Time Weighed Average

To determine the time weighed averages (TWA) of toluene and methyl ethyl ketone (MEK) in a paint spraying booth. 

TEV-TWA Threshold limit value-time weighed average... 

A few of these devices rely on air contact with the tube through diffusion, and thus operate passively. Tubes used with battery-operated pump or those that sample the air through diffusion are used to determine time-weighed average concentrations of the workplace air contaminant of interest, while all the tubes used with a hand-operated vacuum pump are intended for measuring more or less instantaneous concentrations. The Safety Equipment Institute (USA) currently certifies chemical indicator tubes based on American National Standards Institute/In-ternational Safety Equipment Association standard 102 (1996). 

TABLE 6.2.2.1 American Conference of Governmental Industrial Hygienists Threshold Limit Values (TLVs) for Selected Chemicals, 1999-2000 " and Occupational Safety and Health Adminstration s Permissive Exposure Limits (PELs) Expressed as Time-Weighed Average (TWA), Short-Term Exposure Limit (STEL), and/or Ceiling (C)"... 

Time weighed exposure. This is the time weighted average concentration for a normal 8-hour workday or 40-hour workweek to which nearly all workers can be exposed, day after day, without adverse effects. Excursions above the limit are allowed if compensated by other excursions below the limit. 

The 2003 ACGIH threshold limit value-time-weigh ted average (TLV-TWA) is 10 mg/ m for total dust containing <1% quartz. 

Time-weigh ted average (TLV-TWA) designates the average concentration of a chemical to which workers may safely be exposed for 8 h per day and 5 days per week. 

Notes Particles can be measured with a variety of techniques. With cyclone or impactor separators, smaller fractions can be collected on filters. Mass can also be measured by using the optical properties of particles. For the most part, measuring particles requires equipment costing several hundred to a few thousand dollars. Filters must be pre- and post-weighed in a temperature- and humidity-controlled room TWA, time-weighted average TLV, threshold limit value TSP, total suspended particles and RSP, respirable particles. NA, not available. 

Lee-Feldstein (1986) studied a cohort of workers employed at a copper smelter. In the cohort employed between 1925 and 1947, the SMR for death from respiratory cancer was 203 for an estimated average exposure of 0.5 mg m-3 arsenic, 292 for exposures of 7 mg m-3 arsenic, and 444 for exposures of 62 mg m-3 arsenic. At a different smelter site, Enterline et al. (1987) reported that the SMR from respiratory cancer increased from 144 at a time-weighed exposure level of 400 pg As m-3 year-1 to 477 at an exposure level of 59 000 pg As m-3 years-1. 

Painters at a large shipyard exposed to EGEE and EGME (time-weigh ted average 0-80.5 mgm and 0-17.7 mg m , respectively) had an increased prevalence of oligospermia and azoospermia. In addition, a significant proportion of the painters were... 

Action level means employee exposure, without regard to the use of respirators, to an airborne concentration of lead of 30 micrograms per cubic meter of air (30 J.g/m ) calculated as an 8-hour time-weigh ted average (TWA). 

This test, which is frequently referred to as the Huey test, was first described and used by W. R. Huey in 1930, and since that time it has had wide application, particularly in the USA. The test consists of exposing the specimens (20-30 cmin fresh boiling 65% HNO3 (constant boiling mixture) for five successive periods of 48 h each under a reflux condenser. The specimens are cleaned and weighed after each period, and the corrosion rate (as a rate of penetration) is calculated for each period of test and for the average over the five periods corrosion rates are expressed as mm/y. The reason for the... 

An open toluene container in an enclosure is weighed as a function of time, and it is determined that the average evaporation rate is 0.1 g/min. The ventilation rate is 100 ft3/min. The temperature is 80°F and the pressure is 1 atm. Estimate the concentration of toluene vapor in the enclosure, and compare your answer to the TLV for toluene of 50 ppm. 

Fish. One year old Mt. Shasta strain rainbow trout (Salmo gairdneri) weighing an averaqe of 140g were used for the study and housed in circular tanks supplied with 15.2 1 of well water/min. at 11-12°C. Fish were fed twice daily for seven months prior to sacrifice at which time they weighed an average of 800g. 

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