Time-weighted averages , process

This knowledge is also important in order to determine if air tests conducted by OSHA compliance officers are valid. For example, if threshold limit value in the health standard is an 8-hour time-weighted average, the air sample should be obtained by sampling over the entire shift in the employee s breathing zone. It cannot be measured by a few short term samples, even if spaced over the full shift unless the worker is in a relatively fixed location with no variation in his work procedure or in the process. Such an event is generally the exception rather than the rule. 

An example of a nonoccupational exposure is methanol, which is formed endogenously, probably as the result of the activities of intestinal flora or enzymatic processes. It is present in a number of consumer products. Methanol may be present in low concentrations in some foods, juices, and alcoholic beverages. Methanol can also be derived from the intestinal enzymatic hydrolysis of the artificial sweetener aspartame, which results in methanol absorption from the intestine (Butchko et al. 2002). It is estimated that a 355-mL serving of aspartame-sweetened beverages and of various fruit and tomato juices may contribute about 20-100 mg of dietary methanol (Butchko et al. 2002). For comparison purposes, exposure at the current Threshold Limit Value time-weighted average of methanol (262 mg/m3) would result in a daily dose of about 1,500 mg, assuming an 8-hour inhaled volume of 10 m3 of air and absorption of 57%. 

The Occupational Safety and Health Administration (OSHA) has set the permissible exposure level (PEL) of vinyl chloride (VC) at 1.0 part per million (ppm) as a maximiun time-weighted average (TWA) for an 8-h workday. The PEL was set at 1.0 ppm because vinyl chloride is a suspected carcinogen. Thus, if vinyl chloride escapes into the air, the concentration of vinyl chloride must be maintained at or below 1.0 ppm. The major source of VC escape into the workplace air in typical process conditions is fugitive emissions from pipe connections such as valves, flanges, and pump seals. 

Unsaturated polyester thermoset systems use styrene as a reactive dilnent (sometimes along with substituted styrenes such as vinyl styrene or methyl methacrylate). Some of the styrene evaporates in the process (the emission factor is estimated at about 1-3% of the styrene for composites and a little higher for continuous lamination) and poses a threat as a hazardous air pollutant. The time-weighted average (TWA) for styrene is 50 ppm (or 213 mg/m ). In 1996 the Occupational Safety and Health Administration (OSHA) endorsed a proposal by the styrene industry to voluntarily adopt the 50-ppm exposure limit. 

PSM Process Safety Management of TWA Time-weighted average... 

Determine the time weighted average concentration of mercury in breathing zone air that gas operations personnel are exposed to during specific work tasks and processes (Personal Exposure Sampling and Analysis Plan)... 

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