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The Nature and Effects of Noise

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The Pervasive Nature of Occupational Noise

Noise is one of the most common of all the occupational hazards. In the United States, for example, more than 9 million workers are exposed to daily average A-weighted noise levels of 85 decibels (abbreviated here as 85 dBA). These noise levels are potentially hazardous to their hearing and can produce other adverse effects as well. There are approximately 5.2 million workers exposed to noise above these levels in manufacturing and utilities, which represents about 35% of the total number of workers in US manufacturing industries.

Hazardous noise levels are easily identified and it is technologically feasible to control excessive noise in the vast majority of cases by applying off-the-shelf technology, by redesigning the equipment or process or by retrofitting noisy machines. But all too often, nothing is done. There are several reasons for this. First, although many noise control solutions are remarkably inexpensive, others can be costly, especially when the aim is to reduce the noise hazard to levels of 85 or 80 dBA.

One very important reason for the absence of noise control and hearing conservation programmes is that, unfortunately, noise is often accepted as a “necessary evil”, a part of doing business, an inevitable part of an industrial job. Hazardous noise causes no bloodshed, breaks no bones, produces no strange-looking tissue, and, if workers can manage to get through the first few days or weeks of exposure, they often feel as though they have “got used” to the noise. But what has most likely happened is that they have started to incur a temporary hearing loss which dulls their hearing sensitivity during the work day and often subsides during the night. Thus, the progress of noise-induced hearing loss is insidious in that it creeps up gradually over the months and years, largely unnoticed until it reaches handicapping proportions.

Another important reason why the hazards of noise are not always recognized is that there is a stigma attached to the resulting hearing impairment. As Raymond Hétu has demonstrated so clearly in his article on rehabilitation from noise-induced hearing loss elsewhere in this Encyclopaedia, people with hearing impairments are often thought of as elderly, mentally slow and generally incompetent, and those at risk of incurring impairments are reluctant to acknowledge either their impairments or the risk for fear of being stigmatized. This is an unfortunate situation because noise-induced hearing losses become permanent, and, when added to the hearing loss that naturally occurs with ageing, can lead to depression and isolation in one’s middle and old age. The time to take preventive steps is before the hearing losses begin.

The Scope of Noise Exposure

As mentioned above, noise is especially prevalent in the manufacturing industries. The US Department of Labor has estimated that 19.3% of the workers in manufacturing and utilities are exposed to daily average noise levels of 90 dBA and above, 34.4% are exposed to levels above 85 dBA, and 53.1% to levels above 80 dBA. These estimates should be fairly typical of the percentage of workers exposed to hazardous levels of noise in other nations. The levels are likely to be somewhat higher in less developed nations, where engineering controls are not used as widely, and somewhat lower in nations with stronger noise control programmes, such as the Scandinavian countries and Germany.

Many workers throughout the world experience some very hazardous exposures, well above 85 or 90 dBA. For example, the US Labor Department has estimated that nearly half a million workers are exposed to daily average noise levels of 100 dBA and above, and more than 800,000 to levels between 95 and 100 dBA in the manufacturing industries alone.

Figure 1 ranks the noisiest manufacturing industries in the United States in descending order according to the percentage of workers exposed above 90 dBA and gives estimates of noise-exposed workers by industrial sector.

Figure 1. Occupational noise exposure—the US experience


Research Needs

In the following articles of this chapter, it should become clear to the reader that the effects on hearing of most types of noise are well-known. Criteria for the effects of continuous, varying and intermittent noise were developed some 30 years ago and remain essentially the same today. This is not true, however, of impulse noise. At relatively low levels, impulse noise seems to be no more damaging and possibly less so than continuous noise, given equal sound energy. But at high sound levels, impulse noise appears to be more damaging, especially when a critical level (or, more correctly, a critical exposure) is exceeded. Further research needs to be performed to define more exactly the shape of the damage/risk curve.

Another area that needs to be clarified is the adverse effect of noise, both on hearing and on general health, in combination with other agents. Although the combined effects of noise and ototoxic drugs are fairly well known, the combination of noise and industrial chemicals is of growing concern. Solvents and certain other agents appear to be increasingly neurotoxic when experienced in conjunction with high levels of noise.

Around the world, noise-exposed workers in the manufacturing industries and the military receive the major share of attention. There are, however, many workers in mining, construction, agriculture and transportation who are also exposed to hazardous levels of noise, as pointed out in figure 1. The unique needs associated with these occupations need to be assessed, and noise control and other aspects of hearing conservation programmes need to be extended to these workers. Unfortunately, the provision of hearing conservation programmes to noise-exposed workers does not guarantee that hearing loss and the other adverse effects of noise will be prevented. Standard methods to evaluate the effectiveness of hearing conservation programmes do exist, but they can be cumbersome and are not widely used. Simple evaluation methods need to be developed that can be used by small as well as large companies, and those with minimal resources.

