Threshold limit value-time weighted average concentration exposure

TLV-TWA is the Threshold Limit Value—time weighted average concentration for a conventional 8-hour workday and a 40-hour workweek, to which it is believed that nearly all workers may be repeatedly exposed, day after day, without adverse effect (ACGIH 2001). In general, TLV-TWA values are used to derive BEIs (Biological Exposure Indices). 

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. 

TIN-TWA Threshold limit value, time-weighted average for 8-h exposures, with ceiling concentrations shown as C, and with skin absorption hazard as S. The OSHA permissible exposure limit (PEL) is the lower of the TWA or the ceiling limit. 

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%. 

PEL (Permissible Exposure Limit) or TLV-TWA (Threshold Limit Value-Time Weighted Average). This is the maximum permitted concentration of the airborne chemical in volume parts per million (ppm) for a worker exposed 8 hours daily. 

The threshold limit value — time-weighted average for chloroform is 10 ppm. The permissible exposure limit is 50 ppm. No reference concentration has been set. 

If a toxic material is dispersed into the air, the engineer/scientist must know how high its concentration can be without causing danger to people. The Occupational Safety and Health Administration (OSHA) has set concentration levels for many substances. This concentration level is called the permissible exposure limit (PEL). The PEL is synonymous in most application with the TLV-TWA (threshold limit value-time-weighted average). 

Table I" summarizes the maximum permissable working concentrations of these substances in air. In Switzerland (CH) md Germany (GFR) they are called MAK-values, in the USA they eu e known as Threshold Limit Value-Time Weighted Average (TLV-TWA). CIBA-GEIGY established its own Permissible Internal Exposure Level (PIEL) for substances, such as bis(chloromethyl)ether, for which official levels are not available. 

Threshold Limit Value/Short-Term Exposure Limit (TLV-STEL). The concentration of a substance to which workers can be continuously exposed for up to 15 minutes at a time, four times a day, every day, without adverse effects. Threshold Limit Value/Time Weighted Average (TLV-TWA). The average concentration of a substance to which workers can be continuously exposed for eight hours a day, five days a week, without adverse effects. 

Threshold Limit Value/Time-Weighted Average-Ceiling (TLV-C) is the concentration that should not be exceeded during any part of the working exposure. 

---