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COSMIC RADIATION
Radiation consists of a flow of atomic and subatomic particles and of waves, such as those that
characterize heat rays, light rays, and X-rays. All matter is constantly bombarded with radiation of both
types from cosmic and terrestrial sources.
Radiation can be ionizing (i.e. capable of turning atoms and molecules in matter and tissue penetrated
into ions2 and thus causing an electrical effect) or non-ionizing.
Cosmic radiation is the collective term used for radiation coming from the sun (the solar component)
and from the galaxies of the universe (the galactic component).
Ionizing Radiation
Matter consists of a number of simple substances called elements which, as mixtures and compounds,
form all the materials present on earth and in the universe. The basic unit of any element is the atom, and
it is the characteristics of atoms that determine the properties of the elements.
Some elements are naturally radioactive, i.e. they change into other elements with the emission of
atomic particles: radiation. Radiation may be thought of as energy in motion or as transfer of energy.
When radiation energy is absorbed in living tissue, it may have a biological effect which depends not only
on the amount of energy absorbed, but also on the specific effect of the wavelength and on the type of
particles (electrons, neutrons, positrons, etc). If ionization takes place, it frequently results in chemical
changes in matter and in living tissue. These changes may affect the behaviour of living cells and the
organism may suffer obvious injury if enough cells are involved. Unlike light and heat, which are also
forms of radiation, ionizing radiations cannot be directly detected by the body’s senses, except that the
dark-adapted eye, during the 5-6 hours of a transatlantic polar flight, may see a few flashes of light as
cosmic rays directly ionize the retina.
Source and type of radiation
The ionizing radiation to which everyone on earth is exposed comes from the universe, partly from outer
space (galactic radiation of constant intensity) and partly from the sun (solar radiation of increased
intensity during solar flare activity). Furthermore, the earth itself produces ionizing radiation (of intensity
varying with geographical location). Even food and drinking water are sources of ionizing radiation.
In addition to this natural background radiation which has existed for millions of years, there are
2ion: an electrically charged atom or molecule.
ICAO Preliminary Unedited Version — October 2008 II-1-12
modern man-made sources of ionizing radiation: building materials in houses, medical and dental X-ray
examinations, radioactive cargoes, fall-out from atmospheric testing of nuclear weapons, and possibly
nuclear power plants.
Unit of measurement
The effect of both electrons, α-particles and γ-radiation on living tissue is to cause ionization. The amount
of radiation energy absorbed is measured in gray (Gy)3, but as the biological effect depends not only on
energy but also on the composition of the radiation (different particles etc.), it is necessary to weight the
absorbed dose to obtain a dose equivalent, a unit of “harmful effect”, called sievert (Sv).4
Background Radiation
Everybody on earth is exposed to radiation. The total normal radiation (background radiation) per person
is virtually constant with a yearly dose equivalent estimated to be about 2 mSv in most countries. But due
to natural radioactivity in soil and rocks, in parts of Brazil the yearly average is as high as 5-10 mSv, and
in Kerala (India) a yearly dose of 28 mSv has been measured. In the industrial countries radiation from
other sources, mainly medical X-rays, is estimated to around 1 mSv. On top of this exposure, totalling
3 mSv/year, may be added “occupational exposure”.
Occupational Exposure
In recent years world-wide attention has been given to the problem of air crew being exposed to ionizing
radiation. In the European Union, following the recommendations of the International Commission on
Radiological Protection (ICRP), specific provisions on the health protection of air crew against dangers
arising from exposure to cosmic radiation have been laid down in legislation since May 2000. There is,
however, still some disagreement about the effects and even the amount of radiation to which air crew are
exposed while on duty.
A substantial part of the cosmic radiation is absorbed by the upper part of the atmosphere or deflected
by the earth’s magnetic shield, but some penetrates to ground level and thus forms part of our natural
environment. The intensity of cosmic radiation increases with height above sea level because the
atmosphere becomes thinner and absorbs less of the radiation (e.g. the intensity of cosmic radiation is
　
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