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.

The Dose-Response Relationship

Understanding the dose-response relationship is essential for multiple scientific studies and disciplines such as pharmacology, chemistry and biology.  In particular, dose-response relationships are studied prevantly in Toxicology closely studies the various predictions and implications of dose-response relationships. The Dose-response relationships are so crucial to the field of toxicology simply because a wide variety of toxic reactions (responses) exist, which correspond to the amount or concentration of a toxic substance (dose) administered to an individual.  Since the human body is so intricate and connected, dose-response relationships must be studied carefully. The Merck Manual importantly notes that "the response to concentration [of dose] may be complex and is often nonlinear".

Before recording data and constructing the curve of dose-response relationships, one must first understand the relationships' implications. Extension Toxicology Network (EXTOXNET) define two implications: that there is almost always a dose in which no response occurs or can be measured below a certain level and that any further increases in a dose or concentration will not result in any increased effect once the maximum response is achieved. These two implications are helpful in defining the threshold of a fatal response for any given toxic substance.

Two types of curves can be constructed to further understand the potential hazard of a given toxic substance: one that displays and describes the responses of a dose to an individual, and one that displays and describes the responses of a dose to a population.  For both curves, the dose is represented on the x-axis as a function of time and the response is represented on the y-axis. Additionally, both curves most always form in a hyperbolic fashion. The visual representation of dose-response relationships in a mathematical form allow toxicologists to define the threshold of responses for a toxic substance more clearly.

The development of dose-response relationship studies have been extremely useful for administering and prescribing drugs in medical environments.  By understanding dose-response relationships, millions of lives have been saved and improved. Toxicologists will continue to utilize curves and dose-response experiments to prevent the detrimental consequences of toxic substances.  

These pictures show warning labels of a bottle nail polish remover and Bacardi rum.  These warnings were most likely based on a population dose-response curve rather than an individual dose-response curve.