A report by the Lawrence Livermore National Laboratory in California in the USA concluded that the Chernobyl nuclear disaster emitted more long-term radiation into the world's air, water and topsoil than has been produced by all the atomic bombs and nuclear tests ever exploded.
Radioactive fallout can affect man in two main ways: by radiation from fallout deposited on the ground; and by radiation from the active substances that man has taken into his body by ingestion (particularly of food) and by inhalation. The last comprehensive estimates by UNSCEAR of the dose commitments up to the year 2000 from fallout from nuclear tests carried out before 1968 indicate that the contributions to the gonadal dose from external radiation from caesium-137 and from shortlived fission products each amount to 36 mrem for the North Temperate Zone, and to 8 mrem for the South Temperate Zone. The contributions from internal radiation from caesium-137 and carbon-14 are 21 mrem and 13 mrem respectively for the North Temperate Zone, and 4 mrem and 13 mrem respectively for the South Temperate Zone. For this dose commitment, it is estimated that more than half the external dose had already been delivered by the end of 1967, and two thirds of that from internally deposited caesium-137. In contrast, only one tenth of the internal dose due to carbon-14 will have been delivered by the year 2000. It is not possible to quote an average exposure per year derived from this dose commitment. The genetic dose for the early seventies was less than 10 mrem per year for the northern hemisphere and 2 mrem per year for the southern hemisphere, with an average genetic dose to the world population of less than 7 mrem per year.
Exposure at this level (it was higher in previous years) will decrease over the subsequent years. This form of radiation differs, therefore, from other man-made radiation; the latter is tending to increase and will continue to exert its effects at least for decades.