Time-weighted average, defined

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. 

Define what Ceiling, Short term limit, and time weighted average values are. 

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. 

Toxicity is referenced to the Threshold Limit Value — Time Weighted Average established for each refrigerant. This is defined in ASHRAE Standard 15-1994 as (refer to the manufacturer s product data for more complete detail) ... 

The next most important factor to consider when assessing dust clouds is the actual amount of dust present. This is known as the concentration, and is defined as follows. A substantial concentration of dust should be taken as concentration of lOmg/m 8-hour time-weighted average of total inhalable dust or 5 mg/m 8-hour time-weighted average of respirable dust where there is no indication... 

Recommended Exposure Limit (REL) was developed by the National Institute for Occupational Safety and Health (NIOSH) and defines the highest allowable airborne concentration that is not expected to injure a worker expressed as a ceiling limit of time-weighted average for an eight- to ten-hour work day. 

The TLV is not a single entity. The ACGIH defines three categories of TLV, namely, the time-weighted average (TLV-TWA), the short-term exposure limit (TLV-STEL), and the ceiling. Because these are proprietary terms, the U.S. Occupational Safety and Health Administration (OSHA) has adopted different terminology for essentially the same concepts, as follows ... 

A STEL is defined as a 15-min time-weighted average exposure which should not be exceeded at any time during a workday even if the 8-h TWA is within the TLV. Exposures at the STEL should not be longer than 15 min and should not be repeated more thanfour times per day. There should be at least 60 minutes between successive exposures at the STEL. An averaging period other than 15 min may be recommended when warranted by observed biological effects. 

A single sample or the time weighted average of several consecutive samples taken for the entire time period for which a standard TLV is defined yields the best estimate of the true average concentration of the airborne contaminant. This type of sample is referred to as a "full-period" sample. Typically, a full-period sample would have to be 8 to 36% above the standard in order to demonstrate noncompliance with 95% confidence. 

As an index for toxicity of gaseous substances, the median lethal concentration (LC 5 0 ) and the threshold limit value (TLV) are used to evaluate acute toxicity. LCso is the gas concentration (ppm) in air which results in the death of 50% of a population of test animals by inhalation. TLV is the threshold limit of exposure which does not cause any serious sickness in workers under normal conditions of an 8 hours day and a 40-48 hour work week. TLV is usually given in combination with the time-weighted-average concentrations for 8 hours per day (TLV—TWA), or with the short time exposure limit (TLV-STEL), which is that exposure which should not be exceeded even for a relatively short period (15-30 minutes), or with the ultimate limit (TLV-C), which defines the exposure level that should not be exceeded under any circumstances. 

OSHA PEL-TWA (Occupational Safety and Health Administration, Permissible E q)osure Limits — Time Weighted Average NIOSH, 2005) is defined analogous to the ACGIH-TLV-TWA, but is for e q)osures of no more than 10 h/day, 40 h/week. 

Occupational exposure to phosgene" has been defined by NIOSH [1484] as exposure above half of the recommended time-weighted average (TWA) environmental limit whilst "overexposure" is defined as known or suspected exposure above either the TWA or ceiling concentration, or any exposure which results in the development of pulmonary symptoms. 

As an air-borne dust, its long term exposure limit (8-hour time-weighted average) is 10 mg/m of total inhalable dust and 5 mg/m of respirable dust (respirable being defined in [34.1]). More stringent standards apply if the crystalline silica content exceeds 1 %. 

Annex II of European directive 98/24/EC is designed to contain a list of biological limits and provisions for health surveillance for defined chemical agents. At the present time, only one limit value for lead and its ionic compounds has been determined, which is 70 pg/100 mL blood. Medical surveillance is required, if the blood lead level is greater than 40 pg/100 mL blood or if the concentration of lead in the air has been found to exceed 0.075 mg m calculated as a time-weighted average over 40 h per week. For more information see Section 6.5.3. 

In the European community, the occupational exposure level is defined as the limit of the time-weighted average of a chemical agent in the air within the breathing zone of a worker in relation to a specified reference period, usually 8 h or a work shift. In addition, to avoid adverse health effects, there is a need to set limits for short-term exposure peaks. In the occupational safety legislation of most countries within the European Community the lists of occupational exposure levels include limits for short-term exposure for many substances, either in the form of definite concentration values or so-called short-term exposure factors. In case of factors, the allowed maximum short-term concentration is calculated by multipHca-tion of the OEL by the short-term exposure factor. Typically, many hazardous substances have factors within the range of 1-4. 

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