|Fact sheet No 258: Occupational and Community Noise - February 2001 (WHO, 2001, 5 p.)|
Occupational sources of noise
The many and varied sources of noise in industrial machinery and processes include: rotors, gears, turbulent fluid flow, impact processes, electrical machines, internal combustion engines, pneumatic equipment, drilling, crushing, blasting, pumps and compressors. Furthermore, the emitted sounds are reflected from floors, ceiling and equipment. Noise is a common occupational hazard in many workplaces.
The major sources of noise that damages hearing are impact processes, material handling and industrial jets. (ref Noise Sources p. 89)
· Air jets - widely used, for example, for cleaning, drying, power tools and steam valves - can generate sound levels of 105 dB. (ref Noise Sources p. 89)
· Workers in a cigarette factory in Brazil involved in compressed air cleaning were exposed to sound levels equivalent to 92 dB for 8 hours. (ref. Noise Sources p.96)
· In the woodworking industry the sound levels of saws can be as high as 106 dB. (ref Noise Sources p. 95)
· Average sound levels range between 92 and 96 dB in industries such as foundries, shipyards, breweries, weaving factories, paper and saw mills. The recorded peak values were between 117 and 136 dB. (ref Noise Sources p. 100)
· In most developing countries, industrial noise levels are higher than those in developed countries.
· Noise-induced hearing impairment is the most common irreversible (and preventable) occupational hazards world-wide.
Cheaper, more cost-effective production is a driving force in economic development. However, new processes introduced on grounds of cost-effectiveness are often noisier than previous ones. The associated rise in noise levels is often overlooked. Thus, even though noise-reducing measures may have been incorporated in the design of machinery, greater output may generate higher noise levels. For example, for every doubling of the speed of rotary machines the noise emission rises by about 7 dB, of warp knitting looms - 12 dB, of diesel engines - 9 dB, of petrol engines - 15 dB, and of fans - between 18 to 24 dB.
· Exposure for more than 8 hours a day to sound in excess of 85 dB is potentially hazardous.(ref. Exposure criteria p. 78)
After exposure to a typical hazardous industrial sound around 90 dB for an 8-hour work day, the ear tires and hearing is temporarily impaired.(ref Pathophysiology of the Ear p.42
· Industrial workers exposed to noise often turn the volume of their car radios up when they leave work, but turn it down in the morning, because it is too loud. After a time, hearing recovery becomes less complete and impairment becomes permanent.(ref Pathophysiology of the Ear p.42) This can be noticeable within 6-12 months of starting a job where levels of sound are hazardous.(ref Pathophysiology of the Ear p.45)
· Transient tinnitus (ringing in the ear) is a common occupational hearing condition, especially in people exposed to impact noise. It should be considered as a warning of excessive exposure to sound and a trigger for appropriate preventive action. (ref. Pathophysiology of the Ear p.42)
Warning sounds: one sound can sometimes interfere with the perception of another. Because lower frequency sounds can mask higher sounds, warning sounds should be pitched at lower frequencies than the dominant industrial background noise. (ref Fundamentals of Acoustics p.22)