Time-weighted average concentrations TWAs

Time-weighted average concentration, TWA, based on a 40-hour work week. 

Time Weighted Average Concentration (TWA) Tlie concentration of a substance to which a person is exposed in the ambient air, averaged over a period, usually 8 hours. 

In the U.K., the Health and Safety Executive (HSE) publish recommended levels for occupational exposure as maximum exposure limits (MEL), which are reasonably practicable for all work activities and occupational exposure standards (OES) which cover realistic plant design, engineering and control of exposure and can be used to help in the selection of personal protective equipment. Further subdivision of limits occurs into long-term (usually 8-hour) and short-term (10-minute). Short-term limits are of use where brief exposure may cause acute effects. Both levels are expressed as time-weighted average concentrations (TWAs) over the specified period. Studies of OELs in other countries reveal that there are some differences, but generally they fall close to each other in industrial societies. ... 

Time weighted average concentrations (TWA) The limits refer to the maximum exposure concentration when averaged over a 10-minute period or an 8 hour day. The time weighted average value (Cm) may be obtained from the following formula ... 

T T,-STEL is a I5-min, time-weighted average concentration to which workers may be exposed up to four times per day with at least 60 min between successive exposures with no ill effect if the TLV-TWA is not exceeded (developed by the ACGIH). 

TLV-TWA is the time-weighted average concentration hmit for a normal 8-h day and 40-h worlweek, to which nearly all workers may be repeatedly exposed, day after day, without adverse effect (developed by the ACGIH). 

TLV-TWA is defined as the time-weighted average concentration for a normal 8-hour workday and a 40-hour workweek, to wliich nemly all workers may be repeatedly c.xposcd, dtiy after day, witliout any long-tenii adverse effect. 

Class A signifies refrigerants for which toxicity has not been identified at concentrations less than or equal to 400 ppm, based on data used to determine Threshold Limit Value-Time Weighted Average (TLV-TWA) or consistent indices from Section 6.1.2. ... 

With regard to limiting the concentration of particular contaminants, these are generally referred to in terms of threshold limit values (TLV), expressed either in terms of a time-weighted average (TLV - TWA) which represents the average concentration for a normal working day over a 40-hour week to which nearly all workers may be exposed on a repeated basis or as a short-term exposure limit (TLV - STEL), which represents the maximum... 

Short-Term Exposure Limit (STEL)—The American Conference of Governmental Industrial Hygienists (ACGIH) maximum concentration to which workers can be exposed for up to 15 min continually. No more than four excursions are allowed per day, and there must be at least 60 min between exposure periods. The daily Threshold Limit Value - Time Weighted Average (TLV-TWA) may not be exceeded. 

Time-weighted average (TWA) TLV - the time-weighted average concentration for a normal 8 hr work day and a 40 hr work week, to which it is believed that nearly all workers may be repeatedly exposed, day after day, without untoward effects. TWA TLVs permit excursions above the TLV provided that they are compensated for by equivalent excursions below the TLV during the work day. The excursion above the TLV is a rule of thumb, as explained in the source reference. 

A direct method for determining worker exposures is by continuously monitoring the air concentrations of toxicants on-line in a work environment. For continuous concentration data C t) the TWA (time-weighted average) concentration is computed using the equation... 

Threshold Limit Value Airborne concentrations of substances devised by the ACGIH that represent conditions under which it is believed that nearly all workers maybe exposed day after day with no adverse effect. TLV s are advisory exposure guidelines, not legal standards, based on evidence from industrial experience, animal studies, or human studies when they exist. There are three different types of TLV s Time Weighted Average (TLV-TWA) Short Term Exposure Limit (TLV-STEL) and Ceiling (TLV-C). (See also PEL)... 

The odor of cresol is recognized at concentrations as low as 5ppm. The 2003 ACGIH threshold limit value-time-weighted average (TLV-TWA) for all isomers of creosol is 5 ppm (22mg/m ) with a notation for skin absorption. 

Collection should befor at least 1 h, orsay 10 times as long as is needed for the color to change from reddish-purple to blue, which occurs after about 30 yg of NaOH has been collected. Collection may have to be for as long as 3 h if the concentration of NaOH is well below the Threshold Limit Value / Time Weighted Average (TLV-TWA), viz. 2 mg/m3 of air. 

TWA Time-weighted average concentration for a conventional 8-hour workday and a 40 hour workweek. 

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