposure. Nonfatal occupational injuries accounted for the overwhelming majority of cases reported for the SOII in Most illness cases fall into the "All other illnesses" category, which includes such things as repetitive motion cases and systemic diseases and disorders. Other consequences of excessive workplace noise exposure include interference with communications and performance. Workers might find it difficult to understand speech or auditory signals in areas with high noise levels.

Noisy environments also lead to a sense of isolation, annoyance, difficulty concentrating, lowered morale, reduced efficiency, absenteeism, and accidents. In some individuals, excessive noise exposure can contribute to other physical effects. These can include muscle tension and increased blood pressure hypertension.

Noise exposure can also cause a stress reaction, interfere with sleep, and cause fatigue. Ultrasound is high-frequency sound that is inaudible i. However, it still might affect hearing and produce other health effects. For more information, see Appendix C. Animal experiments have indicated that combined exposure to noise and solvents induces synergistic adverse effects on hearing.

Experimental studies have explored specific substances, including toluene, styrene, ethylbenzene, and trichloroethylene. In reviewing IMIS data, note that the exposure levels are not necessarily typical of all worksites and occupations within an industry.

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Typically, OSHA identified those jobs as having some potential for noise exposure. A number of epidemiological studies have investigated the noise-solvent relationship in humans. Overall, the evidence strongly suggests that combined exposure to noise and organic solvents can have interactive effects either additive or synergistic , in which solvents exacerbate noise-induced impairments even though the noise intensity is below the permissible limit value.

In addition to the synergistic effects with solvents, noise may also have additive, potentiating, or synergistic ototoxicity with asphyxiants such as carbon monoxide and metals such as lead. See Appendix D for additional information and additional sources of information on this topic. Workplace noise exposure is widespread. Although this time span covers many years, the recent decade is well represented: 58, 27 of the personal noise exposure levels in IMIS were measured in or later.

These tables also present the median noise levels and the percentage of noise measurements over either the action level AL , 85 dBA, or the permissible exposure limit PEL , 90 dBA 2. In addition, 47 of the samples taken in the construction industry exceeded the PEL. In addition to median decibels and percent over the PEL, Table II-5 shows the distribution of manufacturing industry dosimetry measurements at the PEL and higher by decibel level.

Noise is a potential hazard for most jobs that involve abrasive or high-power machinery, impact of rapidly moving parts product or machinery , or power tools. According to IMIS noise measurements, workers in certain occupations within specific industries are exposed to excessive noise more frequently than others. While many jobs have noise exposure, historically, some of the occupations with the most extreme exposures listed by Standard Industrial Classification, or SIC have included:.

Source: Adapted from Seixas and Neitzel, This effort to reduce occupational noise hazards was not far-reaching but was a first attempt to regulate noise hazards. Even though noise energy exposure doubles every 3 dB, OSHA thought it important to account for the time during the workday that a worker was not exposed to noise hazards.

At the time, using a 5-dB exchange rate was viewed as a sufficient way to account for this. In , OSHA published a proposed occupational noise standard, which included a requirement for employers to provide a hearing conservation program for workers exposed to an 8-hour TWA of 85 dBA or more. This provision was adopted as part of the amendments of and While OSHA provided requirements for hearing conservation programs in general industry, the construction industry standard remained less specific in that regard.

More recently, in the recordkeeping standard 29 CFR Part , OSHA clarified the criteria for reporting cases involving occupational hearing loss. In , the U. Environmental Protection Agency EPA developed labeling requirements for hearing protectors, which required hearing protector manufacturers to measure the ability of their products to reduce noise exposure--called the noise reduction rating NRR.

OSHA adopted the NRR but later recognized that the NRR listed on hearing protectors often did not reflect the actual level of protection, which likely was lower than indicated on the label because most workers were not provided with fit-testing, and donning methods in a controlled laboratory setting were not representative of the donning methods that workers used in the field.

EPA is considering options for updating this rule. In special cases, noise exposure originates from noise-generating headsets. See Appendix F for a discussion of the techniques used to evaluate the noise exposure levels of these workers. General Industry: 29 CFR The General Industry standard establishes permissible noise exposures, requires the use of engineering and administrative controls, and sets out the requirements of a hearing conservation program.


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Paragraphs c through n of the General Industry standard do not apply to the oil and gas well-drilling and servicing operations; however, paragraphs a and b do apply. The general industry noise standard contains two noise exposure limit tables. Each table serves a different purpose:. The requirements for permissible noise exposures and controls under the Construction standard are the same as those under the general industry standard Continuing effective hearing conservation programs are required in all cases where the sound levels exceed the values shown in Table D-2 Agricultural Worksites: Although there is no standard for occupational noise exposure in agriculture, the evaluation and control methods discussed in this chapter are still valid.

Maritime Worksites: Marine terminals and longshoring operations fall under the requirements of the general industry noise standard; therefore, employers in such operations must meet the elements of the general industry Hearing Conservation Amendment, 29 CFR Noise controls should minimize or eliminate sources of noise; prevent the propagation, amplification, and reverberation of noise; and protect workers from excessive noise exposure. Ideally, the use of engineering controls should reduce noise exposure to the point where the risk to hearing is significantly reduced or eliminated.

