Ionizing radiation transfers energy into the body tissue and may thereby interfere in the structure of molecules. In living organisms, this energy transfer may disturb or destroy cellular functions (somatic effect – fatal and nonfatal cancer) or it may change the genetic code of cells (hereditary effect). However, concerning the probability of cell changes, two effects can be distinguished either deterministic or stochastic (IAEA, 1996a). For deterministic, the severity of the effects is proportional to the dose (with threshold) and for stochastic effects the probability but not the severity is proportional to the dose (Frischknecht
et al., 2000; DME, 2005; UNSCEAR, 2000; Tsoulfanidis, 1995). Accordingly deterministic
(acute) effects will occur only if the radiation dose is substantial, such as in accidents. Stochastic effects (cancer and hereditary effects) may be caused by damage in a single cell. As the dose to the tissue increases from a low level, more and more cells are damaged and the probability of stochastic effects occurring increases (UNSCEAR, 2000). Radiobiological and clinical studies have shown that deterministic effects only occur above threshold, doses with dose limits and reference values used in radiological protection which are above 100 mSv. However, for low dose range of less than 100 mSv, only genetic and carcinogenic effects are expected. Although, possible radiation effect of doses less than 100 mSv cannot generally be discovered by epidemiology. Similarly, an individual cancer which may have been caused by ionizing radiation cannot be distinguished from cancers which originate from other unknown causes since there is no specific signature existing for radiation-induced cancer. Therefore epidemiology can probably not clarify the connection between cancer induction and radiation in the low dose range (Streffer, 2010). In addition, other health effects may occur in infants as a result of exposure of the embryo or foetus to radiation. These effects include a greater likelihood of leukaemia and for exposure above various threshold doses during certain periods of pregnancy, severe mental retardation and congenital malformations may arise (IAEA, 1996a; NEA, 1994). According to Wahl and Berkeley (2010), exposure to high levels of radiation is known to cause cancer. But the effects on human health from very low doses of radiation such as the doses from background radiation are very hard to determine because there are so many other factors that can mask or distort the effects of radiation. For example, among people exposed to high radon levels, cigarette smokers are much more likely to get lung cancer than non-smokers. Lifestyle choices, geographic locations, and individual sensitivities are difficult to account for when trying to understand the health effects of its radiation. According to DME (2005) studies have shown that the effect of radiation is dependent on many factors including:
the type of radiation (alpha, beta or gamma) the amount received
the rate at which it is received which part of the body is exposed
whether the exposure is chronic (regular, low doses) or acute (short time, high dose)
the age of the irradiated person
Biological effects of radiation are typically classified into two categories (USNRC, n.d). The first category consists of exposure to high doses of radiation over short periods of time producing acute or short term effects (Deterministic) while the second category represents exposure to low doses of radiation over an extended period of time producing chronic or long term effects (Stochastic). The high doses tend to kill cells, while low doses tend to damage or change them. High doses can kill so many cells that will lead to damage of tissues and organs. This may result to a rapid whole body response often called the Acute Radiation Syndrome (ARS).
3.8.1 Acute Respiratory Syndrome
ARS is an acute illness caused by irradiation of the entire body or most part of the body by a high dose of penetrating radiation in a very short period of time usually a matter of minutes (CDCP, 2006; TR, 2011; Akashi et al., 2006). It is sometimes referred to as radiation toxicity or radiation sickness. According to UWEHS (2006) and Heslet et al. (2012), ARS represents the signs and symptoms which result from large doses of radiation – generally over 100 rads (1 Sievert) delivered to a major portion of the body. This type of injury occurs only when the dose is received over a short period of time and the total effect may vary from mild and transient illness to death. The following are the stages in the ARS (Heslet et al. 2012; Akashi
et al., 2006; CDCP, 2006):
3.8.1.1 Prodrome
This is the initial phase of the syndrome, and is usually characterized by nausea, vomiting and malaise (SurvivalIQ, 2008). According to Heslet et al. (2012), the prodromal phase usually occurs in the first 48 hours, but may develop up to 6 days after exposure.
3.8.1.2 Latent Stage
During the latent stage, which may be likened to the incubation period of a viral infection, the subjective symptoms of illness may subside, and the individual may feel well. However, changes may be taking place within the blood-forming organs and elsewhere which will subsequently give rise to the next aspect of the syndrome.
3.8.1.3 Manifest Illness Stage
This phase reflects the clinical picture specifically associated with the radiation injury. Among the possible signs and symptoms are loss of hair (epilation), fever, infection, haemorrhage, severe diarrhoea, prostration, disorientation, and cardiovascular collapse. Observation of the
foregoing phenomena in a given individual is largely dependent upon the radiation dose received.
3.8.1.4 Recovery or Death
With this stage, according to CDCP (2006) and UWEHS (2006), the recovery process lasts from several weeks to two years. Most patients who do not recover will die within several months of the exposure. Hence, in most cases, bone marrow cells will begin to repopulate the marrow. Hence, there should be full recovery for a large percentage of individuals from few weeks up to two years after exposure but death may occur in some individuals at (1.2 Sv).
3.8.2 Deterministic Effects
A short-term dose is the threshold for causing immediate radiation sickness in a person of average physical attributes, but would unlikely cause death above 1000 mSv (Hall, 2012). Accordingly, the damage may result in cell death or modifications that can affect the normal functioning of organs and tissues. Most organs and tissues of the body are not affected by the loss of even considerable numbers of cells. However, if the number lost becomes large, there will be observable harm to the organ or tissue and therefore to the individual. But only if the radiation dose is large enough to kill a large number of cells will such harm occur. Therefore, this type of harm occurs in all individuals who receive an acute dose in excess of the threshold and is called deterministic.
3.8.3 Stochastic Effects
If the cell is not killed but only modified by the radiation damage, the damage in the viable cell is usually repaired. If the repair is not perfect, the modification will be transmitted to daughter cells and may eventually lead to cancer in the tissue or organ of the exposed individual (Shapiro, 2002). However, if the cells are concerned with transmitting genetic information to the descendants of the exposed individual, hereditary disorders may arise. Such effects in the individuals or in their descendants are called stochastic.