Occupational and Community Noise.

In ancient Rome, the clatter of iron wheels on stone pavements disturbed people's sleep and so annoyed citizens that legislation was enacted to control movement. Some cities of medieval Europe prohibited horse and carriage traffic to protect the sleep of the inhabitants.

The noise problems of the past, though, are not comparable to those plaguing modern society: the roar of aircraft, the thunder of heavily laden trucks, and the thumps and whines of industry. Such noise can be not only annoying but also damaging to the health, and it is increasing with economic development.

Health Impact

The recognition of noise as a serious health hazard (as opposed to a nuisance) is a recent development, but the following effects are being remarked:

* Globally, some 120 million people are estimated to have disabling hearing difficulties.

* In 1990, about 30 million people in the United States were exposed daily to occupational noise levels above 85 decibels (dB), compared with more than nine million people in 1981; these people are mostly in the production and manufacturing industries.

* Prolonged or excessive exposure to noise, whether in the community or at work, can cause permanent medical conditions, such as hypertension and ischemic heart disease.

* Noise can adversely affect performance--for example, in reading, attentiveness, problem solving, and memory Deficits in performance can lead to accidents.

* Noise above 80 dB may increase aggressive behavior.

* A link between community noise and mental-health problems is suggested by the demand for tranquilizers and sleeping pills, the incidence of psychiatric symptoms, and the number of admissions to mental hospitals.

Community Noise

Noise levels associated with impairment may be experienced at open-air concerts, discotheques, motor sports events, and so forth. For most people, however, a lifetime of continuous exposure to an average noisy environment (70 dB) will not cause hearing impairment. An adult person's ear can tolerate an occasional noise level of up to 140 dB, but for children, such exposure should never exceed 120 dB.

Sources of indoor noise are ventilation systems, office machines, home appliances, and neighbors. Some typical sources of more general neighborhood noise include restaurants, live or recorded music, sports, playgrounds, car parks, and barking dogs.

Occupational Noise

The major sources of noise that damages hearing are impact processes, materials handling, and industrial jets. Air jets--widely used for cleaning, drying, power tools, and steam valves--can generate sound levels of 105 dB. In the woodworking industry the sound levels of saws can be as high as 106 dB. Average sound levels range between 92 and 96 dB in industries such as foundries, shipyards, breweries, weaving factories, paper mills, and sawmills. The recorded peak values are between 117 and 136 dB. In most developing countries, industrial noise levels are higher than those in developed countries.

Noise-induced hearing impairment is one of the most common irreversible (and preventable) occupational hazards worldwide. Cheaper, more cost-effective production is a driving force in economic development. New processes introduced on grounds of cost-effectiveness are often noisier than previous ones, and the associated rise in noise levels is often overlooked. Even though noise-reducing measures may have been incorporated in the design of machinery, greater output may generate higher noise levels.

Prevention

Transient tinnitus (ringing in the ear) is a common occupational hearing condition. It should be considered as a warning of excessive exposure to sound.

Occupational exposure limits specify the maximum sound pressure levels and exposure times to which nearly all workers may be repeatedly exposed without adverse effect on their ability to hear and understand normal speech. An occupational exposure limit of 85 dB for eight hours should protect most people against a permanent hearing impairment induced by noise after 40 years of occupational exposure.

It is 10 times less expensive (in unit cost per decibel of reduction) to make noise-generating processes quieter than it is to make a barrier to screen the noise. Noise levels can, however, be lowered by the use of noise-control enclosures, absorbers, silencers, and baffles, and by the use of personal protective equipment such as earmuffs.

Protection against hazardous noise exposure should be included in overall hazard prevention and control programs in workplaces. The dangers of noise should be recognized before workers start complaining of hearing difficulties.

Another consideration is the audibility of warning sounds: One sound can sometimes interfere with the perception of another. Because lower-frequency sounds can mask higher-frequency sounds, warning sounds and alarms should be pitched at lower frequencies than the dominant industrial background noise.

WHO's Response to the Noise Problem

The field is marked by a scarcity of literature, especially for developing countries. Some 20 years after its last publication on noise, WHO has issued Guidelines for Community Noise. This publication, the outcome of a WHO expert task force that met in London in March 1999, includes guideline values for community noise and can be found on the Web at [less than]www.who.int./peh/[greater than].