Exposure duration defined

The shear forces are mainly in the range of 1 to lONm. This exposure causes cell death between 20 and 80% depending on the exposure duration which is between a few seconds and several hours. Studies performed in a bioreactor have an exposure duration of several days. The results are partly contradictory. Tramper et al. [30] found a critical stress level of 1.5 Nm" for insect cells, whereas Oh et al. [31] could not show an influence on hybridoma cells even at high stirrer speed. This shows that each cell line reacts different and that there is a necessity for defined stress systems if the results is to be comparable. 

Eor the purpose of assessing the remaining interspecies uncertainty, Vermeire et al. (1999) collected and analyzed data for 184 chemicals tested in different species and via different exposure routes. NOAELs were selected from studies with mice, rats, and dogs exposed to the same chemical via the same exposure route and with the same duration of exposure. Two categories of exposure duration were defined, subacute and (sub)chronic, in order to increase the comparability of the different studies. The definition of these exposure categories is species specific, partly depending on their maximum lifetime. Subacute exposure was defined as 21-50 days for the mouse and rat, and as 28-90 days for the dog (sub)chronic exposure was defined as 90-730 days for the mouse and rat, and as 365-730 days for the dog. The oral NOAELs were adjusted to account for differences in metabolic size, i.e., by the caloric requirement approach (Section 5.3.2.3). 

Probabilistic exposure models attempt to provide inputs to exposure models by representing variability or uncertainty via frequency or probability distributions. Probabilistic methods can be used in the exposure assessment because pertinent variables (e.g., concentration, intake rate, exposure duration, and body weight) have been identified, their distributions can be observed, and the formula for combining the variables to estimate the exposure is well defined. 

Exposure is defined as the contact (at visible external boundaries) of an individual with a pollutant for specific durations of time (IPCS, 2004a). For exposure to occur, an individual must be present and must come in contact with a contaminated medium. Exposure usually results in absorbed dose when chemicals enter the body. Exposure is described in terms of the intensity, frequency, and... 

The time-of-wetness t, the amount of pollution P and the amount of chloride S are defined to estimate the corrosion rates of carbon steel, zinc, copper and aluminum and the resulting values for an exposure duration were obtained. Use of the different degrees of time of wetness, various values of pollution along with different amounts of contaminant such as chloride, the estimated corrosion rates13 of steel, zinc, copper and aluminum for exposure duration of one year are given in Table 2.2. 

After determining the health-effect endpoint to be used in deriving the value(s) for n, the next step is to evaluate the quality and the quantity of the data to be used in the derivation. Obviously, two data points will define the slope of a curve describing the exposure concentration-duration relationship. However, the validity and, hence, the values of n will depend on many factors, including the scientific soundness of the exposure concentration-duration data, the length of the empirical exposure duration(s) relative to the AEGL-specified exposure periods, and the known or perceived similarities in effects and mechanism of action of the chemical at the reported exposure concentrations and durations. Generally, three empirical data points will improve the scien... 

The third step is identification of the conditions of exposure (broadly defined to include intensity, frequency and duration) of the human population group that might be at risk and for which protection is sought( ). The last step involves... 

It is common practice for toxicologists to differentiate exposure to chemicals based on the dose and the duration of exposure. Lour timeframes have been used to define duration of exposures acute, subacute, subchronic, and chronic. It is useful in light of today s interest in long-term, low-levef exposures to clarify these terms. Acute exposure is defined as exposure to a chemical for less than 24 h. Subacute exposure refers to an exposure of 1 month or less, subchronic for 1 to 3 months, and chronic for more than 3 months. These exposures can be by any route for most chemicals it is the oral route with the chemical given in the diet. However, the limited animal studies using nerve agents have usually employed parenteral administration... 

For humans, the chronic exposure duration assumed for reference dose development is defined by EPA (1989) as lasting between 7 years (approximately 10% of a human lifetime) and a full lifetime. Calculation of an RfD requires use of animal or human toxicity data for a dose or expo.sure corresponding to a no-observed-adversc-effect level (NOAEL) or a lowest-observed-adverse-cffcct level (LOAEL) (EPA, 1989). The NOAEL is the exposure level al which there are no statistically or biologically. significant increases in frequency or severity of adverse... 

Toxic Chemical Exposure— the endpoint criterion is defined as a limiting concentration, often expressed in ppm or mg/m, over an exposure duration. Typical exposure durations may range from a few minutes to a period of several hours. 

Ideal probability of tumor response as a function of steady state drug concentrations (Css) over time. The therapeutic index is defined by the minimum effective steady state concentration (MECss) and the maximum tolerated steady state drug concentration (MTCss). The theoretical therapeutic index (Tt) lies between the MECss and the MTCss. The probability of tumor response can be evaluated as a function of Css and drug exposure duration (DED), which can be defined by the area under the concentration time curve (AUC). 

In this study, self-rescue is defined as being able to save oneself by evacuation to a safe location. In order to judge the effect of self-rescue improving measures, a model has been developed to quantify the effect of self-rescue, depending on exposure to fire or toxic chemicals. The model distinguishes heat radiation, smoke and several types of toxic chemicals and quantifies their effect on the walking velocity and maximinn exposure duration (see Fig. 1). 

Compared to the procedures proposed by other researchers [85], the procedure in this study can be used to obtain short-term data for GFRP bars successfully and efficiently. The proposed procedure can easily be carried out by defining simple plots and performing regression analysis. The results indicated that increasing the number of exposure temperatures and using longer exposure durations in accelerated tests can lead to more precise predictions. 

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