Short-term concentrations measurements

Limited information was located regarding reducing body burden following exposure to CDDs in humans. A recent study examined the influence of short-term dietary measures on CDD and CDF concentrations in human milk (Pluim et al. 1994c). The authors hypothesized that mobilization of fatty acids from adipose tissue cause the concomitant release of CDDs and CDFs, which will then be eliminated in the breast milk. Two diets were tested for their ability to reduce the concentration of CDDs... 

Pluim HJ, Boersma ER, Kramer I, et al. 1994c. Influence of short-term dietary measures on dioxin concentrations in human milk. Environ Health Perspect 102 968-971. 

Because one has to consider unpredictably higher concentrations, especially in the case of the last two jobs mentioned, the use of respiratory protection is often necessary. For selection of the most suitable devices, concentration measurements have to be performed. It is important to consider the surrounding environment of workplaces if other persoimel occasionally work in these areas without respiratory protection. If jobs are of short duration, short-term measurements need to be performed to assess the exposure. A risk assessment of short-term exposures on the basis of an 8-h measurement is only possible in exceptional cases. For short-term exposure measurements, not every sampling technique is suitable. In this case, special sampling methods must be used in order to achieve a sufficiently low detection limit during short measurement times, especially for complex mixtures of substances. In many cases of this type thermodesorption has been proved to be the method of choice. 

Several measurement techniques exist for the determination of short-term concentrations. Depending on the exposure situation, there is a choice of easily usable, directly indicating sampHng devices (see Section 6.9.3.1), e.g., substance-unspecific analytical instruments Hke photoionization detectors (PID), flame ionization detectors (FID), infrared analyzers (see Section G.9.3.4), and specific sampling techniques based on adsorption on a substrate and thermodesorption (Section 6.9.4). 

Especially for the detection of short-term concentration peaks, sources of emission, and leaks, several types of analytical instruments have been developed which are able to determine substance-specific concentrations or sum parameters. Selection of the most appropriate device must be based on the measurement task. Not all instruments can be used for determination of the average shift concentration. 

A cocurrent evaporator train with its controls is illustrated in Fig. 8-54. The control system applies equally well to countercurrent or mixed-feed evaporators, the princip difference being the tuning of the dynamic compensator/(t), which must be done in the field to minimize the short-term effects of changes in feed flow on product quality. Solid concentration in the product is usually measured as density feedback trim is applied by the AC adjusting slope m of the density function, which is the only term related to x. This recahbrates the system whenever x must move to a new set point. 

When spills and releases of hazardous gases or liquids occur, the concentration of the hazardous material in the vicinity of the release is often the greatest concern, since potential health effects on those nearby will be determined by the concentration of the substance at the time of the acute exposure. There are many models of routine continuous discharges (e.g., discharges arising from leaky valves in chemical plants), but these carmot be applied to single episodic events. Research on the ambient behavior of short-term environmental releases and the development of models for concentration profiles in episodic releases are cmcial if we are to plan appropriate safety and abatement measures. 

Marttila et al. (1995) also examined the relationship between daily exposure to malodorous sulfur compounds (measured as total reduced sulfur [TRS]) from pulp production and experience of symptoms in a small population living in the vicinity of a pulp mill. The major components of the malodorous sulfur compounds are hydrogen sulfide, methyl mercaptan, and methyl sulfides. This work was initiated due to the observation that an unusually high short-term exposure to malodorous sulfur compounds (maximum 4-hour concentrations of hydrogen sulfide at 135 g/m3 [96 ppb]) led to a considerable increase in the occurrence of ocular, respiratory, and neuropsychological symptoms (Haahtela et al. 

Most of the recent attention has been given to the development of subcutaneously implantable needle-type electrodes [14, 15, 34, 38], Such devices track blood glucose levels by measuring the glucose concentration in the interstitial fluid of the subcutaneous tissue (assuming the ratio of the blood/tissue levels is constant). Subcutaneously implantable devices are commonly designed to operate for a few days and be replaced by the patient. Success in this direction has reached the level of short-term human implantation ... 

No human studies of sufficient exposure duration with measured concentrations producing irreversible or life-threatening effects were located in the available literature. However, the data of Barcroft (1931), a 1.5-min exposure at 500-625 ppm, and Bonsall (1984), a 6-min exposure at approximately 450 ppm, with recovery from symptoms and effects, can be considered short-term upper limits for healthy adults. 

It is assumed that the contaminant enters the water table or aquifer at a concentration near its solubility limit, although there is no practical means to verify this. This method is more favorable when the release occurred as a single, short-term episode. A long-term release from a continuing source would result in a date that more closely represents the last date upon which the contaminant entered the aquifer at or near its solubility limit. Should the contaminant enter the aquifer below its solubility limit, then a date earlier than the actual event would result. Conversely, should the contaminant enter the aquifer as NAPL for a period of time, a date in which all the NAPL dissolved in groundwater would result. If NAPL was present when measurements were obtained, then the zone of highest concentration would... 

To use a short-term detector tube for measurement of instantaneous concentration of a contaminant, the pointed ends are broken off and one end of the tube is inserted into a special hand pump supplied with the kit. The pump is then used to pump a specified amount of air through the tube. The technique is relatively simple and effective, although the results may not as precise as with more sophisticated methods. Depending on the contaminants and the conditions of use and the skill, there can be as much as a 25 percent error in some situations. 

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