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Effect of Occupational Noise Pollution on Workers

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Published: Mon, 26 Feb 2018

Noise pollution

CHAPTER 1 INTRODUCTION

Background

Noise means unpleasant sound that gives a disturbing and annoying effect to the listener. Noise pollution is any unwanted, disturbing, or harmful sound that impairs or interferes with hearing, causes stress, hampers concentration and work efficiency, or causes accident. Noise can block, distort, change or interfere with the meaning of a message in both human and electronic communication (Wikipedia, 2009). Agricultures workers are one of the highest contributors in the rate of noise pollution among all occupation. Any person who is exposed to an excessive noise pollution in long period may suffer hearing loss. The amount of damaged caused by noise depends on the total amount received over time. The degree of risk is affected by the intensity (loudness) and the frequency (pitch) of the noise, as well as the duration and pattern of exposure and the individual susceptibility to hearing impairment (CCOH, 2009).

The increased spread of hearing loss from high frequencies through low frequencies with age and noise exposure is common for this population. Hearing loss of farmers is very characteristic of a sensor neural, bilateral sloping configuration resulting from both noise and aging. When age group data were compared to the hearing sensitivity values of the U.S. Occupational Safety and Health Administration (OSHA), all farmer populations, age 20-60, showed more dramatically hearing loss than the comparison group. This also was true when the data were compared to the International Organization for Standardization (ISO) 1990 data. Physical ear discomfort to noise exposure starts from sound pressure level of 80-100 decibels (dB). A continuous noise level of 85 dB can result in hearing damage as well as create other various negative effects on health (League for the Hard of hearing, 2002). Noise induced hearing loss occurs gradually and without pain. Noise is often recorded as decibels dB (A) to approximate how the human ears respond to noise.

In Malaysia, noise exposure in work place is legislated under the Factories and Machinery Act (Noise Exposure) Regulation 1989, and Occupational and Safety Health Act (OSHA) 1994. This regulation makes it mandatory for noise level and workers’ exposure to noise is measured, assessed, and controlled. Malaysian permissible exposure level (PEL) refers to the limit of exposure that must not be exceeded by any employee over a specified time limit. These limits denote concentration levels above which exposure to chemicals hazardous to health must be controlled. To protect against chronic health effects of chemicals, the limits set are eight hours time-weighted average values. Excursions above the eight-hour time weighted average limit is allowed as long as it does not exceed three times this set limit.

The farm tractor has a central role in field operations and often in stockyards and buildings too. It pulls lifts, powers and supports; it provides personal transport and shelter from the weather. It is often the main status symbol of the agricultural enterprise; this is why tractors are styled (like cars), why they are loved by enthusiasts (like steam railway engines), and why individual farmers often praise and are faithful to one particular make.

Two aspects of tractor noise cause concern: the environmental noise heard by bystanders, either fellow road users or local residents, and the potentially harmful noise to which the operator is subjected. Environmental noise was the subject of a formal OECD test. In the 1960s the Institute harmonized details of the test procedure by comparing measurements in all European test stations. Noise screens for engines were successfully developed, but the dominant environmental noise arises from exhausts, where a balance must be found between silencer effectiveness and power loss. A separate study of the annoyance caused by farm noise of all kinds rated the tractor as much less annoying than axed plant such as grain driers.

Objectives

The study is focused on highlight that prolong to high level of occupational noise which can affect the hearing ability to agricultures worker who are using tractors and machines.

  1. To characterize noise exposure profile among agricultures workers at Seksyen Kejuruteraan Ladang, Taman Pertanian Universiti,Universiti Putra Malaysia.
  2. To evaluate safety practices among workers.

Significant of study

The purpose of this study is to determine and observe if the noise from heavy machine, agriculture machine, industry machine and agricultures tools that are related to noise can contribute to the incidence of hearing loss.

CHAPTER 2 LITERATURE REVIEW

Noise

Noise is a word often used to mean unpleasant sound that the listener does not want to hear, although there are no physical characteristics distinguishing noise from wanted sound (Plog et. al., 2002). Noise means unpleasant sound that give a disturbing and an annoying effect to the listener. Noise pollution is any unwanted, disturbing, or harmful sound that impairs or interferes with hearing, causes stress, hampers concentration and work efficiency, or causes accident). Noise can block, distort, change or interfere with the meaning of a message in both human and electronic communication (Wikipedia, 2009).

