Hazard PreventionHazard Prevention W Whole - Body

www.asse.org JULY 2005 PROFESSIONAL SAFETY 35 WHOLE-BODY VIBRATION (WBV) is the vibration transmitted to a person’s entire body via his/her contact with a vibration source, usually through sitting or standing on a vibrating surface. WBV is a common occupational problem for workers in highvibration environments, particularly when the exposure represents a significant part of their working day, not simply an intermittent event. Exposure to vibration appears to be increasing throughout the working world, with high-vibration environments now representing a significant proportion of all occupations. In Europe, Canada and the U.S. for example, it has been estimated that up to seven percent of all workers are regularly exposed to WBV (Bovenzi and Hulshof). In Great Britain, approximately nine million people are exposed to some form of WBV every week [Palmer, et al(a)]. More than 400,000 cases of low back pain (LBP) in the U.K. may be attributable to occupational WBV [Palmer, et al(b)]. By category, high-vibration workplaces are numerous and may include crane operators [Bongers, et al(a)]; agricultural tractor drivers (Futatsuka, et al; Bovenzi and Betta; Boshuizen, et al; Sjoflot); freight container tractor drivers (Nishiyama, et al); heavy equipment operators [Waters, et al; Teschke, et al; Kittusamy(b); Kittusamy and Miller]; all-terrain vehicle drivers (Rehn, et al); rally car drivers (Mansfield and Marshall); taxi drivers (Chen, et al; Funakoshi, et al) and garbage truck drivers (Maeda and Morioka). Similarly, pilots—particularly helicopter pilots—may be exposed to WBV at significant rates [Lopez-Lopez, et al; de Oliveira, et al; Bongers, et al(b)]. Other studies have shown that WBV exposure can also originate from less-well-defined sources such as high-speed trains (Sumitomo, et al); manual wheelchairs [Maeda, et al(b)]; foundry work (Armstrong, et al); road-traffic noise [Maeda, et al(a)]; and nonspecific low-frequency noise (Takahashi, et al). Table 1 highlights some industries and occupations affected by WBV. Biomechanics of Whole-Body Vibration Many biodynamic models have been developed to help understand the biomechanics of WBV (Kitazaki and Griffin; Seidel, et al; Yue and Mester). A key concept is the fact that all objects have a speed at which they naturally vibrate. This phenomenon, which depends greatly on the physical characteristics of the object, is termed its resonant frequency (RF). When objects reach their RF, the maximum amplitude of their vibration increases. For humans, there is no single, definitive RF because various parts of the body with differing physical characteristics (such as density and mass) tend to vibrate at different frequencies. Nevertheless, a critical range has been proposed, with vibrations between 0.5 Hz (cycles per second) and 80 Hz generally regarded as having significant effects on the human body. For vertical vibration, the most important RF appears to be between 4 and 8 Hz. Although the RF of specific body parts varies, vibration amplitudes between 3 and 5 Hz are known to generate strong resonances in the neck. When seated, it has been suggested that frequencies between 4 and 7 Hz are the most important for the spine (El-Khatib and Guillon). For the lower back, the natural frequency appears to be around 4.5 Hz (Pope, et al). At frequencies between 20 and 30 Hz, RF amplitude may increase up to 350 percent between the head and shoulders (Hedge). The physical process between vibrating energy transfer and the human body essentially involves two components. First, energy flows from the vibrator and into the human body via a Derek R. Smith, Ph.D., M.P.H., is a researcher at the National Institute of Industrial Health in Japan. Smith is also a visiting senior research fellow at James Cook University in Australia. He earned his B.S. from Griffith University, Master of Health Science (Occupational Health) from Queensland University of Technology, Master of Public Health from James Cook University and Ph.D. in Occupational Health from the University of Southern Queensland.