Time-weighted average calculation

Another reason for a large amount of monitoring is to establish a baseline of exposure. Environmental standards requiring time-weighted-average calculations require monitoring for a specified time period. Some standards also require monitoring at specified time intervals. 

This exposure is not excessive since 7 ppm is below the 8-hour time-weighted average of 10 ppm. Mixtures of air contaminants are additive and calculated in the manner described previously that is, the fractions (concentration divided by limit for each material) are added together and if the sum does not exceed unity, the exposure is not excessive. 

Calculation of the occupational exposure concentration (OEC) depends on the type of OEL. For example, when the limit value has been set as an eight-hour time-weighted average, the cumulative exposure for an eight-hour work shift should be computed as follows ... 

THRESHOLD LIMIT VALUE DS2 is made of two major components (EGME DETA) with different toxicities and physical properties. The TLV of the mixture (calculated) is 5.2 mg/m3 as an 8-hour time weighted average (TWA). To date the Occupational Safety and Health Administration (OSH A) has not promulgated a permissible exposure limit for DS2 nor has the value proposed been officially adopted as a part of a special occupational safety and health standard for DS2 according to DOD 6055.1. 

Threshold limit value-time-weighted average (TLV-TWA) is the TWA concentration for a normal 8-hour workday or 40-hour workweek to which nearly all workers may be repeatedly exposed, day after day, without adverse effect. Time-weighted averages allow certain permissible excursions above the limit provided that they are compensated by equivalent excursions below the limit during the workday. In some instances the average concentration is calculated for a workweek rather than for a workday. 

Worker exposure to alkanes, especially the lower-molecular-mass compounds, is most likely to come from inhalation. In an effort to set reasonable values for the exposure by inhalation of vapors of solvents, hydrocarbons, and other volatile organic liquids, the American Conference of Governmental Industrial Hygienists sets threshold limit values (TLVs) for airborne toxicants.1 2 The time-weighted average exposure (E) is calculated by the formula... 

According to Busch and Liedel (13), one can easily calculate an Upper Confidence Limit (UCL) at 95% confidence for a single 8 hour-time weighted average sample by the following equation ... 

Effects noted in study and corresponding doses Time-weighted average doses (as mg U/kg/day) calculated by the authors from fluid intake data were males 0.05, 0.20, 0.88,4.82, and 28.70 mg U/kg/day females 0.49, 1.32, and 43.02 mg U/kg/day. Four males showed evidence of Pasteurella multocida infection and were excluded from the study. Two other males in the highest dose groups died prematurely, one from apparent mucoid enteritis and one from apparent acute renal failure. Two others were removed after developing hairball obstructions of the GI tract. No evidence of Pasturella infection... 

Within the refinery fence Une, maximum observed and/or calculated airborne concentrations of chemicals reported in the airnual TRI were below OSHA action and permissible exposure levels for 8-hour time-weighted average exposures. Impacts on air quality by the refinery were calculated using air dispersion modeling techniques and the emissions inventory developed during this study. 

Comparison of the urinary trichloro-compound levels with tetrachloroethylene in the environment revealed that while the metabolite levels increased essentially parallel to PERC concentrations up to 100 ppm, leveling off was apparent in the metabolite excretion when the exposure to PERC was more intense (e.g., more than 100 ppm), indicating that the capacity of humans to metabolize this chlorinated hydrocarbon is rather limited. A tentative calculation indicated that at the end of an 8 h shift with exposure to tetrachloroethylene at 50 ppm (time-weighted average, TWA), 38% of the PERC absorbed through the lung would be exhaled unchanged, <2% would be metabolized to be excreted in the urine, while the rest would remain mostly in the fat stores of the body to be eliminated later. 

The solid sorbents are desorbed with a suitable solvent in a similar manner as discussed for tube/pump sampling. The desorption efficiency (DE) must also be determined for each compound. Once the mass of the contaminant (M) is determined, its time-weighted-average concentration (C) in air can be calculated ... 

In general, all work involving possible exposure to lead has to be assessed, and the level of exposure has to be determined, either in the air or in the blood. The maximum allowed concentration of lead in the air is set at 150 pg m , calculated as a time-weighted average over 40 h per week. For blood, the maximum allowed lead level is 70 pg/100 mL blood of an individual worker. If one of these occupational exposure levels is exceeded, immediate protective measures are necessary to remedy the situation, including intensive medical surveillance. 

Annex II of European directive 98/24/EC is designed to contain a list of biological limits and provisions for health surveillance for defined chemical agents. At the present time, only one limit value for lead and its ionic compounds has been determined, which is 70 pg/100 mL blood. Medical surveillance is required, if the blood lead level is greater than 40 pg/100 mL blood or if the concentration of lead in the air has been found to exceed 0.075 mg m calculated as a time-weighted average over 40 h per week. For more information see Section 6.5.3. 

Measured or calculated in relation to a reference period of eight hours as a time-weighted average. 

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