Engineering and administrative controls are essential to an effective hearing loss prevention program. They are technologically feasible for most noise sources, but their economic feasibility must be determined on an individual basis. In some instances the application of a relatively simple noise-control solution reduces the hazard to the extent that the other elements of the program, such as audiometric testing and the use of hearing protection devices, are no longer necessary.

In other cases, the noise reduction process may be more complex and must be accomplished in stages over a period of time. Even so, with each reduction of a few decibels, the risk of hearing loss is reduced, communication is improved, and noise-related annoyance is reduced.

The first step in noise control is to identify the noise sources and their relative importance. This can be difficult in an industrial setting with many noise sources. It can be accomplished through several methods used together: obtain a frequency spectrum from an octave band analyzer, turn various components in the factory on and off or use temporary mufflers or enclosures to isolate noise sources, and probe areas close to equipment with a sound level meter to pinpoint areas where sound is dominant.

These measures will aid in identifying the sound sources that affect workers the most and should be prioritized when implementing noise controls. Once the noise sources have been identified, it is possible to proceed in choosing an engineering control, administrative control, or a form of personal protective equipment to reduce the noise level if noise exposure is too high Driscoll, Principles of Noise Control. The hierarchy of controls for noise can be summarized as: 1 prevent or contain the escape of the hazardous workplace agent at its source engineering controls , 2 control exposure by changing work schedules to reduce the amount of time any one worker spends in the hazard area administrative controls , and 3 control the exposure with barriers between the worker and the hazard personal protective equipment.

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This hierarchy highlights the principle that the best prevention strategy is to eliminate exposure to hazards that can lead to hearing loss. Corporations that have started buy-quiet programs are moving toward workplaces where no harmful noise will exist. Many companies are automating equipment or setting up procedures that can be managed by workers from a quiet control room free from harmful noise. When it is not possible to eliminate the noise hazard or relocate the worker to a safe area, the worker must be protected with personal protective equipment.

The rest of this section, until the discussion of administrative controls, presents information adapted from material developed under contract for the Noise eTool by Dennis Driscoll in Much industrial noise can be controlled through simple solutions. It is important, however, that all individuals administering abatement projects have a good understanding of the principles of noise control and proper use of acoustical materials.

Reducing excessive equipment noise can be accomplished by treating the source, the sound transmission path, the receiver, or any combination of these options. Descriptions of these control measures follow. The best long-term solution to noise control is to treat the root cause of the noise problem. For source treatment to be effective, however, a comprehensive noise-control survey usually needs to be conducted to clearly identify the source and determine its relative contribution to the area noise level and worker noise exposure.

At least four methods exist for treating the source: modification, retrofit, substitution, and relocation. For the most part, industrial noise is caused by mechanical impacts, high-velocity fluid flow, high-velocity air flow, vibrating surface areas of a machine, and vibrations of the product being manufactured. To reduce noise caused by mechanical impacts, the modifications outlined below should be considered.

For any of these options to be practical, however, they must not adversely affect production:. High-velocity fluid flow can often create excessive noise as the transported medium passes through control valves or simply passes through the piping. A comprehensive acoustical survey can isolate the actual noise source so that the appropriate noise-control measures can be identified. When deemed practical, some effective modifications for high-velocity fluid-flow noise include:.

One of the most common noise sources within manufacturing equipment is pneumatic- or compressed-air-driven devices such as air valves, cylinders, and solenoid valves. High-velocity air is also a major contributor to worker noise exposure where hand-held air wands or guns are used to remove debris from work areas. Finally, compressed air nozzles are often used to eject parts from a machine or conveyor line. All these forms of pneumatic systems generate undesirable noise as the high-velocity air mixes with the atmospheric air, creating excessive turbulence and particle separation.

It is important to note that the intensity of sound is proportional to the air flow velocity raised to the 8th power. Therefore, as a source modification, it is recommended that the air-pressure setting for all pneumatic devices be reduced or optimized to as low a value as practical. As a general guideline, the sound level can be reduced by approximately 6 dBA for each 30 reduction in air velocity. Additional noise controls for high-velocity air are presented in the retrofit and relocation sections below.

Machine casings or panels can be a source of noise when sufficient vibratory energy is transferred into the metal structure and the panel is an efficient radiator of sound. Typically, machine casings or large metal surface areas have the potential to radiate sound when at least one dimension of the panel is longer than one-quarter of the sound's wavelength. Conducting a thorough noise-control survey will help in identifying the source of vibration and in determining the existence of any surface-radiated sound.

When a machine casing or panel is a primary noise source, the most effective modification is to reduce its radiation efficiency. The following noise-control measures should be considered:. A variety of commercially available acoustical products and applications can be applied on or relatively close to noise sources to minimize noise. The Noise and Vibration Control Product Manufacturer Guide should be consulted for a partial list of the manufacturers of these products and applications.

