Radiation threshold model

Another line of thinking suggests that there may be no adverse effects at all from exposure to low levels of radiation that there may be a threshold, below which we see no risk. Under threshold models, there is a certain level of exposure that is completely safe, and it is only above that threshold that we begin to see an increase in cancer risk. Virtually all known harmful agents exhibit threshold effects. 

Once a dose metric is selected and estimated, a dose extrapolation model can be applied to estimate cancer risk. The choice of the model will be driven by the likely mechanism of action of the chemical or agent. For example, if the substance is a genotoxic material, such as radiation, a linear model would be used. A threshold model or nonlinear model might be used if the chemical or agent is not genotoxic (Paustenbach 2002 Williams and Paustenbach 2002). The general theory behind both models is discussed below. 

Two models of radiation damage, illustrated in Fig. 19.17, have been proposed the linear model and the threshold model. The linear model postulates that damage from radiation is proportional to the dose, even at low levels of exposure. Thus any exposure is dangerous. The threshold model, on the other hand, assumes that no significant damage occurs below a certain exposure, called the threshold exposure. Note that if the linear model is correct, radiation exposure should be limited to a bare minimum (ideally at the natural levels). If the threshold model is correct, a certain level of radiation exposure beyond natural levels can be tolerated. Most scientists feel that since there is little evidence available to evaluate these models, it is safest to assume that the linear hypothesis is correct and to minimize radiation exposure. 

A recent study concluded that any amount of radiation exposure can cause biological damage. Explain the differences between the two models of radiation damage, the linear model and the threshold model. 

Figure 19.17 The two models for radiation damage. In the linear model, even a small dosage causes a proportional risk. In the threshold model, risk begins only after a certain dosage.

Specific health effects resulting from an acute dose appear only after the victim exceeds a dose threshold. That is, the health effect will not occur if doses are below the threshold. (Note that this is significantly different from the LNT model used to predict stochastic effects.) After reaching the acute dose threshold, a receptor can experience symptoms of radiation sickness, also called acute radiation syndrome. As shown in Table 3.2, the severity of the symptoms increases with dose, ranging from mild nausea starting around 25-35 rad (0.25-0.35 Gy) to death at doses that reach 300-400 rad (3-4 Gy). Table 3.2 shows that the range of health effects varies by both total dose and time after exposure. 

The existing evidence does not exclude the existence of a threshold for some (perhaps even aU) forms of cancer, but the available epidemiological and laboratory data do not favor such a possibility. Hence, the interpolation models used by national and international experts for estimating the carcinogenic risks of low-level ionizing radiation are generally based on the assumption of a non-threshold dose-incidence relationship (ICRP, 1977 UNSCEAR, 1977 NAS/NRC, 1980 NCRP, 1980 Sinclair, 1981 Rail eta/., 1985). 

In addition, the observed width of an Auger line is also affected by the spectrometer resolution. However, the bandpass of the incident radiation which produces the initial state for the Auger decay does not play a role, unlike in the case of the width of an observed photoline. (This statement only holds for the two-step model of inner-shell ionization and subsequent Auger decay. In the vicinity of the inner-shell ionization threshold it significantly fails due to postcollision interaction (Section 5.5) and the resonant Raman Auger effect (Section 5.1.2.1).) Hence, Auger transitions often appear in the spectra of ejected electrons as lines much sharper than the corresponding photolines. 

The aim of this section is to show that the experimentally obtained ionization thresholds displayed in Fig. 7.3 can be reproduced by numerical quantum calculations conducted within the framework of the SSE model discussed in detail in Chapter 6. Since the SSE model is one-dimensional, it is surprising that it should be possible to use this model with any hope of success for the description of the manifestly three-dimensional experiments with real Rydberg atoms in the laboratory. Therefore, the main point here is to motivate and to justify the use of a one-dimensional model for the description of Rydberg atoms in a strong linearly polarized radiation field. 

These studies were later extended to the ignition of specially prepared cellulose sheets" as a model for the broad class of kindling fuels. These sheets were made from a single batch of wood a-cellu-lose, with various proportions of carbon black added to provide a variation in optical properties from white to black. The thickness of the sheet varied within the range of 0.002 to 0.03". Furthermore, the samples were prepared in two densities, which gave two different sets of heat-conduction properties. Thus, the experimental samples had the same chemical properties but a considerable latitude for variation in physical properties. The samples were exposed to constant thermal radiation at levels of 2—23 cal.cm. sec. , to establish the relationship between the threshold of ignition (with the exposure parameter) and the fuel properties. 

Knotek and Feibelman [94] examined the modification to a surface when exposed to ionising radiation and assesed the damage that can be produced. They addressed the stability of ionically bonded surfaces, where the KF mechanism applies, and concluded that Auger induced decomposition only occurs when the cation in the solid is ionised to relatively deep core levels. In the case of non-maximal oxides as with NiO, Freund s group [95] showed that whilst desorption of neutral NO and CO from NiO(lOO) and (111) surfaces has thresholds at the C Is, N Is and O Is core levels, it proceeds mainly on the basis of the MGR model, involving an excited state of the adsorbate. An overview of electronic desorption presented by Feibelman in 1983 [96] examined particularly the stability of the multiple-hole final state configuration leading to desorption. The presence of multiple holes, and associated hole-hole correlation... 

The (unacceptable) linear extrapolation model of radiation risk represents only one of several examples showing that the old dogma based on the assumption of no threshold for genotoxic carcinogens does not hold true in all cases. Such considerations stimulated opposition against the no threshold assumption inherent in extrapolating risk linearly to the zero dose intercept. Several examples have been published in Science ... 

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