15% of the population in the member countries of the OECD, that is over 100 million people, are exposed to an external noise (apart from the noise in the place of work) exceeding 55 dB. Continued daily exposure to noise over a period of years can cause hearing loss which may vary from partial to 'complete'. It has been estimated that there are about 500 professions and occupations which under the present conditions of industrial production involve the danger of impairment of learning due to noise.
Sound is measured in units called decibels (dB). Sound is generally considered to be dangerously loud when long-term exposure exceeds an 85 dB level. Due to the upward spiral in industrial technology and the increase of labour saving gadgets, noise sources have greatly proliferated during the past 20 years. Some countries including the USA have set the permissible noise exposure for workers at 90 dB for a duration of 8 hours a day. Even under such standards, it has been found that one-fifth of the exposed work force will suffer a disabling loss of hearing, and several countries have therefore lowered the limit to 80 dB (for example, the Netherlands). Present knowledge indicates that an upper limit of 75 dB would considerably reduce the risk for noise-induced hearing loss, as well as for other adverse effects of noise. Efforts to lower the decibel level have been intensified as a result of the remarkable increase in the number of industrial workers with hearing impairment. In Sweden, for example, 16,000 cases of hearing loss due to exposure to industrial noise were reported in 1977, compared to 5,000 in 1973.