Time weighted averages

In adopting the TLVs of the ACGIH, OSHA also adopted the concept of the TWA concentration for a workday. The 8h TWA is the average concentration of a chemical in air over an 8h exposure period. 

To illustrate the formula described above, assume that a substance has an 8 h TWA PEL of lOOppm. Assume that an employee is subject to the following exposure  

Two hours exposure at ISOppm Two hours exposure at 75 ppm Four hours exposure at 50 ppm 

Since 81.25 ppm is less than 100 ppm, the 8h TWA limit, the exposure is acceptable. 

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Time-weighted exposure. This is the time-weighted average concentration for a normal 8-hour workday or 40-hour workweek to... 

Ozone s presence in the atmosphere (amounting to the equivalent of a layer 3 mm thick under ordinary pressures and temperatures) helps prevent harmful ultraviolet rays of the sun from reaching the earth s surface. Pollutants in the atmosphere may have a detrimental effect on this ozone layer. Ozone is toxic and exposure should not exceed 0.2 mg/m (8-hour time-weighted average - 40-hour work week). Undiluted ozone has a bluish color. Liquid ozone is bluish black and solid ozone is violet-black. 

It is very poisonous, 50 mg constituting an approximate fatal dose. Exposure to white phosphorus should not exceed 0.1 mg/ms (8-hour time-weighted average - 40-hour work week). White phosphorus should be kept under water, as it is dangerously reactive in air, and it should be handled with forceps, as contact with the skin may cause severe burns. 

Exposure to chlorine should not exceed 0.5 ppm (8-hour time-weighted average - 40 hour week.)... 

Hydrogen selenide in a concentration of 1.5 ppm is intolerable to man. Selenium occurs in some solid in amounts sufficient to produce serious effects on animals feeding on plants, such as locoweed, grown in such soils. Exposure to selenium compounds (as Se) in air should not exceed 0.2 mg/m3 (8-hour time-weighted average - 40-hour week). 

Care should be taken in handling and using iodine, as contact with the skin can cause lesions iodine vapor is intensely irritating to the eyes and mucus membranes. The maximum allowable concentration of iodine in air should not exceed 1 mg/nu (8-hour time-weighted average -40-hour). 

Exposure limits (threshold limit value or TLV) are those set by the Occupational Safety and Health Administration and represent conditions to which most workers can be exposed without adverse effects. The TLV value is expressed as a time weighted average airborne concentration over a normal 8-hour workday and 40-hour workweek. 

The U.S. Department of Labor (OSHA) has ruled that an employee s exposure to dimethyl acetamide in any 8-h work shift of a 40-h work week shall not exceed a time-weighted average of 10 ppm DMAC vapor in air by volume or 35 mg/m in air by weight (7). If there is significant potential for skin contact with DMAC, biological monitoring should be carried out to measure the level of DMAC metaboHtes in urine specimens collected at the end of the shift. One industrial limit is 40 ppm DMAC metaboHtes, expressed as AJ-methylacetamide [79-16-3] for individuals, and 20 ppm metaboHte average for workers on the job (8). 

American Conference of Government Industrial Hygienists (ACGIH) threshold limit value = 5 ppm on an 8-h time-weighted average. 

The time-weighted average (TWA) concentrations for 8-h exposure to bromine ttifluoride, bromine pentafluoride, chlorine ttifluoride, chlorine pentafluoride, and iodine pentafluoride have been estabHshed by ACGIH on a fluoride basis to be 2.5 mg/m. NIOSH reports (121) the foUowing inhalation toxicity levels for chlorine ttifluoride LC q monkey, 230 ppm/h LC q mouse, 178 ppm/h for chlorine pentafluoride LC q monkey, 173 ppm /h mouse, 57 ppm/h. 

Handling and Toxicity. Tungsten hexafluoride is irritating and corrosive to the upper and lower airways, eyes, and skin. It is extremely corrosive to the skin, producing bums typical of hydrofluoric acid. The OSHA permissible exposure limits is set as a time-weighted average of 2.5 mg/kg or 0.2 ppm (22). 

OSHA has a standard time-weighted average (TWA) of 2.5 mg/m based on fluoride. NIOSH has issued a criteria document (24) on occupational exposure to inorganic fluorides. 

Threshold limit value, time-weighted average. 

Health and Safety Factors. MEK is slightly more toxic than acetone, but is not considered highly toxic, and nor does it exhibit cumulative toxicological properties. The OSHA time weighted average iu air is 200 ppm other measured toxicity values are shown iu Table 3. Methyl ethyl ketone is highly flammable. 

The 1994—1995 threshold limit values as recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) are given in Table 8. These time-weighted average values are those levels to which nearly all workers may be exposed for an 8-h workday and a 40-h work week without adverse effect (99). 

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). 

Phenol fumes are irritating to the eyes, nose, and skin. According to the National Institute for Occupational Safety and Health (NIOSH), exposure to phenol should be controUed so that no employees are exposed to phenol concentrations >20 mg/m, which is a time-weighted average concentration for up to a 10-h work day, 40-h work week. Phenol is very toxic to fish and has a nearly unique property of tainting the taste of fish if present in marine... 

Threshold limit value—time weighted average. Defined as the maximum time weighted average concentration to which a worker may be exposed repeatedly and without adverse effects for a normal 8 h/d, 40 h/wk period. 

Phosphine has an 8-h time-weighted average exposure limit of 0.3 ppm (13). Under alkaline conditions the rate of PH formation is high. At neutral or acidic pH, the PH generation is slow but stiU very ha2ardous if the PH is allowed to accumulate in a confined vapor space. The safest commercial handling conditions for molten phosphoms are generally considered to be from pH 6 to 8 at 45—65°C. 

Phosphoms(V) sulfide is a mild skin irritant and may cause dermatitis in sensitive individuals. The primary health ha2ard results from the Hberation of hydrogen sulfide after contact with moisture. Contact with moisture also forms phosphoric acid. A secondary ha2ard is the formation of sulfur dioxide when phosphoms(V) sulfide bums. The oral LD q of in rats is 389 mg/kg the OSHA standard time-weighted average (TWA) is 1 mg /m (33). 

Ceiling value concentration which should not be exceeded not appropriate to use a time-weighted average. 

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