Noise environments of the type and severity associated with combustion engines and other noises arising from mechanisms or animals may have the following principle effects on the person exposed. The noise may be annoying to varying degrees, from being just objectionable to being unbearable. The performance may be affected due to a lowering of concentration, fatigue caused by longer exposed, rhythm disturbance, interference with sound cues associated with the work or interference with worker-to-worker communication in a team. Damage to hearing noise may be caused by noise; the character and to a lesser extent, the mechanism of this damage is now being understood. Both temporary and permanent components of hearing threshold shift are possible (Matthews, 1968).

Noise is unwanted electrical or electromagnetic energy that degrades the quality of signals and data. Noise occurs in digital and analog systems, and can affect files and communications of all types, including text, programs, images, audio, and telemetry. In a hard-wired circuit such as a telephone-line-based Internet hookup, external noise is picked up from appliances in the vicinity, from electrical transformers, from the atmosphere, and even from outer space.

Normally this noise is of little or no consequence. However, during severe thunderstorms, or in locations were many electrical appliances are in use, external noise can affect communications. In an Internet hookup it slows down the data transfer rate, because the system must adjust its speed to match conditions on the line. In a voice telephone conversation, noise rarely sounds like anything other than a faint hissing or rushing.

Noise is a more significant problem in wireless systems than in hard-wired systems. In general, noise originating from outside the system is inversely proportional to the frequency, and directly proportional to the wavelength. At a low frequency such as 300 kHz, atmospheric and electrical noise are much more severe than at a high frequency like 300 megahertz. Noise generated inside wireless receivers, known as internal noise, is less dependent on frequency. Engineers are more concerned about internal noise at high frequencies than at low frequencies, because the less external noise there is, the more significant the internal noise becomes.

Communications engineers are constantly striving to develop better ways to deal with noise. The traditional method has been to minimize the signal bandwidth to the greatest possible extent. The less spectrum space a signal occupies, the less noise is passed through the receiving circuitry. However, reducing the bandwidth limits the maximum speed of the data that can be delivered. Another, more recently developed scheme for minimizing the effects of noise is called digital signal processing (digital signal processing). Using fiber optics, a technology far less susceptible to noise, is another approach (Techtarget, 2010).

Noise Pollution generally refers to unwanted sound produced by human activities unwanted in that it interferes with communication, work, rest, recreation, or sleep. Unlike other forms of pollution, such as air, water, and hazardous materials, noise does not remain long in the environment. However, while its effects are immediate in terms of annoyance, they are cumulative in terms of temporary or permanent hearing loss. Society has attempted to regulate noise since the early days of the Romans, who by decree prohibited the movement of chariots in the streets at night. In the United States, communities since colonial days have enacted ordinances against excessive noise, primarily in response to complaints from residents. It was not until the late 1960s, however, that the federal government officially recognized noise as a pollutant and began to support noise research and regulation. Federal laws against noise pollution included the National Environmental Policy Act of 1969, especially sections concerning environmental impact statements; the Noise Pollution and Abatement Act of 1970; and the Noise Control Act of 1972, which appointed the Environmental Protection Agency (EPA) to coordinate federal research and activities in noise control.

Charged with developing federal noise-emission standards, identifying major sources of noise, and determining appropriate noise levels that would not infringe on public health and welfare, the EPA produced its so-called Levels Document, now the standard reference in the field of environmental noise assessment. In the document, the EPA established an equivalent sound level (Leq) and a day–night equivalent level (Ldn) as measures and descriptors for noise exposure. Soon thereafter, most federal agencies adopted either the Leq, Ldn, or both, including levels compatible with different land uses. The Federal Aviation Administration (FAA) uses Ldn as the noise descriptor in assessing land-use compatibility with various levels of aircraft noise. In 1978 the research findings of Theodore J. Schultz provided support for Ldn as the descriptor for environmental noise. Analyzing social surveys, Schultz found a correlation between Ldn and people who were highly annoyed by noise in their neighborhoods. The Schultz curve, expressing this correlation, became a basis for noise standards.

