Ionizing radiation is either penetrating (X-rays, ‘}’-rays or neutrons) or non-penetrating (a or f3 particles). Penetrating radiation affects the whole body, while nonpenetrating radiation only affects the skin. All radiation effects, however, depend on the type of radiation, the distribution of dose and the dose rate.
Absorption of doses greater than 100 rads of y-radiation, e.g. following survival from a nuclear explosion or nuclear power plant accident, causes acute radiation syndromes of varying severity. Long-term effects also occur, sometimes decades after exposure, as radiation increases the rate of mutagenesis.
Radiation dosage is measured in joules per kilogram (J kg-I); 1 J kg” ‘ is also known as 1 gray (1 Gy). This is equivalent to 100 rads. Radioactivity is measured in becquerels (Bq); 1 Bq is equal to the amount of radioactive material in which there is one disintegration per second. 1 curie (Ci) is equal to 3.7 x 1010 Bq. Radiation differs in the density of ionization it causes. Therefore a dose equivalent called a sievert (Sv) is used.
This is the absorbed dose weighted for the damaging effect of the radiation. The annual background radiation is approximately 2.5 mSv.
Excessive exposure to ionizing radiation occurs following accidents in hospitals, industry, nuclear power plants and strategic nuclear explosions.
Mild acute radiation sickness
Nausea, vomiting and malaise follow doses of approximately 1 Gy (75-125 rad). Lymphopenia occurs within several days, followed 2-3 weeks later by a fall in all white cells and platelets. There is a late risk of leukaemia and solid tumours.
Severe acute radiation sickness
Many systems are affected; the extent of the damage depends on the dose of radiation received. The effects of radiation are summarized.
Acute myeloid leukaemia
Absorption of doses between 2 and 10 Gy (200-1000 rad) is followed by early and transient vomiting in some individuals, followed by a period of relative well-being. Lymphocytes are particularly sensitive to radiation damage and severe lymphopenia develops over several days. A decrease in granulocytes and platelets occurs 2-3 weeks later as no new cells are being formed by the damaged marrow. Thrombocytopenia with bleeding develops and frequent, overwhelming infections occur, with a very high mortality.
Absorption of doses greater than 6 Gy (600 rad) causes vomiting several hours after exposure. This then stops, only to recur some 4 days later accompanied by severe diarrhoea. Owing to radiation inhibition of cell division, the villous lining of the intestine becomes denuded. Intractable bloody diarrhoea follows, with dehydration, secondary infection and death.
Exposures above 30 Gy (3000 rad) are followed rapidly by nausea, vomiting, disorientation and coma; death due to severe cerebral oedema follows in 36 hours.
Skin erythema, purpura, blistering and secondary infection occur. Total loss of body hair is a bad prognostic sign and usually follows an exposure of at least 5 Gy (500 rad). Late effects of radiation exposure The survivors of the nuclear bombing of Hiroshima and Nagasaki have provided information on the long-term effects of radiation. The risk of developing acute myeloid leukaemia or cancer, particularly of the skin, thyroid and salivary glands, increases. Infertility, teratogenesis and cataract are also late sequelae of radiation exposure.
Acute radiation sickness is a medical emergency. Hospitals should be immediately informed of the type and length of exposure so that suitable arrangements can be made to receive the patient. The initial radiation dose absorbed can be reduced by removing clothing contaminated by radioactive materials.
Treatment of radiation sickness is largely supportive and consists of prevention and treatment of infection, haemorrhage and fluid loss. Storage of the patient’s white cells and platelets for future use should be considered, if feasible.
Accidental ingestion or exposure to bone-seeking radioisotopes (e.g. strontium-90 and caesium-137) should be treated with chelating agents (e.g. EDTA) and massive doses of oral calcium. Radioiodine contamination should be treated immediately with potassium iodide 133 mg daily. This will block 90% of radioiodine absorption by the thyroid if given immediately before exposure.