The technology exists to abate most noise problems, as mentioned above, but there is a large gap between the existing technology and its application. Methods need to be developed by which information on all kinds of noise control solutions can be disseminated to those who need it. Noise control information needs to be computerized and made available not only to users in developing nations but to industrialized nations as well.

Future Trends

In some countries there is a growing trend to place more emphasis on non-occupational noise exposure and its contribution to the burden of noise-induced hearing loss. These kinds of sources and activities include hunting, target shooting, noisy toys and loud music. This focus is beneficial in that it highlights some potentially significant sources of hearing impairment, but it can actually be detrimental if it diverts attention from serious occupational noise problems.

A very dramatic trend is evident among the nations belonging to the European Union, where standardization for noise is progressing at an almost breathless pace. This process includes standards for product noise emissions as well as for noise exposure standards.

The standard-setting process is not moving rapidly at all in North America, especially in the United States, where regulatory efforts are at a standstill and movement toward deregulation is a possibility. Efforts to regulate the noise of new products were abandoned in 1982 when the Noise Office in the US Environmental Protection Agency was closed, and occupational noise standards may not survive the deregulatory climate in the current US Congress.

The developing nations appear to be in the process of adopting and revising noise standards. These standards are tending toward conservatism, in that they are moving toward a permissible exposure limit of 85 dBA, and toward an exchange rate (time/intensity trading relation) of 3 dB. How well these standards are enforced, especially in burgeoning economies, is an open question.

The trend in some of the developing nations is to concentrate on controlling noise by engineering methods rather than to struggle with the intricacies of audiometric testing, hearing protection devices, training and record keeping. This would appear to be a very sensible approach wherever feasible. Supplementation with hearing protectors may be necessary at times to reduce exposures to safe levels.

The Effects of Noise

Certain of the materials which follow have been adapted from Suter, AH, “Noise and the conservation of hearing”, Chapter 2 in Hearing Conservation Manual (3rd ed.), Council for Accreditation in Occupational Hearing Conservation, Milwaukee, WI, USA (1993).

Loss of hearing is certainly the most well-known adverse effect of noise, and probably the most serious, but it is not the only one. Other detrimental effects include tinnitus (ringing in the ears), interference with speech communication and with the perception of warning signals, disruption of job performance, annoyance and extra-auditory effects. Under most circumstances, protecting workers’ hearing should protect against most other effects. This consideration provides additional support for companies to implement good noise control and hearing conservation programmes.

Hearing impairment

Noise-induced hearing impairment is very common, but it is often underrated because there are no visible effects and, in most cases, no pain. There is only a gradual, progressive loss of communication with family and friends, and a loss of sensitivity to sounds in the environment, such as birdsong and music. Unfortunately, good hearing is usually taken for granted until it is lost.

These losses may be so gradual that individuals do not realize what has happened until the impairment becomes handicapping. The first sign is usually that other people do not seem to speak as clearly as they used to. The hearing-impaired person will have to ask others to repeat themselves, and he or she often becomes annoyed with their apparent lack of consideration. Family and friends will often be told, “Don’t shout at me. I can hear you, but I just can’t understand what you’re saying.”

As the hearing loss becomes worse, the individual will begin to withdraw from social situations. Church, civic meetings, social occasions and theatre begin to lose their attraction and the individual will choose to stay at home. The volume of the television becomes a source of contention within the family, and other family members are sometimes driven out of the room because the hearing-impaired person wants it so loud.

Presbycusis, the hearing loss that naturally accompanies the ageing process, adds to the hearing handicap when the person with noise-induced hearing loss becomes older. Eventually, the loss may progress to such a severe stage that the individual can no longer communicate with family or friends without great difficulty, and then he or she is indeed isolated. A hearing aid may help in some cases, but the clarity of natural hearing will never be restored, as the clarity of vision is with eyeglasses.

Occupational hearing impairment

Noise-induced hearing impairment is usually considered an occupational disease or illness, rather than an injury, because its progression is gradual. On rare occasions, an employee may sustain immediate, permanent hearing loss from a very loud event such as an explosion or a very noisy process, such as riveting on steel. In these circumstances the hearing loss is sometimes referred to as an injury and is called “acoustic trauma”. The usual circumstance, however, is a slow decrease in hearing ability over many years. The amount of impairment will depend on the level of the noise, the duration of the exposure and the susceptibility of the individual worker. Unfortunately, there is no medical treatment for occupational hearing impairment; there is only prevention.