As part of its effort to identify major noise sources in the United States, the EPA set about determining the degree to which noise standards could contribute to noise reduction. During the 1970s, EPA-sponsored research on major noise sources led to regulation of the products that most affected the public, including medium and heavy trucks, portable air compressors, garbage trucks, buses, and motorcycles. Missing from the list was aircraft, which was considered the responsibility of the FAA. During the administration of President Ronald Reagan in the 1980s, the power of the EPA and its Office of Noise Abatement and Control was curtailed and most of its noise regulations rescinded. Even so, efforts continued to curb noise pollution. The Department of Transportation maintains standards for highways, mass transit, and railroads, as well as aircraft. The environmental review process, mandated by the National Environmental Policy Act of 1969, remains the single most effective deterrent to noise pollution (Answer, 2010).

Noise exposure

Noise is one of the most important environment factors, which affects the workers’ health and efficiency. Noise can increase the overall workload of operators during a specific task and can affect the performance. As the result, noise affects workers’ health directly and indirectly (Parsons, 2000). Exposure to intense noise has been shown to damage the human hearing process and noise has been labeled as the most pervasive hazardous agent in the workplace (Milz et al., 2008). Among these effects are weariness, backhoe, nervousness, nausea, careless, etc (Tör, 1989; Anonymous, 2002; Ekerbicer and Saltik, 2008). According to McBride et. al (2003), it is known that people working in agricultural facilities are exposed to some noise sources, but the risk appeared in the people who have been exposed to noise for many years have not been fully characterized yet. The reduction in the hearing loss does not decrease below 1000Hz (Akyildiz, 2000). It was showed that noise induced hearing loss increase up to 7dB in the first 10 years at 1000 Hz and 100 dB (A), and then gradually increases to 12 dB losses for exposure time of 40 years. The hearing loss is about 30 dB for first ten years exposure at 4000 Hz and 100 dB (A). It is clear that at 100 dB (A), the ear is much more sensitive to 4000 Hz compared to 1000 Hz. Maximum sound pressure level for 8 h/day exposure is accepted to be 85 dB at frequencies higher than 1000 Hz. At levels lower than this value, the risk of noise becomes the least (Grandjean, 1988).

Lonsbury and Martin (2004) states that “the beginning region of impairment involves the sensitive mid-frequency range, primarily of impairment involves the sensitive mid-frequency range, primarily between 3 and 6 kHz, and the corresponding impairment is classically described as the 4-kHZ notch. This particular pattern of appears regardless of the noise exposure environment.” Sumer et al. (2006) explains that, there is a tendency of reducing daily noise exposure to below 90 dBA for an 8-h shift, and hence exposure level of 85 dBA is informally acknowledged to be the informal threshold sound pressure level. Therefore it is crucial to keep sound pressure levels within safety limits to avoid health related disturbances and work related inefficiencies.

Sanders and McCormick (1992) explained that the ear is more sensitive to noise at frequencies over 2000 Hz and the sensitivity increase with age. Miyakita and Ueda (1997) wanted to determine the number of persons exposed to loss of hearing at levels above 40 dB at a frequency of 4 Hz; and as a result, estimated that 360,000 people working in agricultural facilities in Japan impaired their hearing abilities. This feature makes the agriculture their second biggest sector after the construction sector, which causes the loss of hearing abilities.

Hearing loss from noise exposure

Exposure to occupational noise has been linked to variety of physical effects such as work absenteeism and stress. The most profound effect of prolonged exposure to noise is the noise induced hearing loss (NIHL). NIHL is an irreversible sensory-neural hearing impairment caused by prolonged exposure to noise. NIHL causes communication interference that can substantially affect social integration and the quality of life. The development of NIHL depends on exposure time, intensity, frequency, type of noise, and the use of personal protective equipment (Ismail et al, unpublished).

Noise-induced hearing loss (NIHL) is a well and long recognized occupational hazard but methods of influencing attitudes towards noise hazard and prevention of hearing loss as a result a poor (World Health Organization, 1997). Although the effects of noise on hearing are not precisely defined and uncertainties remain, there is sufficient information to permit development of predictive indices of the hazardous effects of noise on human hearing sensitivity. The effects of noise on hearing may be divided into three categories which are acoustic trauma, noise-induced temporary threshold shift (NITTS) and noise-induced permanent threshold shift (NIPTS).

