Concentration limits, chemical

Flammable Limits in Air - This is a concentration expressed as percent by volume of the chemical in air, whereby spontaneous combustion will be supported. The lowest concentration where combustion will be supported is known as the lower flammability limit (LFL) or lower explosion limit (LEL). LEL and LFL are considered interchangeable. The upper concentration limit is the UFL (Upper Flammability Limit) or UEL (Upper Explosion Limit). 

Can work without conditioning chemicals Thickening solids concentration limited... 

Many very hazardous solvents, such as benzene and carbon tetrachloride, were widely used until the 1970s. The situation was very similar for the use of pesticides. Among the toxic pesticides that were still in wide use 20 years ago were chlorophenols, DDT, lindane, and arsenic salts, all of which are classified as human carcinogens as well as being acutely toxic. Fortunately, use of these kinds of very toxic chemicals is now limited in the industrialized world. However, because the number of chemicals used in various industries continues to increase, the risks of long-term health hazards due to long-term exposure to low concentrations of chemicals continues to be a problem in the workplace. 

While occupational hygiene measurements always measure only the concentrations of chemical compounds present in the occupational environment, i.e., the potential dose, the analysis of biological specimens predominantly reflects the body burden. Furthermore, biological monitoring is always limited to assessment of individual exposure. Personal occupational hygiene sampling takes into consideration only some of the individual factors, e.g., working... 

In addition, it may be necessary to limit permissible upward excursions from the TWA. In practice, concentrations of chemical agents in workplace air fluctuate frequently and to a considerable extent. The amount by which the OEL-TWA may be exceeded for short periods without impairment of health depends upon several factors, such as the nature of the substance, the frequency with which high concentrations occur, and the duration of such periods. 

Threshold limit value (TLV) The limits of airborne concentration of chemical substances that are allowed in workplaces published by the American Conference of Governmental and Industrial Hygienists (ACGIH). Also known as MAC. 

A complication here, however, is noted with those drugs that exhibit a limited chemical stability in either acidic or alkaline fluids. Since the rate and extent of degradation is directly dependent on the concentration of drug in solution, an attempt is often made to retard dissolution in the fluid where degradation is seen. There are preparations of various salts or esters of drugs (e.g., erythromycin) that do not dissolve in gastric fluid and thus are not degraded there but which dissolve in intestinal fluid prior to absorption. A wide variety of chemical derivatives are used for such purposes. 

Besides the LCA approach, also risk assessment can be performed analysing the chemical compounds or modelling via predictive exposure models. Both types of approaches have their justification to measure environmental concentrations of chemicals in the environment with laboratory measurement is still the most reliable way for determination. But it goes along with the disadvantage of high investments concerning time and money. Besides that laboratory approaches are limited in terms of space and time, and in consequence, the survey of many micro-pollutants and their... 

Characteristically, within certain concentration limits, if a chemical is absorbed by passive diffusion, then the concentration of toxicant in the gut and the rate of absorption are linearly related. However, if absorption is mediated by active transport, the relationship between concentration and rate of absorption conforms to Michaelis-Menten kinetics and a Lineweaver-Burk plot (i.e., reciprocal of rate of absorption plotted against reciprocal of concentration), which graphs as a straight line. 

The estimated ambient vapor-phase concentrations of chemicals in indoor air were, in general, below the NIOSH time-weighted average exposure limits for a 40 hr workweek. However, the authors of this study concluded that the consequences of long term human exposure to these complex mixtures of airborne... 

There are several standard tests for determining the flammability of materials (ASTM, 2004). For example, the upper and lower concentration limits for the flammability of chemicals and waste can be determined by standard test methods (ASTM D4982, E681), as can the combustibility and the flash point (ASTM D1310, E176, E502). With these definitions in mind it is possible to divide ignitable materials into two subclasses ... 

In another study, the average amount of 1,2-diphenylhydrazine consumed by wild deer mice over a 3-day period without killing more than 50% of the mice was determined to be 1213 mg/kg/day (Schafer and Bowles 1985). The validity of this finding is uncertain however, as the dose was estimated from consumption of seeds treated with only one concentration of chemical, only five mice were treated, and the actual number of deaths was not reported. Because of these limitations, the 1213 mg/kg/day dose is not a reliable effect level for lethality due to acute duration exposure. 

In occupational settings, the concentrations of chemicals are often monitored in the working environment to monitor compliance with occupational exposure limits as required by various national laws. Moreover, medical surveys of workers are often performed including analyses of biomarkers for exposure and/or effects. In addition, workers also generally have the possibility to report signs and symptoms of nuisances related to their working environment. Such data, which in some cases are available in the open literamre, are relevant for use in a hazard assessment. 

The T25 was originally proposed as a simplihed carcinogenic potency index as a practical method for potency considerations in carcinogen classihcation systems (Dybing et al. 1997) and is used within the EU context of classihcation and labeling of chemical substances (see Section 2.4.1.8) for inclusion of potency considerations in setting specihc concentration limits for carcinogens in Annex I of Directive 67/548/EEC (EC 1999). 

As discussed in detail in Sections C.3.d and C.3.e, the fastest atmospheric reactions of S02 are believed to be with H202 and perhaps with Os at higher pH values. Under extreme conditions of large fog droplets (—10 yu,m) and very high oxidant concentrations, the chemical reaction times may approach those of diffusion, particularly in the aqueous phase. In this case, mass transport may become limiting. However, it is believed that under most conditions typical of the troposphere, this will not be the case and the chemical reaction rate will be rate determining in the S(IV) aqueous-phase oxidation. 

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