【Basic Knowledge of Occupational Health】Time-Weighted Average Permissible Concentration

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This post was last edited by slll611 on 2016-4-24 17:37. What is the time-weighted average allowable concentration? GBZ2.1-2007 \"Occupational Exposure Limits for Hazardous Factors in the Workplace _ Part 1 _ Chemical Factors\" defines it as the average allowable exposure concentration over an 8-hour workday and a 40-hour workweek, with time taken as the weighting factor. The character is abbreviated as PC-TWA. To understand what weighting means, one first needs to understand what a “weight” is. In ancient times, a “weight” referred to the heavy iron piece on a scale that could be moved around to determine the mass. Mencius, Liang Hui Wang I says: “Only by weighing can one know what is heavy and what is light.” ”That’s exactly what it means. For example: When a school calculates final grades, the mid-term exam accounts for 30%, the final exam accounts for 50%, and homework accounts for 20%. If someone gets 84 points on the mid-term exam, 92 points on the final exam, and 91 points on homework, then the arithmetic average would be (84+92+91)/3 = 89 ； So after applying the weighting, the result is 84*30% + 92*50% + 91*20% = 89.4, and this is when the weights are known. What about in the case of unknown weights? I want to find the weighted average score for chemistry in two classes. Class 1 has 50 students with an average score of 80, while Class 2 has 60 students with an average score of 82. The arithmetic average is (80+82)/2 = 81. The weighted average is (50*80 + 60*82)/(50+60) = 81.09. There is another scenario similar to the first one, which is also a matter of human decision-making. For example, if you consider experts to have the highest influence, followed by teachers, with students having the lowest influence, and for a particular opinion rated out of 10, an expert assigns a score of 8, a teacher assigns 6, and a student assigns 7. But if you think the weights for experts, teachers, and students should be 0.5:0.3:0.2, then the weighted score would be 8*0.5 + 6*0.3 + 7*0.2 = 7.2, while the arithmetic average would be (8+6+7)/3 = 7. According to statistics, when calculating indicators such as the mean in statistical analysis, values that play a role in assigning importance to various variable values are called weights. GBZ2.1-2007 \"Occupational Exposure Limits for Hazardous Factors in the Workplace _ Part 1 _ Chemical Factors\" provides supplementary explanations regarding the application of PC-TWA: The 8-hour time-weighted average allowable concentration (PC-TWA) is a key indicator for assessing the environmental health conditions in the workplace as well as the exposure levels of workers. Evaluation of the effectiveness of controls against occupational disease hazards is necessary, such as during the completion inspection of construction projects, regular hazard assessments, and systematic exposure evaluations. When changes in production processes, raw materials, equipment, etc. require a re-evaluation of the impact on the working environment, it is particularly important to conduct tests and evaluations related to TWA. Individual testing is an ideal method for determining the TWA, especially suitable for assessing the actual exposure levels of workers; it represents the fundamental limit value for occupational exposure to hazardous factors in the workplace. Fixed-point monitoring is also a method for determining the TWA; it involves collecting samples at various times throughout a workday at a specific workplace. The time-weighted average concentration (TWA) for an 8-hour workday is calculated by taking the sum of the products of the duration of exposure at each time period and the corresponding concentration values, and then dividing this sum by 8. Fixed-point monitoring not only reflects individual exposure levels but is also applicable to assessing the hygiene conditions of the workplace environment.