Stochastic effects are usually associated with exposures to low levels of radiation exposure over a long period of time (e.g. years).  The term stochastic literally means ‘random’, the implication being that low levels of radiation exposure are not certain to produce an effect.  The induction of cancer and genetic defects are two of the most familiar consequences attributed to stochastic effects.  The description of stochastic effects is subject to a degree of controversy (owing to the difficulties in separating the effects of low-level radiation exposure from the effects of other carcinogens, e.g. tobacco smoke, non-radioactive species, etc), but the currently accepted theories lead to the following conclusions about stochastic effects: 

1.              There is no threshold level of radiation exposure below which we can say with certainty that cancer or genetic effects will NOT occur;

                 Doubling the radiation dose doubles the probability that a cancer or genetic effect will occur. 

Taken together, radiation experts refer to these two conclusions as the ‘linear-no-threshold’ hypothesis.  This hypothesis is questioned from time to time; however, it provides a pragmatic means of estimating radiation risks, and is consistent with the (limited) data that are available. 

Deterministic effects are associated with much higher levels of radiation exposure, usually incurred over a much shorter period of time (fractions of a second to tens of days) than is the case for stochastic effects.  As for stochastic effects, deterministic effects have two characteristic features: 

                  There is a threshold radiation dose, below which the deterministic effects are not observed;

                  The severity of the deterministic effect increases with the magnitude of the radiation dose

There are a variety of deterministic effects that can be observed after an acute exposure to radiation.  These include (in order of increasing severity): 

1.               Hemopoietic syndrome – an effect related to the effects of radiation on blood-forming tissues, normally indicated by changes in blood cell counts;

2.                 Gastrointestinal syndrome – an effect signaling the destruction of the gastrointestinal epithelium (the lining of the gastrointestinal tract);

3.                    Central nervous system syndrome – an effect seen at very high radiation doses in which the central nervous system undergoes irreparable damage. 

The usual symptoms following an acute radiation dose include nausea, vomiting and general fatigue.  In the case of the hemopoietic syndrome, medical intervention may be capable of saving the victim.  With the gastrointestinal syndrome, the most likely outcome is death within several weeks.  Anyone suffering the central nervous syndrome will die within a few hours to a few days of exposure.

Exposure to high levels of radiation, for example through placing a powerful radiation source next to the skin, can also give rise to some nasty burns and other externally visible symptoms.  The following photos illustrate some of these:

	This chest burn was produced when a powerful radiation source was placed in a shirt pocket.
	This damage was caused by handling a powerful radiation source, without protection.
	These burns are on the legs of a fireman who was involved in the aftermath of the Chernobyl accident, and were caused by beta radiation.

The moral of these photographs is that radiation and radioactivity needs proper handling if serious side-effects are to be avoided.  Proper handling was certainly not applied in the first two cases.

At various stages in this discussion, we have referred to ‘levels’ or ‘magnitude’ of radiation dose.  Expressing the size of a radiation dose is most conveniently done by specifying the amount energy deposited by the incident radiation.  The basic measure of radiation dose is called absorbed dose and is equal to the amount of energy deposited in the body, divided by the mass of the body volume irradiated.  The unit is the Gray, and one Gray = one joule per kilogram.  One joule is quite a small amount of energy (to heat 1 litre of water by 1 degree Celsius requires 4200 joules of energy), but 1 Gray of radiation dose is quite a substantial dose (the hemopoietic syndrome sets in after a whole-body dose of about 2 Gray).  This hints at one of the curious factors about radiation – even high radiation doses cannot be felt by the human senses, at the time of irradiation. 

Other more sophisticated measures of radiation dose take account of the differing abilities of the different types of radiation to induce cellular damage (alpha particles are more effective than beta particles and gamma rays), and the different sensitivities of different tissues in the body.  Of particular importance is a measure called the Sievert.  The Sievert defines a quantity called effective dose, which takes into account both of these considerations.

In assessing effective doses, the calculated or measured effective dose is usually compared with dose limits.  Dose limits are maximum allowed values, and are prescribed by a body known as the International Commission on Radiological Protection (ICRP).  For members of the public, the dose limit is 1 mSv/yr (0.001 Sieverts per year), and for occupational workers (workers who work in the nuclear industry), the dose limit is prescribed as 20 mSv/yr (0.02 Sieverts per year).  These dose limits exclude the effects of background radiation, as described in the next section.

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