Threshold limit values defined

Immission is the term for the air contamination affecting humans, animals or property. The maximum allowable workplace concentration, or TLV (threshold limit value) defines still-harmless levels of substances inherently dangerous to health under exposure for 8 h/d and 45 h/week. 

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. 

Inhalation. The threshold limit value of HCN is 4.7 ppm. This is defined as the maximum average safe exposure limit for a 15-min period by the Occupational Safety and Health Administration. Exposure to 20 ppm of HCN in air causes slight warning symptoms after several hours 50 ppm causes disturbances within an hour 100 ppm is dangerous for exposures of 30 to 60 min and 300 ppm can be rapidly fatal unless prompt, effective first aid is adininistered. There is always a small concentration of cyanide (0.02 to 0.04 mg/L) in the blood, and the body has a mechanism for continuous removal of small amounts, such as from smoking, by converting it to thiocyanate, which is discharged in the urine. 

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 Chemical Substances Threshold Limit Values Committee classifies certain substances found in the occupational environment as either confirmed or suspected human carcinogens. The present listing of substances that have been identified as carcinogens takes two forms those for which a TLV has b n assigned and those for which environmental and exposure conditions have not been sufficiently defined to assign a TLV. Where a TLV has been assigned, it does not necessarily imply the existence of a biological threshold however, if exposures are controlled to this level, we would not expect to see a measurable increase in cancer incidence or mortality. 

Filter samples can be prepared to airborne workplace concentrations by spiking each filter with aqueous solution containing elements with concentrations gravimetrically traceable to ultrapure metals or stoidiiometricaUy well defined oxides. The amormts correspond for some of the materials to current threshold limit values of contaminants in workroom atmospheres provided that the simulated filter has been exposed to one cubic meter of air. The certified values are based on a gravimetric procedure, i.e. weight per volume composition of the primary reference material dissolved in high purity sub-dis-tiUed acids. The National Institute of Occupational Health in Oslo, Norway, has produced several batches of such materials certified for 20 elements. Additionally, information values are reported for four other elements see Table 6.2. 

The numerical value shows the threshold limit value, which is defined as the maximum permissible vapor concentration that the average person can be exposed to for 8 h/day, 5 days/week without harm, in ppm (cm of solvent per 1 m of air). The mark T shows the solvent has Hsted in Title 111 of the Clean Air Act (42 U.S.C. 7401-7626 Public Law 159 from July 14,1955 69 Stat. 322) and the Amendments of 1990 as a hazardous air pollutant. Source Compton, S. and Brownlee, R., Biotechniques, 6,432-440,1988. With permission. 

Threshold Limit Value—Short-Term Exposure Limit. These are the concentrations to which workers can be exposed continuously for a short period of time without suffering (1) irritation, (2) chronic or irreversible tissue damage, or (3) narcosis of sufficient degree to increase the likelihood of accidental injury, or materially reduce work efficiency, and, provided that the daily TLV-TWA is not exceeded. A STEL is defined as a 15-minute TWA exposure that should not be exceeded at any time during a workday even if the 8-hour TWA is within the TLV-TWA. Exposure above the TLV-TWA up to the STEL should not be longer than 15 minutes and should not occur more than four times a day. There should be at least 60 minutes between successive exposures in this range. 

Evaluation of worker exposure requires samples in the breathing zone and in general room air or rest area. To define a potential hazard, check compliance with regulations or obtain data for control purposes, samples would normally be collected in the vicinity of the operation itself. In general, samples are collected in the vicinity of the workers directly exposed and also workers remote from the exposure who voice complaints. Sample duration requires that the sample contain sufficient matter for an accurate analysis and is based on the sensitivity of the analytical procedure and the estimated air concentration, as well as the current threshold limit value for the contaminant. Table 1 contains appropriate sampling duration, sample flow rates and sample volume. 

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. 