Acoustic Trauma (immediate organic damage to the ear from excessive sound energy) is restricted to the effects of a single exposure or relatively few exposures at high sound levels. Extremely intense sound reaching the structures in the inner ear may exceed the physiologic limits of those structures producing a complete breakdown and disruption of the organ of Corti. Some degree of permanent hearing loss usually results from acoustic trauma. The precipitating episode is frequently dramatic so the person involved has no difficulty in specifying the onset of the resulting hearing problem.

Noise-Induced Temporary Threshold Shift (NITTS) results in an elevation of hearing levels such as loss of hearing sensitivity, following shift the hearing loss is reversible. In Noise-induced Permanent Threshold Shift (NIPTS) the hearing loss is nonreversible; it remains throughout the lifetime of the affected person. There is no possibility of further recovery. Permanent threshold shift may result from acoustic trauma or may be produced by the cumulative effect of repeated noise exposure over periods of many years. The majority of those persons experiencing permanent hearing losses from noise sustain such losses from long periods of repeated noise exposure.

Hearing ability decreases as age progresses. Age has been identified as one of the individual risk factors for sensory neural hearing loss (SNHL) among forest workers who handled chainsaws. Hearing loss induced among elderly miners in Romania was more pronounced compared to younger miners. The mean hearing threshold level (HTL) for the 40- 46 age group workers produced a decrease in HTL at frequencies 4, 6 and 8 kHz. Age is positively associated to hearing loss among metal processing factory workers in Brazil, with prevalence ratio of 4.02 for workers older than 40 years old (Ismail et. al, unpublished). Animals’ studies have shown that these chemicals interact synergistically with noise or potentiate its effect on auditory system. Workers exposed to chemicals have significantly poorer pure-tone thresholds compare to those not expose (Morata et al., 2003).

Lonsbury and Martin (2004) gave audiogram results that show audiometric patterns of hearing levels from patients in beginning stages of noise induced hearing loss and examples were given for males and females exposed to noise in different environments including industrial noise. Hearing loss was not observed at frequencies below 1000 Hz and was sharpest above 2000 Hz for a male industrial worker. Patients working in different sectors showed that the hearing loss might not be observed below 2000 Hz in different work environments while others might experience hearing loss at about 1000 Hz. The sensitivity is also affected by gender and the number of years worked in a particular environment.

Occupational health hazard of noise

In Malaysia, noise exposure in the workplace is legislated under the Factories and Machinery Act (Noise Exposure) Regulation 1989, and the Occupational and Safety and Health Act 1994. These regulations make mandatory for noise levels and workers’ exposure to noise to be measured, assessed and controlled (Leong, 2005).

International Labor Organization (ILO) accepts 85 dBA as warning limit and 90 dBA as danger limit for continuous work for 8 h. A-weighted equivalent sound pressure level of 85 dBA results in temporary hearing losses and 90 dBA increases the blood pressure, accelerates the pulse and breathing, decreases brain liquid pressure, causes tension in muscles, and withdrawal of blood in the skin (Sabanci and Uz, 1984).

Lonsbury and Martin (2004) gave audiogram results that show audiometric patterns of hearing levels from patients in beginning stages of noise induced hearing loss and examples were given for males and females exposed to noise in different environment including industrial noise. Hearing loss was not observed at frequencies below 1000 Hz and was the sharpest above 2000 Hz for male industrial worker. Patients working in different sectors showed that the hearing loss might not be observed below 2000 Hz in different work environments while others might experience hearing loss at about 1000 Hz. The sensitivity is also affected by gender and the number of years worked in a particular environment.

According to Dewangan et al. (2005), the exposure to noise may have both immediate and long-term effects on hearing of the tractor drivers and other farm workers. High noise levels can cause headaches; dizziness; nervousness and stress; sleeping problems; and loss of concentration. Noise can also increase human error, contributing to accidents by masking audible alarms, verbal messages, etc.; harder to process complex information for difficult task; and harder to monitor and interpret unusual events, by narrowing the span of attention. Although this kind of the damage is quite difficult to measure, it is always present and can manifest: in the gastro enteric tract with an increase in acid secretion; in the nervous system with states of fatigue and depressions; in the psyche, with insomnia and headaches (Febo et al., 1983).

Jansen (2003) observed that sounds in the range or 70-90 dB cause tiny blood vessels in the toes, fingers, skin and abdominal organs to contract. This narrowing of small blood vessels can reduce blood flow to affected body parts by as much as one-half. Studies have indicated that workers exposed to high levels of industrial noise for 5-30 years have increased blood pressure and statistically significant increases in risk for hypertension, compared to workers in control areas (Passchier and Vermeer, 1993).

