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Health effects of radioactivity in the human body

Stochastic and non-stochastic effects

There are stochastic and non-stochastic effects of nuclear radiation. Non-stochastic effects occur at very high doses within a short period and are due to cell killing on a massive scale. The effects become evident within hours or days. Demonstrable relationships exist between the effects and the magnitude of the received radiation dose. Non-stochastic effects are important in case of nuclear explosions and large nuclear accidents, typically during nuclear conflicts.

Stochastic effects occur at random and at lower radiation doses. The effects become evident only after months or ywears or even decades. The classical radiobiology assumes a linear relationship between dose and adverse effects. However it not certain if an individual will develop a cancer or other adverse effect. If a large number of individuals receive the same dose, one can predict the number of individuals who will develop an adverse effect, but not which individuals. With regard to stochastic effects there is no threshold of the received dose below which adverse effects certainly will not occur.

Targeted, non-targeted and delayed effects

The classical explanation for stochastic radiation health effects was that they were mostly caused by structural DNA damage (i.e.single- and double-strand DNA breaks) which resulted in mutations in the cellŐs genetic information that, without repair or elimination, would end eventually in cancers. This is the target theory of radiation effects, the target being specific sequences in DNA and chromosomes.

Relatively recent studies proved the existence of 'non-targeted' and 'delayed' radiation effects. Probably these effects had been observed in earlier studies but they had been unrecognised as they fell outside the then accepted paradigm of radiation effects. Non-targeted effects, which arise as a result of damage/changes to unknown areas in the cell, are termed 'non-targeted' because they mainly do not cause damage/changes to DNA or chromosomes, heretofore believed to be the main site for radiation's lesions. Non-targeted effects include:

• genomic instability,

•bystander effects (effects in unirradiated cells situated close to irradiated cells),

• clastogenic effects (causing chromosome disruption or breakages in blood plasma that result in chromosome damage in non-irradiated cells),

• heritable effects of parental irradiation that occur in succeeding generations.

Presently there is no mechanical explanation for how the non-targeted effects actually occur. The observed phenomena pose many fundamental questions to be answered and result in a paradigm shift in the understanding of radiation biology. Evidently the relationship between dose and adverse effects is not linear.

Biochemical aspects of radioactivity

The standards for the public exposure to nuclear radiation were (and probably still are) based on the experience with diagnostic X-rays and gamma rays from external sources and originate from the early 1950s. Not included in the early models are the fact that the adverse effect of radiation is tens to hundreds of times more serious for the developing infant in the motherŐs womb and for young childern than for the adults who have been studied following medical X-ray exposures.

Not until the early 1970s it was discovered that the effects of prolonged low radiation exposures, as from long-lived radionuclides accumulating in the body, is much greater than from the same total dose received in a short X-ray exposure.

Some radionuclides have a specific biological behaviour and tend to accumulate in a special organ or tissue. In that case, the radioactivity is not evenly distributed among the body and doubling of the radioactivity of the body as a whole, means locally a sharp increase in radiation. The chemical properties of an element are not affected by the radioactivity of its atoms. For example, the biochemical machinery of the human body cannot distinguish between a normal water molecule or a water molecule with one or two radioactive tritium atoms.

High concentrations of a specific radionuclide in a specific organ are possible as a consequence of its biochemical properties. Radioactive iodine atoms for example, seek out the thyroid gland, together with its non-radioactive sister atoms, and damage the production of key growth hormones and cause thyroid cancer. Strontium-90 and plutonium tend to cumulate in the bones, where they irradiate the bone marrow, causing leukemia in newly forming red blood cells as well as damage to crucial white cells of the immune system, with all consequences of that. Cesium-137 collects in soft tissue organs, such as the breasts an reproductive organs of females and males, leading to various types of cancer in the individuals and their childern as well as in later generations.

Radiation-induced diseases

In the regions contaminated with radioactivity after the Chernobyl disaster a greatly increased incidence of a many different maligne and non-maligne diseases and disorders are observed, such as:

multimorbity classified as radiation-induced premature senescence,

cancers and leukaemia

thyroid cancer and other thyroid diseases

damage to nervous system,

mental disorders

heart and circulatory diseases

infant mortality

congenital malformations

endocrinal and metabolic illnesses

diabetes

miscarriages and pregnancy terminations

genetic damage, hereditary disorders and diseases

teratogenic damage, such as:

anencephaly,

open spine,

cleft lip/palette,

polydactylia,

muscular atrophy of limbs,

Down's syndrome

In some areas in Belarus and Ukraine nearly all habitants are suffering from one or more radiation-induced diseases.

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