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1. In some cases, a small radiation dose reduces the impact of a subsequent, larger
radiation dose. This has been termed an 'adaptive response' and is related to hypothetical
mechanisms of hormesis.
Radiation hormesis is the conjecture that a low level of ionizing radiation (i.e., near the
level of Earth's natural background radiation) helps "immunize" cells against DNA damage from
other causes (such as free radicals or larger doses of ionizing radiation), and decreases the risk of
cancer. The theory proposes that such low levels activate the body's DNA repair mechanisms. This
assertion is very difficult to prove in humans (using, for example, statistical cancer studies) because
the effects of very low ionizing radiation levels are too small to be statistically measured amid the
"noise" of normal cancer rates.

1) Acute
Acute radiation exposure is exposure to ionizing radiation that occurs during a
short period of time. Extreme examples include:
 Instantaneous flashes from nuclear explosions
 Exposures of minutes to hours during handling of highly radioactive
sources
 Laboratory and manufacturing accidents
 Intentional and accidental high medical doses.
The effects of acute events are more easily studied than those of chronic
exposure.

2) Chronic
Exposure to ionizing radiation over an extended period of time is called
chronic exposure. The term chronic (greek: Cronos – time ) refers to the duration, not the
magnitude or seriousness. The natural background radiation is chronic exposure, but
a normal level is difficult to determine due to variations. Geographic location and
occupation often affect chronic exposure.

Monitoring and controlling exposure
Radiation has always been present in the environment and in our bodies. The human body
cannot sense ionizing radiation, but a range of instruments that are capable of detecting even very
low levels of radiation from natural and man-made sources exists.

Dosimeters measure an absolute dose received over a period of time. Ion-chamber
dosimeters resemble pens and can be clipped to one's clothing. Ion-chamber dosimeters must be
periodically recharged, and the result logged. Film-badge dosimeters enclose a piece of
photographic film, which will become exposed as the radiation passes through it. Another type of
dosimeter is the TLD (Thermoluminescent Dosimeter). These dosimeters contain crystals that emit
visible light when heated, in direct proportion to their total radiation exposure. Like ion-chamber
dosimeters, TLDs can be re-used after they have been 'read'.

Geiger counters and scintillation counters measure the dose rate of ionizing radiation
directly.

In the process of studying the influence of ionizing radiation on the human
body were found the following features:
1) high destructive power of absorbed radiation energy;
2) the presence of a latent period of welfare;
3) the effects of low doses can accumulate. This effect is called cumulation;

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