Sensor neural hearing loss has been common complaint among farmers seen by rural otolaryngologist (Gregg, 1972). Farmers to be among the most hearing impaired workers and most expect to experience significant hearing loss by age 50 (Mahon, 1988).

Occupational noise limit

In the light of scientific data which shows the negative effects of noise on human health, new legal regulations were made in order to eliminate these effects. One of these regulations is “Noise Control Regulation”. In this regulation, it is stated that beside the intensity of the noise, the exposure duration in noisy medium can be effective on human health. Therefore it is emphasized that working hours should be determined according to sound pressure level (Aybek et al., 2007). Duration of exposure is also a consideration as well as the frequency content and A-weighting curve is used in practical applications denoted by dB(A) and 85-90 dB(A) have been proposed to be limiting values for 8 hours exposures (Parson, 2000).

The effect is more profound to certain frequencies of noise (Parson, 2000). The farmers worked on average 14 hours a day and their exposure was 86 decibels on A-weighted scale (dB (A)) as an 8-hour time-weighted average (8HR TWA). Except for breakfast and lunch breaks, the farmers had nearly continuous noise exposure that exceeded the Occupational Safety and Health Administration (OSHA) action level of 85 dB (A) (Milz et al., 2008). Legislation, effective since June, 1976, limits the maximum noise level at the driver to 90 dB (A) for all tractors sold in UK (Talamo, 1979). In Malaysia, to protect the workers from excessive exposure to noise, the hearing conservation program was introduced under the Factories and Machinery (Noise Exposure) Regulation 1989. Under this regulations, workers are protected from excessive noise exposure and reducing the risk of NIHL.

According to the Factories and Machinery (Noise Exposure) Regulation 1989, for the permissible exposure limit, the employee shall not be exposed to noise level exceeding equivalent continuous A-weighted sound pressure level of 90 dB(A) or exceeding the limits specified in the First Schedule or exceeding daily dose of unity. No employee shall be exposed to noise level exceeding 115 dB (A) at anytime. The 85 dB (A) is adopted as a criterion for action (action level). When the action level is reached or exceeded, it necessitates (NIOSH, 2006).

According to the U.S. National Institute of Occupational Safety and Health, the recommended exposure limit (REL) for occupational noise exposure is 85 dB (A) time weighted average (TWA). Exposures at or above this level are considered as hazardous. They differed from the U.S. Occupational Safety and Health Act (OSHA) which uses 90 dB (A) TWA. The rationale is to offer greater protection to noise-exposed workers, citing research that indicates an 8% excess risk of hearing loss at the 85 dB (A) TWA limit as opposed to 25% at 90 dB(A). The TWA is the averaging of different exposure levels during an exposure period. The REL for an 8-hr work shift is a TWA of 85 dB (A) using a 3-decibel (dB) exchange rate. The Malaysian Noise Exposure Regulation 1989 adopted a 5 dB exchange rate (OSHA, 2006).

In the last 10 years, a considerable number of laws and rules regarding the control of noise in the working environment, also involving the agricultural sector, have come into force. The main purpose of the individual laws and rules were different, but all emphasized the hazard of noise for workers, and consequently the need to control and, if necessary, to reduce the relevant levels. In the European Union, and specifically in Italy, some of them held an important role. For agricultural machinery, the EN 1553 represents a particular reference, while specially for tractors, the EC 74/150 (environmental noise) and EC 77/311 (driver’s ear noise) Directives were issued in the 1970s (Domenico and Matteo, 2000).

Tractors and machines

Agricultural and forestry tractors operate both as vehicles and as engines which provide power on the farm and in the forest. With respect to the noise which they emit, it is generally accepted that use as vehicles has the greater impact on the population at large. Hence the type of the test used to asses noise emission is equivalent to that used for other road vehicles, in which sound pressure level at a bystander position is measured under specified drive-past conditions, as opposed to the measurement of sound power, which is used for lawn-mowers and various items in construction equipment (Stayner, 1988). Parallel to the development in technology, the use of machines in mechanization processes of agricultural production has brought about the factors such as noise, vibration, gas , etc. which affect the working environment of users and inspectors of those machines (Aybek et al., 2007).

