TWA samples

An 8 hour time-weighted average limit (TWA) of 0.2 fiber per cubic centimeter (f jcc) of air based on an 8 hour time-weighted average (TWA) sampling period. This is the maximum level of airbome asbestos, on average, that any employee may be exposed to over an 8 hour period (normal work shift). 

With kinetic or TWA sampling, it is assumed that the rate of mass transfer to the sorption phase is linearly proportional to the difference between the chemical activity of the contaminant in the water phase and that in the sorption phase. During the initial phase of sampler exposure, the rate of desorption of analyte from the sorption phase to water is negligible and the sampler works in the linear uptake mode. The amount of analyte accumulated is therefore linearly proportional to its TWA concentration in water, even for situations where aqueous concentrations fluctuate over time (Figure 3.2). In this case Equation 3.1 reduces to... 

TWA sampling can be used in situations where analyte concentrations are variable and can be used to measure episodic pollution events. As integrative samplers permit the measurement of concentrations over extended time periods, they can provide a more realistic picture of contaminant levels than can be achieved by the collection of discrete spot samples of water. 

A number of common TWA samples were analysed in the participating laboratories. The results of these analyses are given in Table 6. Good agreement in the results for the analysis of these samples was obtained in view of their nature and the different detection systems employed. These data emphasise the superior specificity of the fluorescence detection system. 

TWA samples—Air is drawn through a tube containing a collecting solution or solid adsorbent like silica gel, charcoal, or a molecular sieve. 

Health and Safety. Remover formulas that are nonflammable may be used in any area that provides adequate ventilation. Most manufacturers recommend a use environment of 50—100 parts per million (ppm) time weighted average (TWA). The environment can be monitored with passive detection badges or by active air sampling and charcoal absorption tube analysis. The vapor of methylene chloride produces hydrogen chloride and phosgene gas when burned. Methylene chloride-type removers should not be used in the presence of an open flame or other heat sources such as kerosene heaters (8). 

Air samples are collected over an 8 hour work shift for MEK. Measurements showed 3 ppm after 2 hours, 24 ppm after 4.5 hours, 12 ppm after 6 hours, and 2 ppm after 8 hours. Caleulate the TWA concentration and compare this value against the OSHA safe level of exposure. 

An acceptable SSAHP will specifically address hazards of concerns, action levels, and protective measnres and techniqnes workers mnst nse. This will inclnde the nse of direct reading instrnments conpled with TWA-type sampling when warranted. 

The more usual case is for intermittent samples to be obtained, representing worker exposures at fixed points in time. If we assume that the concentration C, is fixed (or averaged) over the period of time 7), the TWA concentration is computed by... 

Equation 2.2 shows that Cs increases hnearly with time when the aqueous concentration is constant. This is why the initial stage of the nptake process is called the linear nptake phase (Figure 2.3). Integrating Eq. 2.2 over time shows that sampling is integrative , and that Cs is linearly proportional to the TWA concentration in the water phase (Cw.twa)... 

Perhaps the ultimate failing of the 0SHA/NI0SH scheme is that it bases important decisions on relatively small amounts of data. Intuitively, such a scheme would lead to incorrect conclusions in many cases. Table III gives the number of samples expected to be required for making decisions in various environments (calculated from the relationship derived in the appendix). As in the previous example the PEL is 10 and the AL is 5. In virtually all cases the number of samples is two or less. With such small sample sizes accurate prediction of the long-term rates of exposure is impossible without additional information or assumptions. Stated in slightly different terms, the interday variability of 8-hr TWA values cannot be measured or controlled for with information based strictly on such small sample sizes. 

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