These are non-stochastic effects that manifest them­selves in the individual exposed within a few weeks of the exposure. They are all the result of fairly massive acute doses of radiation. Although changes in the blood cell count could be detected by laboratory test, an exposed individual would not feel unwell after doses of up to 0.5 Sv. At higher doses the following effects will become more pronounced until at doses of more than 4 Sv most people would die without specialised medical treatment.

Haematopoietic or bone marrow syndrome

The long lifetime of red blood cells results in the number of circulating red cells not being immediately affected by a large exposure, whereas the shorter-lived white cells show’ a rapid drop in number over the first week. A reduction in the white cell count clearly lowers the body’s defences against infection. The red cell count eventually falls because of parent cell death.

A whole-body dose of 7 Sv will render sufficient numbers of the white parent cell population repro­ductive^ non-viable to cause death from uncontrolled infections in the irradiated individual. Death usually occurs within one or two months of irradiation, but the exact time will depend on the dose and the phy­sical state of the individual exposed.

Gastro-intestinal tract syndrome (GI)

Doses of several sieverts and higher, significantly im­pair the cell renewal system of the gastro-intestinal tract by ‘killing’ the parent cell population. Death may result from a combination of infection by gut organisms, reduced nutrient uptake and loss of blood to the intestine. Death can occur within a few days or weeks for doses over 10 Sv with early symptoms such as vomiting, diarrhoea and dehydration.

The central nervous system syndrome (CNS)

The CNS syndrome only becomes apparent after doses of several tens of sieverts. Tremors, convulsion and general loss of control of movement are the most common symptoms and sometimes occur within min­utes, although death may not occur for a day or two. In this case, unlike the two previous syndromes, the cells involved physically break up.