Stochastic effect

In opposition to the effects known as deterministic, the stochastic effects does not appear according to the principle of a cause which always induces the same effect. They relate to for example the effects of low dose of poisons, or the harmful ionizing rays to the Santé. The Cancer is an example of disease, often multifactorielle resulting from stochastic effects.

Contrary to a deterministic Effect, a stochastic effect is not related to a limiting threshold (of exposure to a poison or noxious radiations for example); one can associate to him only one statistical estimate of the “risk” to see the effect to appear at a patient. There exists nevertheless a threshold of significativity.

It is not possible to quantify an amount for which a given effect is certain to appear - on the other hand, its probability of appearance (and thus the frequency) increases according to the received amount, but in a random way.

Estimate of the risk and the effects

It is possible to consider a risk average of appearance of effect stochastic by studying rather vast samples of a human, animal, vegetable population. subjected to a physiological stress (toxic, ionizing rays.).

Epidemiological investigations

The epidemiology or the ecoepidemiology is first means of seeking, of qualifying and of quantifying the induced effects, and for example to evaluate relations amount-frequency, for a given effect. An excess of risk can thus be given. This method nevertheless is limited by:

- difficulties of choosing and of following over one long life the pilot troops and populations.

to take into account synergies between various causes and

effects to measure the received amount or to which an individual is exposed

the difficult taking into account of the associated factors (smoker or not smoker, or genetic predisposition for example in the case of the man).

Some examples of troops: In the field of the study of the effects of various forms of irradiation (external/intern.);

the survivors of Hiroshima
Tchernobyl
the workers of the Nuclear
Certain areas of the world or the natural exposure is very high (Iran, India…)

Animal experimentation

The experimentation In vitro or in laboratory makes it possible to insulate a cell, an organization or an animal and precisely to know the received amount (as well as the type of poison or radiation, if necessary).

For technical reasons (lifespan, place, facilitated reproduction of the animals) and of cost, it is done mainly on Rat S of laboratory. The population used is relatively homogeneous and from this point of view “ identical ” to a pilot population (all the rats result from a consanguineous stock and present same genetic inheritance), which facilitates the description of the stochastic effects. However, extrapolation with the man (or another species) is not always correct for certain effects and in particular for the effects of low dose.

Problem of low dose

In the field of the radiation, the stochastic effects seem mainly induced by microphone Lésion S of the molecule of DNA, following a more or less long and strong exposure to the ionizing rays. Problems posed by this type of effect are obviously the estimate of the risk for very weak amounts. Indeed, the probability that a cell modified by a viable change of the DNA gives a cancer is weak. The real risk incurred by a person exposed to very limited radiations (in amount and/or duration) is nonquantifiable, but not no one. Indeed, the extrapolation of the effects of the strong amounts towards low dose is not possible, the mechanisms being various and more complexes or more sensitive to the Hasard. It is necessary, indeed, to take into account:

genetic instability : appearance of modification genetic several cellular divisions after the irradiation.
over-sensitiveness with low dose : defect of repair of the DNA for weak amounts.
the adaptive answer .
the effect of proximity : modification of not irradiated cells induced by other cells which, they, have summers irradiated.

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