Fallout is deposited from the atmosphere of radioactive aerosols arising from nuclear weapons tests. Fallout are distinguished: local, tropospheric and stratospheric.
Local R. O. represents a large, mostly melted particles that fall under the force of gravity near the place of explosion. Main sanitary significance is defined as sources of gamma radiation. Tropospheric fallout - radioactive particles of micron and submicron size received by a nuclear explosion in the troposphere. For 2-6 weeks they are carried by air currents around the globe, gradually deposited on the earth's surface. They contain mostly short-lived isotopes, of which the largest sanitary danger of radioactive iodine. A decisive role in cleansing the troposphere play precipitation (especially drizzling rain). Stratospheric (or global) radioactive fallout - radioactive particles produced by a nuclear explosion in the upper atmosphere (the stratosphere) and slowly settling to the ground. The time of their stay in the stratosphere varies from 2 to 5 years. They contain mostly long-lived isotopes (strontium-90, cesium-137, cerium-144 and others). The density of the global radioactive fallout uneven at different latitudes. The maximum loss after stopping in 1963, mass testing of nuclear weapons took place between 20-60 ░ n. Because of the peculiarities of the transfer of air masses are seasonal fluctuations of the density of depositions with a maximum of spring - beginning of summer. Further migration fell on the earth's surface radioactive isotopes on the biological chain is determined by their biological availability. Unlike local fallout, consisting mainly of large melted insoluble particles, stratospheric radioactive fallout, consisting of fine fractions, higher degree (strontium-90, cesium-137) biological availability. The solubility of these particles can reach 100%. In the first years after the nuclear weapon tests pollution of ground vegetation everywhere was due to direct deposition R. O. on the surface of plants. In the subsequent, more specific importance migration them in the plant root path from the soil. The highest density of radioactive fallout treated 1963, resulting in maximum exposure doses of population due to stratospheric precipitation occurred in 1963-1964 However, even in this period they do not exceed the dose limit established for the population. By reducing the density of radioactive fallout and radioactive decay of a radioactive isotope is decreasing year to year. Accordingly decrease the absolute value of the exposure doses of the people. For example, the dose of bone in adult residents of Moscow in 1968 due incorporated strontium-90 was 2.6 mrad/year, i.e. less than 10% of the dose limit.
No real danger to health from such doses eliminates any preventive or curative measures.
However, control over the radiation situation caused by the global precipitation on the territory of Russia is carried out continuously to study regularities. The monitoring object are the atmospheric air, soil, surface waters, vegetation, food products. There are also constant monitoring of the content of radioactive substances in the body of different age groups and population dose due to the global radioactive fallout.

Fallout - fallout from the radioactive cloud formed as a result of explosion of a nuclear device.
There are local radioactive fallout, trailing and global. R. O. local are particles of the order of tens of microns and more; rolled on the ground explosions in several tens of hours and spread them over the direction of the wind at 500-550 km from the centre of the explosion. Lagging (polypolyline, tropospheric, continental) P. O. particles have a size of about 1-5 MK; fall in the weeks since the moment of explosion (usually up to 5 months) and distributed in the latitudinal direction. Global stratospheric R. O. have a particle size of less than 1 micron; fall for a number of years, usually more intensively in spring.
The nature of education and drop R. O. depends on the nature of the explosion (land, air, surface), TNT equivalent, a nuclear device, the nature of the soil in the area of explosion and meteorological factors.
At ground explosion of a nuclear device with a TNT equivalent of about 1 MT to ordinary substances that make up the fireball (fission products, the shell charge and other parts, preheated to a temperature of several million degrees), is added around 20,000 those of the lost ground. In addition, the air flows, accompanying explosion, raise a significant amount of dust and other solid particles composing "leg" specific "mushroom cloud of a nuclear explosion.

Radioactive contamination as a result of this explosion in an hour after it covers the territory about 28 thousand km2. Local precipitation is about 90% of the total raised at land mass explosion of the soil.
Fine part of the up in the air ground enters the stratosphere, with a further basis for the formation of the global radioactive fallout. When the air explosions (fireball does not touch the surface of the earth) the formation of local rainfall does not occur, and the bulk of radioactive debris raised in the stratosphere, forms a further global precipitation.
Thus, as a result of explosions of nuclear devices in the atmosphere reaches a large number of different radioactive isotopes, which are carried by air, polluting most remote from the blast area.
For many years, Sr -90, Cs -137 and other radioactive isotopes formed during the explosion, would be transported by air currents. The highest density of radioactive contamination create local R. O., the isotopic composition of which is represented mainly by short-lived radioactive debris, primarily radioactive J131.
The decline of radioactivity in the first period after a nuclear explosion (up to 100 days) obeys the law t-1,2. Isotopic composition trailing R. O. less diverse, however, and J131 plays a rather significant role. The composition of the global fallout radioactivity presents long-lived debris - Sr -90, Cs -137, CE -144, Pr144, Pm147 and some others, however, the biological significance mainly represent Sr -90 and Cs -137.
Falls to the surface of the soil and plants, radioactive fallout come in cycles continuously passing on Earth biological processes, it is difficult migrating in various parts of the ecological chain (see Ecology, radiation). Investigation of the mechanism of penetration Sr90- component of the global R. O. has shown that up to 80% of it is concentrated in the surface layer of uncultivated land in the thickness of 5 cm In arable lands, he distributed on the whole depth of plowing. With the continued loss R. O. amount of Sr90falling in the human diet, largely dependent on the direct contamination of the leaves, flowers and lower parts of perennial plants than from its root uptake from soil. If the rate of radioactive fallout decreases the uptake by roots begins to dominate.
A number of radioactive substances formed in the explosions of nuclear devices, enters the human body and accumulates. Especially great biological importance Sr90, with 28-year half-life and accumulate in the human skeleton. The main accumulation of strontium is growing in parts of the bones - the epiphysis, turning into a kind of strontium "depot", where is a constant exposure to nearby sites of bone and bone marrow (see Radiation toxicology).
In connection with the biological significance R. O. in the USSR and several other countries, developed and implemented a system for monitoring the levels of radioactive fallout, migration and receipt in an organism of the person the most important radioactive fragments of nuclear fission.
In the result of the Treaty banning nuclear tests in the atmosphere number R. O. has decreased significantly and continues to decline. Cm. Radiation hygiene.