Nuclear explosions, whether for military or peaceful purposes, and explosions involving nuclear material, like the one at the nuclear reactor at Chernobyl give rise to radioactive fission products which (in the case of the larger particles) may settle immediately to the ground or which may be transported by wind to other areas at distances well beyond the range of destruction and casualties caused by the blast, heat and initial radiation. Having reached these regions they may present a radiation hazard long after the explosion. This is particularly true of the smaller particles which are carried by the winds in the troposphere or stratosphere, generally in the directions of the prevailing westerlies. The debris in the troposphere circles the globe in about 2 weeks, while material injected into the stratosphere may travel much faster. North-South spreading is a much slower process. The radioactive debris is brought down to the surface of the planet mainly in precipitation, after remaining about 30 days in the troposphere. With high-yield explosions a single detonation may contaminate a huge area with radioactivity and make it uninhabitable for a long time.
Most of the knowledge about global fall-out is derived from the large scale series of atmospheric tests of nuclear weapons carried out mainly by the USA and the former Soviet Union, before the Partial Test Ban Treaty (1963). Since then 63 further atmospheric tests (and 52 underground tests) have been carried out by China and France. The other nuclear-weapon states have carried out about 670 tests since the Treaty but they were all underground explosions. By the end of 1980 the total number of tests of all kinds was estimated to be about 1,270. The number of tests which contributed to the global fall-out, that is to atmospheric, ground surface, water surface, and underground with venting, is believed to be about 550. The majority were fission bombs, many of a yield less than 1 kt, but the smaller number of thermonuclear bombs ranging up to 60 Mt contributed most of the explosive yield. The total yield was nearly 300 Mt, about half of which was due to fission. The majority of tests took place in the northern hemisphere and therefore most of the fall-out radioactivity remained in that hemisphere, predominantly in the temperate zone.
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.
Those substances that decay rapidly (that is, which have a short half-life) vanish very fast, but radioisotopes with a long half-life, such as strontium-90 and caesium-137 still fall in considerable quantities many years after the detonation of a nuclear explosion.
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.