Radiation exposure, models used

Under LNT, the risks of developing cancer from occupational radiation exposure are about the same as the risks of any other occupational illness or injury—about 1 in 10,000. By comparison, the background cancer death rate is about 1,600 in 10,000 (16%), and about 1 person in 7,000 dies each year in traffic accidents (more than 40,000 in the year 2000). For the vast majority of radiation workers, the drive to work is far more hazardous than their occupational radiation exposure, even using the LNT model. 

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 general approach of graded radiation exposure can also be used to examine light driven processes such as photopolymerization [19]. For example, Lin-Gibson and coworkers used this library technique to examine structure-property relationships in photopolymerized dimethacrylate networks [38] and to screen the mechanical and biocompatibility performance of photopolymerized dental resins [39]. In another set of recent studies, Johnson and coworkers combined graded light exposure with temperature and composition gradients to map and model the photopolymerization kinetics of acrylates, thiolenes and a series of co-monomer systems [40 2]. 

Radiation-induced genomic instability and bystander effects are now well-established consequences of exposure of living cells to ionizing radiation. Cells not directly traversed by radiation may still exhibit radiation effects. This phenomenon, known as bystander effect, has become a major activity in radiation biology and in some cases has challenged the conventional wisdom. An example is the currently accepted models used for low-dose extrapolation of radiation risks. The currently used models assume that cells in an irradiated population respond individually rather than collectively. If bystander effects have implications for health risks estimates from exposure to ionizing radiation, then the question of whether this is a general phenomenon or solely a characteristic of a particular type of cell and the radiation under test becomes an important issue. 

UCL takes into account measurement uncertainty in the study used to estimate the dose-response relationship, such as the statistical uncertainty in the number of tumors at each administered dose, but it does not take into account other uncertainties, such as the relevance of animal data to humans. It is important to emphasize that UCL gives an indication of how well the model fits the data at the high doses where data are available, but it does not indicate how well the model reflects the true response at low doses. The reason for this is that the bounding procedure used is highly conservative. Use of UCL has become a routine practice in dose-response assessments for chemicals that cause stochastic effects even though a best estimate (MLE) also is available (Crump, 1996 Crump et al., 1976). Occasionally, EPA will use MLE of the dose-response relationship obtained from the model if human epidemiologic data, rather than animal data, are used to estimate risks at low doses. MLEs have been used nearly universally in estimating stochastic responses due to radiation exposure. 

Models of radiation damage have been explored and are well developed. For many years, accidental releases of ionizing radiation have been universally feared. Alternatively, radiation has been used in controlled, defined amoimts as a therapy for certain tumors in both companion animals and humans. Taken together, all studies provide a cohesive and comprehensive picture of radiation toxicity. There are sufficient details of radiation effects to make credible estimates of risk resulting from radiation exposure (Harley 2001, 2008). 

Two types of models are often used for conducting statistical analysis of cancer risks (1) absolute-risk models and (2) relative-risk models. With absolute-risk models, the excess risk due to exposure to radiation does not depend on the normal risk that would arise when there is no radiation exposure. With relative-risk models, the relative risk is a multiple of the normal risk. Unlike absolute risk, which is measured on a scale that starts at 0 and goes to 1, relative risk values begin at 1 and go to infinity (i.e., very large numbers). A value of 1 for the relative risk means that there is no excess risk. 

Stable isotopes are advantageous both because there is no radiation exposure to study subjects and the problems of disposing of radionuclides are avoided. The combined use of such tools as mathematical models and stable isotopes is a powerful approach for understanding the dynamics of nutrient metabolism and for tailoring their requirements to physiologic state and age. 

These values together with simple, robust modelling have been used for a number of years to derive segregation tables for different modes of transport. Assessments of radiation exposures arising indicate that continued use of these values is acceptable. In particular, surveys of exposure occurring in air and sea... 

There are many considerations and conditions specific to the transport mode which should be factored into the models used to calculate segregation distances. These include consideration of how the relationship between accumulated transport indices in a location and radiation levels in occupied areas is affected by shielding and distance, and how exposure times for workers and members of the public depend upon the frequency and duration of their travel in conjunction with radioactive material. These may be established by programmes of work using questionnaires, surveys and measurements. In some circumstances exposure for a short... 

In areas significantly contaminated with radionuclides, external exposure of critical groups can be measured with individual gamma radiation dosimeters worn by group members for some days or weeks. The results of individual measurements should be used mainly for the vahdation of models used for the assessment of external doses. 

The purpose of the dose assessment for the public Uving in conditions of chronic (prolonged) radiation exposure is usually the justification of remedial actions that involve considerable expense associated with them. The doses of critical population groups should therefore be estimated on the basis of realistic, not screening, dosimetric models. To the extent possible, available data from environmental measurements and selective data from individual measurements, such as data from whole body counting for internal dosimetry and individual doses for external dosimetry, should be used to validate these models. 

The methodology leading to the presented risk estimates has serious hmitations. Firstly, the risk models are based on extrapolated data of the consequences of radiation exposure, and secondly, the appHed Monte Carlo methods use standardized geometrical phantoms not corresponding to different types of patient morphology. However, as Einstein states, ...this study provides a simplified approach, albeit one that we beheve is the best available from current data. ... 

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