In order to increase the work success of the machines and to provide the users with safety and comfort these machines must be designed with respect to the human characteristics (Lijedahl et al., 1996). The machines used in agricultural operations such as tractors, combines, shellers, elevators, driers, etc. exposed noise of high level. More hearing loss is encountered among people who work in agricultural facilities than other jobs (Baker, 2002). The mechanization of agriculture; including the use of the internal combustion engine, greatly increased the noise exposure of the US farm population (Matthew, 1968).

Equipment manufactured prior to the institution of noise reduction features is often still in use on US farms, as demonstrates by the New York Farm Family Health and hazard Surveillance Program (Beckett et al., 2000). The majority of noise exposure seemed to come from mechanical equipment (Dennis et al., 1995). The mechanization of agriculture, and in particular the large-scale employment of the internal combustion engine, has led to a serious increase during the last few decades of farm workers to noise. For the most damaging environment, noise measurements were made on tractors (Matthews, 1968).

Noise isolating enclosure for tractor drivers have the same conflicting performance requirements as hearing defenders in that they are required to exclude harmful noise and yet allow the passage of important information. The tractors drivers and his counterpart in industry may depend on acoustical signals for warnings of danger and for maneuvering instructions when he is working to close limits or when hitching implements to the three point linkage. Other external sounds may be important for example, the tone generated by a component such as a fan may be the most important source of feedback to the driver of machine performance. A change in noise produced also vital where it is associated with the operation of a safety device such as an overload clutch or with the failure of the component. Many tractors in use on farms, however, have a much higher noise level and since tractors have a long life it will be many years before they are phased out (Talamo, 1979).

The management of very old tractors is uneconomical. They may be dangerous on the roads, unsafe for the driver and for other vehicles, mainly due to deterioration in performance. Very often the operators are exposed to unfavorable climatic and working conditions (noise, vibrations, etc.), and they are therefore subject to a higher level of stress and to a higher risk of accident or occupational disease. Despite the limit of technical obsolescence of agricultural tractors being commonly fixed at 10 yr (corresponding to an optimum of 1000 working hours per year) a large number of older machines are still in use in Italy. Repair and maintenance operations are frequently neglected on these vehicles, because the farmer does not consider the relevant cost appropriate (Domenico and Matteo, 2000).

Hearing protectors

In many noise environments it is not practical, economical, or feasible to reduce noise levels at the ear of a listener to within acceptable limits using engineering controls. In these situations, an acceptable noise level may be achieved with the use of personal either single or in combination. The widespread attention given to noise as a pollutant has stimulated the use of hearing protectors in industry as well as around the home and in recreational and in sports activities. This chapter describes the types of ear protectors, characteristics of ear protectors which influence their effectiveness and acceptance, other practical considerations for users of ear protectors, and estimations of the hearing protection which provide. Various types of ear protectors’ like’s earplugs, earmuffs, ear cups on hardhat, ear cups on welding mask, communication headsets and helmets. Although an ear protector can reduce effectively the ambient sound at the ear of the wearer, factors other than hearing protection may actually determine its suitability and acceptability (Nixon, 1979).

According to Domenico and Matteo (2000), the function of a hearing protection device (HPD) is to cover or to fill the ears so that the noise reaching the ear drum is attenuated. It is important to emphasize that the HPDs should not be the sole or primary means by which the worker noise exposures are reduced. HPDs should be used only when engineering controls and work practices are not feasible for reducing noise exposures. This seems to be the typical situation for very old and worn tractors. Nevertheless, the degree of attenuation that a HPD provides is dependent on the technical characteristics of the HPD and generally on a range of other factors, such as the wearing time, the motivation and training of the worker. The use of HPDs has been practiced since the 1950s; the first standards to measure their attenuation were issued about 40 years ago. All of them are finalized to obtain an index, in order to define the attenuation of the HPD: noise reduction rating (NRR) and single number rating (SNR) are those more frequently considered. Wearing time is also an important parameter since it can decrease the effective protection provided by a HPD. For example, if a HPD with an NRR of 20 dB is not worn for as little as 30 min in an 8 h work shift, its effective NRR is reduced by 5 dB (Berger, 1980). It is in fact taken into account that wearers may be prone to remove and replace some HPDs more than others, depending on various factors such as comfort, ease of donning and removal, and the interference of the protector with the auditory communications.

Many types of H


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