The auditory effects of noise are well documented and there is little controversy over the amount of continuous noise that causes varying degrees of hearing loss (ISO 1990). That intermittent noise causes hearing loss is also uncontested. But periods of noise that are interrupted by periods of quiet can offer the inner ear an opportunity to recover from temporary hearing loss and may therefore be somewhat less hazardous than continuous noise. This is true mainly for outdoor occupations, but not for inside settings such as factories, where the necessary intervals of quiet are rare (Suter 1993).

Impulse noise, such as the noise from gunfire and metal stamping, also damages hearing. There is some evidence that the hazard from impulse noise is more severe than that from other types of noise (Dunn et al. 1991; Thiery and Meyer-Bisch 1988), but this is not always the case. The amount of damage will depend mainly on the level and duration of the impulse, and it may be worse when there is continuous noise in the background. There is also evidence that high-frequency sources of impulse noise are more damaging than those composed of lower frequencies (Hamernik, Ahroon and Hsueh 1991; Price 1983).

Hearing loss due to noise is often temporary at first. During the course of a noisy day, the ear becomes fatigued and the worker will experience a reduction in hearing known as temporary threshold shift (TTS). Between the end of one workshift and the beginning of the next the ear usually recovers from much of the TTS, but often, some of the loss remains. After days, months and years of exposure, the TTS leads to permanent effects and new amounts of TTS begin to build onto the now permanent losses. A good audiometric testing programme will attempt to identify these temporary hearing losses and provide for preventive measures before the losses become permanent.

Experimental evidence indicates that several industrial agents are toxic to the nervous system and produce hearing loss in laboratory animals, especially when they occur in combination with noise (Fechter 1989). These agents include (1) heavy metal hazards, such as lead compounds and trimethyltin, (2) organic solvents, such as toluene, xylene and carbon disulphide, and (3) an asphyxiant, carbon monoxide. Recent research on industrial workers (Morata 1989; Morata et al. 1991) suggests that certain of these substances (carbon disulphide and toluene) can increase the damaging potential of noise. There is also evidence that certain drugs which are already toxic to the ear can increase the damaging effects of noise (Boettcher et al. 1987). Examples include certain antibiotics and cancer chemotherapy drugs. Those in charge of hearing conservation programmes should be aware that workers exposed to these chemicals or using these drugs may be more susceptible to hearing loss, especially when exposed to noise in addition.

Non-occupational hearing impairment

It is important to understand that occupational noise is not the only cause of noise-induced hearing loss among workers, but hearing loss can also be caused by sources outside the workplace. These sources of noise produce what is sometimes called “sociocusis”, and their effects on hearing are impossible to differentiate from occupational hearing loss. They can only be surmised by asking detailed questions about the worker’s recreational and other noisy activities. Examples of sociocusic sources could be woodworking tools, chain saws, unmuffled motorcycles, loud music and firearms. Frequent shooting with large-calibre guns (without hearing protection) may be a significant contributor to noise-induced hearing loss, whereas occasional hunting with smaller-calibre weapons is more likely to be harmless.

The importance of non-occupational noise exposure and the resulting sociocusis is that this hearing loss adds to the exposure that an individual might receive from occupational sources. For the sake of workers’ overall hearing health, they should be counselled to wear adequate hearing protection when they engage in noisy recreational activities.


Tinnitus is a condition that frequently accompanies both temporary and permanent hearing loss from noise, as well as other types of sensorineural hearing loss. Often referred to as a “ringing in the ears”, tinnitus may range from mild in some cases to severe in others. Sometimes individuals report that they are more bothered by their tinnitus than they are by their hearing impairment.

People with tinnitus are likely to notice it the most in quiet conditions, such as when they are trying to go to sleep at night, or when they are sitting in a sound-proof booth taking an audiometric test. It is a sign that the sensory cells in the inner ear have been irritated. It is often a precursor to noise-induced hearing loss and therefore an important warning signal.

Communication interference and safety

The fact that noise can interfere with or “mask” speech communication and warning signals is only common sense. Many industrial processes can be carried out very well with a minimum of communication among workers. Other jobs, however, such as those performed by airline pilots, railroad engineers, tank commanders and many others rely heavily on speech communication. Some of these workers use electronic systems that suppress the noise and amplify the speech. Nowadays, sophisticated communication systems are available, some with devices that cancel unwanted acoustic signals so that communication can take place more easily.

