Threshold effect, radiation

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. 

Exposure to radiation can cause detrimental health effects. At large doses, radiation effects such as nausea, reddening of the skin or, in severe cases, more acute syndromes are clinically expressed in exposed individuals within a short period of time after the exposure such effects are called deterministic because they are certain to occur if the dose exceeds a threshold level. Radiation exposure can also induce effects such as malignancies, which are expressed after a latency period and may be epidemiologically detectable in a population this induction is assumed to take place over the entire range of doses without a threshold level. Hereditary effects due to radiation exposure have been statistically detected in other mammalian populations and are presumed to occur in human populations also. These epidemiologically detectable effects—malignancies and hereditary effects—are termed stochastic effects because of their random nature. 

Improved understanding of the health effects of low levels of radiation, where currently standards are set by simple linear extrapolation of health effects observed at large doses, may potentially result in reassessment, either up or down, in the safety of nuclear power and the consequences of nuclear accidents. The demonstration of a threshold for radiation effects, postulated by some researchers, would result in a major decrease in the calculated consequences of severe accidents, and would also affect design requirements for radioactive waste disposal. 

Breckow J (2006) Linear-no-threshold is a radiation-protection standard rather than a mechanistic effect model. Ra-diat Environ Biophys 44 257-260 Brenner DJ, Sachs RK (2006) Estimating radiation-induced cancer risks at very low doses rationale for using a linear no-threshold approach. Radiat Environ Biophys 44 253-256... 

Much of the data confirm the concept that there is a generalized increase in the threshold of radiation effects. However, there are specific data that... 

Another phenomenon that was inexplicable in classical terms was the photoelectric effect discovered by Hertz in f 887. When ultraviolet light falls on an alkali metal surface, electrons are ejected from the surface only when the frequency of the radiation reaches the threshold... 

In air, PTFE has a damage threshold of 200—700 Gy (2 x 10 — 7 x 10 rad) and retains 50% of initial tensile strength after a dose of 10" Gy (1 Mrad), 40% of initial tensile strength after a dose of 10 Gy (10 lad), and ultimate elongation of 100% or more for doses up to 2—5 kGy (2 X 10 — 5 X 10 rad). During irradiation, resistivity decreases, whereas the dielectric constant and the dissipation factor increase. After irradiation, these properties tend to return to their preexposure values. Dielectric properties at high frequency are less sensitive to radiation than are properties at low frequency. Radiation has veryHtde effect on dielectric strength (86). 

Volume of vessel (free volume V) Shape of vessel (area and aspect ratio) Type of dust cloud distribution (ISO method/pneumatic-loading method) Dust explosihility characteristics Maximum explosion overpressure P ax Maximum explosion constant K ax Minimum ignition temperature MIT Type of explosion suppressant and its suppression efficiency Type of HRD suppressors number and free volume of HRD suppressors and the outlet diameter and valve opening time Suppressant charge and propelling agent pressure Fittings elbow and/or stub pipe and type of nozzle Type of explosion detector(s) dynamic or threshold pressure, UV or IR radiation, effective system activation overpressure Hardware deployment location of HRD suppressor(s) on vessel... 

Thermal effects depend on radiation intensity and duration of radiation exposure. American Petroleum Institute s Recommended Practice 521 (1982) reviews the effects of thermal radiation on people. In Table 6.5, data on time to reach pain threshold are given. As a point of comparison, the solar radiation intensity on a clear, hot summer day is about 1 kW/m (317 Btu/hr/ft ). Criteria for thermal damage are shown in Table 6.6 (CCPS, 1989) and Figure 6.10 (Hymes 1983). 

Radiation is carcinogenic. The frequency of death from cancer of the thyroid, breast, lung, esophagus, stomach, and bladder was higher in Japanese survivors of the atomic bomb than in nonexposed individuals, and carcinogenesis seems to be the primary latent effect of ionizing radiation. The minimal latent period of most cancers was <15 years and depended on an individual s age at exposure and site of cancer. The relation of radiation-induced cancers to low doses and the shape of the dose-response curve (linear or nonlinear), the existence of a threshold, and the influence of dose rate and exposure period have to be determined (Hobbs and McClellan 1986). 

Threshold hypothesis A radiation-dose-consequence hypothesis that holds that biological radiation effects will occur only above some minimum dose. 

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. 

Some radiation effects result from nonlethal damage to a single cell. These effects are called stochastic. They have the property that there is no threshold for these effects to occur. It is the probability of occurrence rather than its severity which increases with dose. The causation of some cancers may be rooted in a stochastic effect. 

The current health risks associated with exposure to low-dose radiation are extrapolated from high-dose data taken from the Life Span Study of the Japanese atomic bomb survivors. Currently, a linear no threshold extrapolation is recommended. The numerous technical reports and scientific papers about the Japanese A-bomb survivors were widely interpreted as showing that the effects of occupational exposures to radiation would be too small to detect in epidemiological studies. However, questions about the reliability of the A-bomb results were presented by Stewart and Kneale [2]. Their Oxford Childhood Study observed that children whose in utero exposures were as little as 10 to 20 mSv had 40% more childhood leukemias than those who were not exposed. No similar effects are reported in the A-bomb data. Of course, the finding of no effect is not a compelling argument for or against a safe dose. 

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