Exposure dose

The linear relation between the PSL signal and neutron fluence was established to exist over three decades up to 6. lO cm. The IP-ND can be used as fast and efficient neutron monitors at rather low neutron fluxes. However, for neutron dosimetry individual IP-ND must be calibrated individually since their sensitiviy can differ from one plate to another. In Gd/film based direct NR the film fog below the exposure dose of about 8.10 cm is the limiting factor. 

If possible, there should be measurement of the toxic effect in order quantitatively to relate the observations made to the degree of exposure (exposure dose). Ideally, there is a need to determine quantitatively the toxic response to several differing exposure doses, in order to determine the relationship, if any, between exposure dose and the nature and magnitude of any effect. Such dose—response relationship studies are of considerable value in determining whether an effect is causally related to the exposure material, in assessing the possible practical (in-use) relevance of the exposure conditions, and to allow the most reasonable estimates of hazard. 

In addition to the effect of biological variabihty in group response for a given exposure dose, the magnitude of the dose for any given individual also determines the severity of the toxic injury. In general, the considerations for dose—response relationship with respect to both the proportion of a population responding and the severity of the response are similar for local and systemic effects. However, if metabohc activation is a factor in toxicity, then a saturation level may be reached. 

Evaluate the hazard for noncarcinogcnic clTccls as a ratio of exposure dose to the recommended RfD. 

Variability arises from true heterogeneity in characteristics such as dose-response differences within a population, or differences in contaminant levels in tlie enviromiient The values of some variables used in an assessment change witli time and space, or across tlie population whose exposure is being estimated. Assessments should address tlie resulting variability in doses received by members of the target population. Individual exposure, dose, and risk can vary widely in a large population. The central tendency and high end individual risk descriptors are intended to capture tlie variability in exposure, lifestyles, and other factors tliat lead to a distribution of risk across a population. 

Table 2 Fate and effects of metals in a stream receiving a point-source of metals (upper part of the table) or diffuse input via urban runoff (lower part of the table). Summary of the expected influence of four different hydrological situations base-flow in a rainy period a flood after a rainy period low-flow after a long period of low rainfall (water scarcity) and a flood produced after this drought. Metal concentration (M) metal retention efficiency (measured on the basis of the nutrient spiraling concept) exposure (dose and duration) bioaccumulation (in fluvial biofilms) and metal sensitivity (of biofihns)...

Fig. 4. Response of E.coU recA y.lux strain on UV exposure (on the horizontal axis - the exposure dose, J/m ) — — the absolute intensity of bioluminescence (I) " the number...

Ecotoxicology deals with the study of the harmful effects of chemicals in ecosystems. This includes harmful effects upon individuals, although the ultimate concern is about how these are translated into changes at the levels of population, commnnity, and ecosystem. Thns, in the conclnding sections of the chapter, emphasis will move from the distribntion and environmental concentrations of pollutants to conseqnent effects at the levels of the individnal, population, community, and ecosystem. The relationship between environmental exposure (dose) and harmful effect (response) is fundamentally important here, and full consideration will be given to the concept of biomarkers, which is based on this relationship and which can provide the means of relating environmental levels of chemicals to consequent effects npon individuals, populations, communities, and ecosystems. 

DNA - deoxyribonucleic acid, dosage - cumulative exposure equivalent to the concentration of chemical agent to which an individual is exposed integrated over the time of exposure, dose - quantity of agent having entered the body. 

A lower exposure dose is required for polymer radical formation. 

Table 4 Internal Exposure (Dose) of Propoxur Expressed as Excreted Amount of 2-Isopropoxyphenol (nmol IPP)...

More traditional approaches have calculated exposure doses from a particular medium via a specific route (ATSDR, 1992). Such exposure doses can then be compared with a reference value derived for the same substance via the same route of exposure. Usual assumptions are ingestion rates of 100 mg dust/day and 200 mg soil/day, child body weight of 15 kg, and continuous exposure scenarios. This approach assumes a threshold for the effects of lead and does not reflect the fullest possible use of the wealth of human data on PbB levels. 

Exposure assessment is one of the most important steps in risk assessment. It is the process that predicts or estimates the amount of the substance under study that reaches the human body. To assess the exposure, it is necessary to define in detail the exposure pathway, the route of exposure, the concentration of the pollutant in the particular media, the contact rate, the frequency of exposure, and the population exposed (age, gender, and vulnerable population, among others). A general equation (Eq. 1) to determine the exposure dose is as follows ... 

The sensitivity a expressing its ability to have its structure modified as a result of exposure (more sensitive resists require lower exposure doses). 

The nature and behavior of free Rn-d ions or atoms, i.e. not attached to atmospheric condensation nuclei, is still subject of controversy, particularly with regard to the influence of environmental atmospheric conditions, such as humidity and presence of other gases (Busigin et al., 1981). Free Rn-d atoms are one of the most critical parameters for the exposure-dose conversion. This can be of particular importance in indoor exposure situations with a large ratio of unattached to attached Rn-d. 

Due to the superposition of various other biological, physiological and physical parameters used in modelling, the published exposure-dose conversion factors range from 2 to 120 mGy per WLM. However, a sensitivity analysis indicated that for most indoor exposure situations compensatory effects can reduce this range to about 5 to 10 mGy/WLM for the indoor situations occurring most frequently (OECD/NEA, 1983). 

Physical characteristics of the air inhaled (e.g. fraction of unattached Rn-d), ventilation of the room and physiological parameters (e.g. breathing rate) can influence significantly the exposure-dose conversion for the individual inhabitant, with the most frequently occurring value for indoor Rn-d exposure of about 5 mSv/WLM. 

A third possibility consists of comparing the theoretically calculated lung cancer rate based on risk coefficients derived from miners with the actual cancer occurrence among non-miners, derived from Rn-d exposure assessment in dwellings and using appropriate exposure-dose conversion factors (Steinhausler et al.. 1983 ... 

Although in most real multimaterial fires, exposure doses of CO produced normally far exceed those of other toxicants, significant combined effects have certainly been demonstrated with rodents in the laboratory. In addition to those described here with CO, HCN and HC1, such effects have also been reported due to C02 and to low oxygen when in combination with the narcotic toxicants (22,25). These all need to be studied further, preferably with nonhuman primates, in order to determine their impact on hazards to humans. Smoke atmospheres are likely to be much more hazardous than one would initially suspect from consideration of the concentrations of the individual toxicants taken separately. Perhaps, the major concern should not be so much the toxicity of HC1 or HCN, but rather, the toxicity of combinations of these gases with CO, C02 and low oxygen as may be present in smoke. 

Suppose a person tried to escape at 365 seconds (Figure 8) where the CO encountered is 10,000 ppm and he/she travels at. 5 rn/sec along line D-D. In this process, the person would encounter CO, shown as D-D in Figure 9. This is the exposure dose. 

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