Vibration damping materials are an effective retrofit for controlling resonant tones radiated by vibrating metal panels or surface areas. In addition, this application can minimize the transfer of high-frequency sound energy through a panel. The two basic damping applications are free-layer and constrained-layer damping.

Free-layer damping, also known as extensional damping, consists of attaching an energy-dissipating material on one or both sides of a relatively thin metal panel. For thicker machine casings or structures, the best application is constrained-layer damping, which consists of damping material bonded to the metal surface covered by an outer metal constraining layer, forming a laminated construction. Each application can provide up to 30 dB of noise reduction. It is important to note that the noise reduction capabilities of the damping application are essentially equal, regardless of which side it is applied to on a panel or structure.

Also, for practical purposes, it is not necessary to cover of a panel to achieve a significant noise reduction. For example, 50 coverage of a surface area will provide a noise reduction that is roughly 3 dB less than coverage. In other words, assuming that coverage results in 26 dB of attenuation, 50 coverage would provide approximately 23 dB of reduction, 25 coverage would produce a dB decrease, and so on. Next, for free-layer damping treatments, it is recommended that the application material be at least as thick as the panel or base layer to which it is applied. For constrained-layer damping, the damping material again should be the same thickness as the panel; however, the outer metal constraining layer may be half the thickness of the base layer.

Finally, just because a surface area vibrates, it is not safe to assume it is radiating significant noise. If fact, probably less than 5 of all vibrating panels produce sufficient airborne noise to be of concern in an occupational setting. For damping materials to be successful, at a minimum, the two following conditions must be satisfied determine by a comprehensive noise-control survey :.

When selecting the right type of damping material, it is recommended that the person making the decision refer to the expertise of the product manufacturer or their designated representative s. Typically, the supplier will need to obtain specific information from the buyer, such as the temperature and size of the surface area to be treated and the substrate thickness.

The supplier will then use the input data to select the most effective product for the particular application. The vendor can also provide the buyer with estimates of noise reduction and costs for procuring the material. Most industrial equipment vibrates to some extent. As machines operate, they produce either harmonic forces associated with unbalanced rotating components or impulsive forces attributed to impacts such as punch presses, forging hammers, and shearing actions.

Quite often, vibration problems are clearly identified by predictive-maintenance programs that exist within most industrial plants. Assuming that the root cause or source cannot be effectively modified, the next option for controlling undesirable vibration is to install vibration isolation. Isolators come in the form of metal springs, elastomeric mounts, and resilient pads. These devices serve to decouple the relatively "solid" connection between the source and the recipient of the vibration.

As a result, instead of the vibratory forces being transmitted to other machine components or the building, they are readily absorbed and dissipated by the isolators. When selecting the appropriate isolation device s , the person making the decision should consider the expertise of trained professionals. It is critical to note that improper selection and installation of isolators can actually make a noise and vibration problem worse. Many manufacturers of vibration isolation equipment have useful websites for troubleshooting problems and finding solutions see the Noise and Vibration Control Product Manufacturer Guide for a partial list of manufacturers.

Silencers are devices inserted in the path of a flowing medium, such as a pipeline or duct, to reduce the downstream sound level. For industrial applications, the medium typically is air. There are basically four types of silencers: dissipative absorptive , reactive reflective , combination of dissipative and reactive, and pneumatic or compressed air devices. This section will address the absorptive and reflective type; a separate section will discuss the pneumatic or compressed air silencers. The type of silencer required will depend on the spectral content of the noise source and operational conditions of the source itself.

Dissipative silencers use sound-absorbing materials to surround or encompass the primary airflow passage. These silencers' principal method of sound attenuation is by absorption. The advantages and disadvantages of dissipative silencers include:. Reactive silencers use sound reflections and large impedance changes area variations to reduce noise in the airflow. The principal method of attenuation is through sound reflection, which cancels and interferes with the oncoming sound waves. The advantages and disadvantages of reactive silencers include:.

The combination dissipative and reactive silencer is essentially a reactive silencer with sound-absorption added to provide high-frequency attenuation capabilities. The advantages and disadvantages are similar to those listed for each type. To determine which type of silencer is best for a particular application, a trained professional should be consulted. The manufacturer or a designated representative will need to work closely with the facility engineering representative s to clearly identify all operational and physical constraints.

The Noise and Vibration Control Product Manufacturer Guide contains a partial list of silencer manufacturers and their websites. In the earlier High-Velocity Air Flow section, it was mentioned that pneumatic or compressed air is a very common noise source in manufacturing plants.

Silencers - Principles and Evaluations

Assuming sufficient noise reduction cannot be achieved by optimizing the air-pressure setting, the second step for controlling this class of noise source is to use commercially available silencers. For retrofitting pneumatic devices, selecting the appropriate silencer type is critical for this control measure to succeed over time. If the source is a solenoid valve, air cylinder, air motor, or some other device that simply exhausts compressed air to the atmosphere, then a simple diffuser-type silencer will suffice.

The disadvantage of these types of devices is that they can cause unacceptable back pressure. Therefore, when selecting a diffuser silencer, it is important that the pressure-loss constraints for the particular application be satisfied.