In many cases, workers just have to make do, straining to understand communications above the noise and shouting above it or signalling. Sometimes people may develop hoarseness or even vocal nodules or other abnormalities on the vocal cords from excessive strain. These individuals may need to be referred to for medical care.

People have learned from experience that in noise levels above about 80 dBA they have to speak very loudly, and in levels above 85 dBA they have to shout. In levels much above 95 dBA they have to move close together to communicate at all. Acoustical specialists have developed methods to predict the amount of communication that can take place in industrial situations. The resulting predictions are dependent upon the acoustical characteristics of both the noise and the speech (or other desired signal), as well as the distance between talker and listener.

It is generally known that noise can interfere with safety, but only a few studies have documented this problem (e.g., Moll van Charante and Mulder 1990; Wilkins and Acton 1982). There have been numerous reports, however, of workers who have got clothing or hands caught in machines and have been seriously injured while their co-workers were oblivious to their cries for help. To prevent communication breakdowns in noisy environments, some employers have installed visual warning devices.

Another problem, recognized more by noise-exposed workers themselves than by professionals in hearing conservation and occupational health, is that hearing protection devices may sometimes interfere with the perception of speech and warning signals. This appears to be true mainly when the wearers already have hearing losses and the noise levels fall below 90 dBA (Suter 1992). In these cases, workers have a very legitimate concern about wearing hearing protection. It is important to be attentive to their concerns and either to implement engineering noise controls or to improve the kind of protection offered, such as protectors built into an electronic communication system. In addition, hearing protectors are now available with a flatter, more “high fidelity” frequency response, which may improve workers’ abilities to understand speech and warning signals.

Effects on job performance

The effects of noise on job performance have been studied both in the laboratory and in actual working conditions. The results have shown that noise usually has little effect on the performance of repetitive, monotonous work, and in some cases can actually increase job performance when the noise is low or moderate in level. High levels of noise can degrade job performance, especially when the task is complicated or involves doing more than one thing at a time. Intermittent noise tends to be more disruptive than continuous noise, particularly when the periods of noise are unpredictable and uncontrollable. Some research indicates that people are less likely to help each other and more likely to exhibit antisocial behaviour in noisy environments than in quiet ones. (For a detailed review of the effects of noise on job performance see Suter 1992).


Although the term “annoyance” is more often connected with community noise problems, such as airports or race-car tracks, industrial workers may also feel annoyed or irritated by the noise of their workplace. This annoyance may be related to the interference of speech communication and job performance described above, but it may also be due to the fact that many people have an aversion to noise. Sometimes the aversion to noise is so strong that a worker will look for employment elsewhere, but that opportunity is not often feasible. After a period of adjustment, most will not appear to be bothered as much, but they may still complain about fatigue, irritability and sleeplessness. (The adjustment will be more successful if young workers are properly fitted with hearing protectors from the start, before they develop any hearing loss.) Interestingly, this kind of information sometimes surfaces after a company starts a noise control and hearing conservation programme because the workers would have become aware of the contrast between earlier and subsequently improved conditions.

Extra-auditory effects

As a biological stressor, noise can influence the entire physiological system. Noise acts in the same way that other stressors do, causing the body to respond in ways that may be harmful in the long run and lead to disorders known as the “stress diseases”. When facing danger in primitive times, the body would go through a series of biological changes, preparing either to fight or to run away (the classic “fight or flight” response). There is evidence that these changes still persist with exposure to loud noise, even though a person may feel “adjusted” to the noise.

Most of these effects appear to be transitory, but with continued exposure some adverse effects have been shown to be chronic in laboratory animals. Several studies of industrial workers also point in this direction, while some studies show no significant effects (Rehm 1983; van Dijk 1990). The evidence is probably strongest for cardiovascular effects such as increased blood pressure, or changes in blood chemistry. A significant set of laboratory studies on animals showed chronic elevated blood pressure levels resulting from exposure to noise around 85 to 90 dBA, which did not return to baseline after cessation of the exposure (Peterson et al. 1978, 1981 and 1983).

Studies of blood chemistry show increased levels of the catecholamines epinephrine and norepinephrine due to noise exposure (Rehm 1983), and a series of experiments by German investigators found a connection between noise exposure and magnesium metabolism in humans and animals (Ising and Kruppa 1993). Current thinking holds that the extra-auditory effects of noise are most likely mediated psychologically, through aversion to noise, making it very difficult to obtain dose-response relationships. (For a comprehensive overview of this problem, see Ising and Kruppa 1993.)

Because the extra-auditory effects of noise are mediated by the auditory system, meaning that it is necessary to hear the noise for adverse effects to occur, properly fitted hearing protection should reduce the likelihood of these effects in just the way it does with hearing loss.



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