Time-Weighted Average

Workers in manufacturing, construction and mining industries are often exposed to loud noise, particles, gases, vapors and other various elements.  Such hazardous physical and chemical factors of a working environment must be inspected to define Permissible Exposure Limits (PELS) and regulated to prevent overexposure.  The most practical method to measure one's daily exposure to such potentially dangerous materials is utilizing the Time-Weighted Average (TWA) calculation.  Sacremento State Water Programs defines the TWA calculation as an equation that takes into account the average levels of the substance or agent and the time spent in the area before being averaged to an eight-hour work shift of a forty-hour work week. 

Time-Weighted Average measurements are most often conducted by OSHA compliance officers who are experienced in different sampling methods.  Commonly, instruments are placed on or in close proximity of individuals at risk to observe and measure an accurate concentration or amount of hazardous material within the workplace. Concentrations of substance or agents are often represented in parts per million or parts per billion, depending on the volume in which they are measured.

Once concentrations are measured and recorded, the TWA calculation can be applied to the data.  First, the concentrations are multiplied by the time interval in which they were measured in.  Next, these results are summed and then divided by eight-hours to achieve a true time-weighted average.  Lastly, these time-weighted averages are then compared to the legal OSHA PELs to determine if exposure levels are within compliance or not.  If workers are discovered to be over-exposed to hazardous materials, OSHA compliance officers take whichever corrective action is necessary to decrease exposure limits.  Heavy fines and citations are often issued as a result of over-exposure observations.  Environmental health and safety managers work to regulate exposure levels throughout the eight-hour work day which increases workers' health and decreases the risk of fines or corrective actions. 

These are two different warnings for the potential risks of an overexposure to carbon monoxide and propane, respectively.  Carbon monoxide and propane are used in many industrial workplaces can produce harmful fumes, flames, and gases if not regulated and controlled.