Deterministic effects

The domain of strong doses and severe, reproducible effects

For high doses above a certain threshold, the damage done is predictable, reproducible and highly widespread. One can predict with a great deal of certainty the effects of high levels of exposure, effects which worsen with increasing dosage. For the whole body effective doses to consider are of the order of sieverts  (Sv).

These effects can appear after a few weeks or even days, not an immediate reaction by any means but far quicker than the years which usually characterise the so-called ‘probabilistic effects‘ of low doses of radiation. The predictable effects of these higher doses are known as ‘deterministic’.

Because their impact can be gauged and reproduced, these higher doses are often used in radiotherapy to kill cancerous cells by concentrating the dosage on the tumour in question. Predictably, the effects of the radiation depend on the quantity absorbed.

The relevant unit for this issue is the amount of energy deposited by radiation per kilogram of body mass – the gray (joule/kg). In radiotherapy and curietherapy, the doses are localised and very high, often in the tens of grays, in order to get rid of the malignant cells. In medicine, radiologists consider the amount of energy deposited per kilo.

In the case of an accident which exposes the whole body, the effective dose must be used. If the effective dose is of the order of 1 gray, then deterministic effects can be noticed: these include nausea (at 0.25 gray), hospitalisation (at 0.5 gray) and certain death (from 5 grays).

Most of the people who ever lost their lives to the ill effects of radiation were those killed by the atomic bombs in Hiroshima and Nagasaki. The radioactivity was not the most deadly consequence, however, and most of those who died were killed by the ensuing fire, suffering severe burns or suffocation.

If we set aside Hiroshima and Nagasaki, one rarely comes across victims of high doses of radiation. The most well-known cases are those of the firefighters at Chernobyl who participated in the dousing of the damaged reactor immediately after the accident. Around forty of these firefighters died in the weeks and months that followed. At Tokaimura, two or three of the Japanese technicians responsible for the 1999 accident also died in the ensuing weeks.

The 2006 poisoning with radioactive substances of the former KGB agent Alexander Litvinenko is another example. Litvinenko was poisoned by consuming a cup of tea containing an infinitesimal amount in grams of polonium 210, but several times the lethal dose in terms of radioactivity. This highly radioactive element is very rare in its natural state, which led to speculation that the polonium was produced in a Russian reactor. It took doctors time to understand what had happened to Litvinenko, as such a poisoning was virtually unheard of. Litvinenko died a few weeks after being exposed.

According to some experts, the dose received was around 2 GBq (50 millicuries), corresponding to the activity of 10 millionths of a gram of polonium 210. The cost of polonium, estimated at 25 million dollars per kilogram by the Berliner Zeitung, fortunately minimises the risks posed by such techniques. For secret agencies, this method of assassination is highly visible and less effective than the classic Bulgarian umbrella dipped in cyanide