Exposure Limits PELs and TLVs. Threshold limit values (TLVs) were developed in the 1940s, long before the existence of OSHA. The TLV is defined as the airborne concentration of a contaminant to which it is believed that most workers may be repeatedly exposed, day after day, without developing adverse health effects. Developed and maintained by the American Conference of Governmental Industrial Hygienists (ACGIH), these values have been determined based on industrial experience and animal and human studies. 

Occupational exposure is defined as chronic exposure in amounts less than the threshold limit value causing material symptoms. As with many teratogens, critical... 

Environmental criteria have been established for many of these, but the utility and applicability of such criteria for indoor environments is controversial for at least four reasons. Eor example, the goals of the threshold limit values often do not include preventing irritation, a primary concern in indoor environments with requirements for close eye work at video display terminals. For most of the pollutant categories, the problem of interactions, commonly termed the multiple contaminants problem , remains inadequately defined. Even for agents that are thought to affect the same receptor, such as aldehydes, alcohols, and ketones, no prediction models are well established. Finally, the definition of representative compounds for measurement is unclear. That is, pollutants must be measurable, but complex mixtures vary in their composition. It is unclear whether the chronic residual odor annoyance from environmental tobacco smoke correlates better with nicotine, particulates, carbon monoxide, or other pollutants. The measure total volatile organic compounds is meanwhile... 

Gas treating is defined here as removal of H2S and CO2. Other sulfur compounds are discussed where applicable. Dehydration and sulfur production are not included, except for when sulfur production in liquid redox processes and selective H2S removal is being discussed. H2S must be removed from natural gas and process streams for health reasons and prevention of corrosion. Natural gas pipeline specifications require no more than 1/4 grain/100 SCF. This is equivalent to 4ppmv or 7 ppmw (for a 0.65 specific gravity gas). By comparison, the human nose can detect 0.13 ppmv and the threshold limit value for prolonged exposure is 10 ppmv. 

Employees are frequently monitored when working in an environment where exposure to toxic metals is a possibility. The most common form of monitoring involves quantification of airborne concentrations of metals in the production process. Threshold limit values for airborne concentrations and time-interval exposure concentrations are defined by the U.S. National Institute for Occupational Safety and Health (NIOSH) to ensure worker safety. Workers may also be monitored by quantification of biological samples. The most common sample used is a random urine sample, and results are expressed in concentration units for the metal of interest per gram of creatinine to normalize for... 

Threshold Limit Value - The term refers to toxicity by inhalation Hie abbreviation used is TLV. The ILV is usually expressed in units of parts per million (ppm) - i.e., the parts of vapor (gas) per million parts of contaminated air by volume at 2S °C (77 °F) and atmospheric pressure. For chemicals that form a fine mist or dust, the concentration is given in milligrams per cubic meter (mg/m. The TLV is defined as the concentration of the chemical in air that can be breathed for five consecutive eight-hour workdays (i.e., 40 hours per week) by most people without suffering adverse health effects. This is the definition given by the American Conference of (jovemmental Industrial Hygienists. 

Several organizations recommend limits of exposure to airborne contaminants in the workplace. These include the Occupational Safety and Health Administration (OSHA), the National Institute for Occupational Safety and Health (NIOSH), and the non-governmental organization, American Conference of Governmental Industrial Hygienists (ACGIH). The threshold limit value (TLV) for a substance is defined as the concentration level under which the majority of workers may be repeatedly exposed, day after day, without adverse effects. The TLV recommendations are given in two forms ... 

For more or less all non-genotoxic carcinogens, a threshold limit value exists where there is no real risk of cancer. Taking this into account, some scientific associations have defined additional categories of carcinogenic substances which have no real carcinogenic potential below their occupational exposure limit. Well-known examples of such substances include sulfuric acid aerosols, hexachloro-benzene, trichloromethane, tetrachloromethane, lindane, dioxane, and 2,3,7,8-tet-rachlorodibenzodioxin (TCDD). 

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