Measurement of the number of ionisations which radiation causes per unit distance as it traverses the living cell or tissue is called the linear energy transfer of the radiation. The concept involves lateral damage along the path, in contrast to path length or penetration capability. Medical X-rays and most natural background radiation are low LET radiation, while alpha particles have high LET. On the average, fission fragments have high LET.
        The density of ionisation causes special problems in sperm and ova because the damage (protein breakage) is concentrated within a few cells. The two-year sterility of Japanese fishermen exposed to fallout from the 1954 hydrogen bomb test is probably an example of this effect. Sperm and the cells which produce sperm were damaged beyond their capability of prompt repair.
        As a young girl in St George, Utah, USA, Elizabeth Catalan used to stand outdoors and watch the mushroom clouds raised by the Nevada nuclear tests float overhead. She has never been able to have children. She, like some other women in St George, is unable to carry a foetus to birth. Elizabeth's father, president of a local college, died prematurely of leukaemia. He used to go horse-riding with three friends and was frequently outside when the grey clouds laden with radioactive chemicals went over. Three of the four men are now dead from cancer.
        Elizabeth's sister died in her late twenties of a thyroid disease which may have been caused by the radioactive iodine released in the atomic blasts. Elizabeth and her mother attribute many of their abnormal health problems, and those of family and friends, to the atomic fallout. No government studies have been undertaken to confirm or deny these claims. However, the situation was so widely recognised as abnormal by the local population that the Governor of Utah has filed a court claim against the US Federal Government for wrongful deaths of the people of Utah. About a thousand individual damage claims have entered the courts in the USA, and as part of the trial preparations Dr Carl Johnson undertook a detailed study of the Mormon population of Utah exposed to the fallout. It is reasonable to conclude that the health problems reported by the people of Utah are typical of what could be expected on the basis of theoretical radiobiology.[10]
        On 10 May 1984, US District Court Judge Bruce S. Jenkins ruled on the first twenty-four claims of US government negligence in its conduct of nuclear testing. He has awarded $2.6 million in damages to ten claimants. This landmark, 489-page, carefully worded decision is expected to be appealed against by the US Federal Government.
        In order to have a quantitative sense of the frequency of the different cell effects caused by radiation exposure, imagine a colony of 1,000 living cells exposed to a 1 rad X-ray (about the dose for one X-ray spinal examination). There would be two or three cell deaths, two or three mutations or irreparable changes in cell DNA and about 100,000 ionisations in the whole colony of cells -- ranging from 11 to 460 ionisations per cell.[11] While cells can repair some damage, no one claims that there is perfect repair even after only one such X-ray.
        A comparable 1 rad exposure to neutrons which have higher linear energy transfer (LET) would be expected to cause more cell deaths and more mutations. The ionisations caused would range from 145 to 1,100 per cell.
        Alpha particles which occur naturally would cause roughly 10 times as many cell deaths and mutations, and 3,700 to 4,500 ionisations per cell. Alpha particles have high linear energy transfer.
        The average number of cell deaths and mutations caused by fresh fission particles (i.e. those present soon after detonation of a nuclear bomb) would be even greater, with the ionisations as frequent as 130,000 per cell.[12] In nuclear reactors, most of these extremely high-energy early fission fragments are enclosed within the fuel rod. In a nuclear bomb blast, they are all released but they decay very quickly and do not persist long in the environment.
        If instead of thinking of a colony of living cells, we think of a person exposed to 1 rad (again about the skin dose from one spinal X-ray) of 1 MeV (million electron volts) energy, this corresponds to 2.2 billion (US) photons per cm^2 acting on the body. In the words of Karl Morgan, `It is inconceivable that all the billions of irradiated and damaged cells would be completely repaired.'[13] This unrepaired damage accumulates, eventually causing a reduction in the level of health that is normal for a particular age.
        Stated very simply, ionising radiation seriously disrupts the chemistry of the cell. It can also kill or permanently change the cell. Every exposure to ionising radiation has this effect, and it is not possible for the body to perfectly repair all of the damage. Whether or not the residual unrepaired damage is of concern to the individual exposed is a personal value judgment. It is not at all clear that ordinary people find the damage `acceptable" unless it initiates a fatal cancer, and yet this is the basis on which radiological safety standards are set in all nations of the world.
        R. M. Sievert, the famous radiologist, who had supervised radiation therapy since 1926 at the Karolinska Institute in Stockholm, pointed out at an international meeting in 1950 that `there is no known tolerance level for radiation'.[14] A tolerance level is a level below which there is no damage (sometimes called a threshold). A safety level is ordinarily a fraction (one-tenth) of the tolerance level.[14]