Measuring The Risk From Whole Body Vibration

Human response to whole body vibration depends on the frequency of vibration, acceleration (or magnitude) of the vibration, and how long a person is exposed to the vibration. Because of the difficulty of evaluating the response to vibration and inconsistencies in quantitative data obtained from research, the International Standards Organization (ISO) 2631/1, Evaluation of human exposure to whole body vibration, has been established. When using these criteria and limits, it is important to bear in mind the restrictions placed upon their application. Some research indicates that the standards are not low enough and that musculoskeletal disorders are caused from exposure to vibration levels below the standard.

   This standard is applicable only to situations involving people of normal health: that is persons who are considered fit to carry out normal living routines, including travel, and to undergo the stress of a typical working day or shift. The standard provides numerical limits for exposure to vibrations transmitted from solid surfaces to the human body in the frequency range of 1 to 80 Hz.

   The standard addresses three different levels of concern: Reduced Comfort, Fatigue Decreased Proficiency, and Exposure Limits.

1. Reduced Comfort Boundary is applicable where passenger comfort is of concern, for instance on trains, subways, and buses. This limit will not be addressed here.

2. Fatigue Decreased Proficiency Boundary is applied to the situations where maintaining operator efficiency of a vehicle is of concern, such as situations where operators are required to work with safe manipulation of controls or to read the gauges accurately.

3. Exposure Limit applies to situations where the health and safety of the worker, such as back injuries and injuries to internal organs, is of concern.

   Vibration is measured in three directions; longitudinal (buttocks to head - az), and two transverse directions (chest to back - ax, and right to left side - ay). When vibrations occur in more than one direction simultaneously, the effect on comfort and performance of the combined motion can be greater than that of any single component. In order to simplify measurements and comparisons of a vibration environment for the frequency range of 1 to 80 Hz, with respect to its effect on the worker, weighted accelerations can be determined. When the weighted acceleration (ax, ay, az) are combined, the resultant acceleration is the vector sum, a. This amount of the vector sum can be used primarily for comparison with the vector sum of other motions.