A It is not difficult for people to encounter sound at levels capable of harming their health. During an ordinary day, someone living in a typical urban environment may experience a wide range of sounds in streets, shopping centres, schools, workplaces, recreational facilities and the home. Even locations that were once quiet have become affected by noise, making it increasingly difficult to escape unwanted sound completely.

In its 1999 Guidelines for Community Noise, the World Health Organisation reported that noise-induced hearing impairment was the world’s most prevalent irreversible occupational hazard and estimated that 120 million people had disabling hearing difficulties. Evidence also indicates that excessive noise can produce numerous other adverse health effects.

Mark Stephenson, a senior research audiologist at the National Institute for Occupational Safety and Health, or NIOSH, explains that his organisation defines hazardous occupational noise as sound exceeding a time-weighted average of 85 dBA over a normal eight-hour working day. Different measurements and definitions may be used in other circumstances.

B According to NIOSH, approximately 30 million workers in the United States are exposed to hazardous sound levels at work. Industries with particularly large numbers of exposed employees include construction, agriculture, mining, manufacturing, utilities, transportation and the military.

Craig Moulton, a senior industrial hygienist with the Occupational Safety and Health Administration, acknowledges that monitoring noise throughout American industry is extremely difficult. Nevertheless, he explains that employers whose workers are overexposed to noise are required to protect them from its harmful effects. Employers must also establish an effective and continuing hearing-conservation programme in accordance with occupational noise regulations.

C Numerous scientific studies have confirmed that exposure to certain sound levels can damage hearing. Prolonged exposure may alter the structure of the delicate hair cells in the inner ear, resulting in permanent hearing loss. It can also cause tinnitus, which may be experienced as ringing, roaring, buzzing or clicking in the ears.

NIOSH research conducted during the mid- to late 1990s found that approximately 90 per cent of coal miners had some degree of hearing impairment by the age of 52, compared with only 9 per cent of the general population. The research also indicated that, by the age of 25, the average carpenter’s hearing could be equivalent to that of an otherwise healthy 50-year-old man who had not been occupationally exposed to noise.

William Luxford, medical director of the House Ear Clinic at St Vincent Medical Center in Los Angeles, points to one encouraging fact. Continued exposure causes hearing loss to progress, but once the exposure ends, the deterioration also stops. A change in a person’s environment can therefore prevent further damage and improve that individual’s prospects for maintaining healthy hearing.

In 2001, researchers from the Centers for Disease Control and Prevention reported the results of audiometric tests involving 5,249 children. They estimated that 12.5 per cent of American children had noise-induced hearing-threshold shifts—sometimes described as dulled hearing—in one or both ears. Most of the affected children had only limited damage. However, continued exposure to excessive noise could eventually cause difficulty with high-frequency sound discrimination. Potentially harmful sources included loud music, concerts, certain toys, lawnmowers and fireworks.

D The effects of noise do not end with the ears. Non-auditory effects are physical or psychological changes that do not involve hearing loss but can nevertheless be measured. These may include raised blood pressure, interrupted sleep, an increased heart rate, cardiovascular constriction, laboured breathing and changes in brain chemistry.

Such effects were being investigated as early as 1930. In a study published by E. L. Smith and D. L. Laird, exposure to noise was found to cause stomach contractions in healthy adults. Later research into the non-auditory consequences of noise has produced results that are sometimes contradictory or controversial.

Noise can also interfere with learning. In 1975, psychologist Arline Bronzaft collaborated on a study involving children attending a school beside an elevated railway line. Half of the pupils studied were taught in classrooms facing the railway, while the remainder were in classrooms in a quieter part of the building. Children on the quiet side performed better in reading tests; by the sixth grade, they were approximately one academic year ahead of pupils taught in the noisier rooms.

Bronzaft and the school principal subsequently persuaded the education authorities to install sound-absorbing acoustic tiles in the classrooms beside the railway. The Transit Authority also modified the nearby track to reduce the amount of noise it produced. A follow-up study published in 1981 found that the children’s reading scores improved after these measures had been introduced.

E Campaigners against noise argue that Europe and several Asian countries are more advanced than the United States in dealing with the problem. Population pressure encouraged European authorities to respond more quickly. In the European Union, cities with populations of at least 250,000 began producing noise maps to help decision-makers develop noise-pollution policies. Paris was among the first cities to complete such maps.

The information shown on the maps could be entered into computer models to predict the likely acoustic consequences of proposed street designs or new buildings before construction began. The maps were therefore intended not merely to record existing noise but also to support future planning decisions.

Some campaigners have called for a stronger government body with greater power to regulate noise. Other experts argue that regulation alone is not sufficient. Bronzaft believes governments around the world should devote more money to noise research and coordinate their efforts more effectively. This would allow health and environmental policies to be founded on reliable scientific evidence.