The terms Time-weighted average, short term exposure limit, exposure ceiling, and biological indicesare essential lexicons in the nomenclature of occupational health and safety.As guidelines of health hazard controls, they are particularly valuable for occupational health professionals who want to understand the concept of exposure thresholds. The purpose of this essay is to spell out their meanings and foci. The author shall give their definitions and a brief elucidation for each.
Time-weighted averages are used to analyze the daily exposure of a worker to hazardous substances like dust, chemicals, gasses, mists, fumes, vapors or even occupational noise (Checkoway & Rice, 1992, p 25-33). This includes the average levels of the specific substance and the time spent in a particular areaduring the work day. Occupational safety and health professionals calculate a time-weighted average by taking the portion of each period, multiplying it by the level of a substance during the period, and dividing it by the length of the workday, which is usually 8 hours.
Short-term exposure limit denotes tolerable exposure in the course of a short time period. The interval is normally 15 minutes and applies only if the time-weighted average is not surpassed (Ferguson, 1976, p 275-284). On the other hand, exposure ceiling is used to indicate the legal limit for exposure to chemical agent or substance that should not be exceeded, even instantaneously. It is normally applied to chemicals and irritants that have immediate effects.
Biological indices are guidance values used in the assessment and measurement of chemical determinant concentration in the biological media of exposed workers. They are generally indicators of the uptake of agents and substances and are used in evaluating biological monitoring reports(Pekari et al., 1992, p 317-322).
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