|Little further international progress was made in the 1940s, but national committees continued to advise on protection as new information became available on radiation quality and the influence of dose rate. The consequences were emerging from an epidemiological study involving hundreds of young women who had been exposed, some since the 1910s, to luminous paint containing radium-226 and mesothorium used for painting numerals on instrument dials. A high proportion of these workers, who ingested the radioactive material by licking their contaminated brushes in order to produce a fine tip for applying the paint, developed jaw-bone necrosis and bone cancers after a minimum latent period of several years.|| In 1941, the U.S. Advisory Committee on X-ray
and Radium Protection recommended the adoption of a maximum body burden
of 0.1 microcurie of radium for workers - (the Curie had been defined in
1910 as the activity emitted from 1 g of radium). For radon gas generators
that were in widespread use for preparing radon seeds for radiotherapy,
the U.S. National Bureau of Standards recommended an intake limit of 10-11
curies per litre of air. The biological basis for this recommendation
was that miners exposed to radon gas could develop lung cancer.
| It was during World War II that
plutonium was first produced for atomic bombs (the USA Manhattan Project)
and many workers on the project at Los Alomos were potentially exposed
to plutonium and gamma-emitting fissionable materials in the laboratories.
The US Advisory Committee attempted to ensure that the internationally
recommended working conditions previously defined were adhered to.
Urgently conducted animal experiments showed that doses of about 1 r per
day had no effect upon fertility, but doses of about 5 r per day impaired
fertility. Hereditary effects of a minor nature were also observed
at accumulated doses of about 100 r delivered at dose rates between 0.1
to 10 r per day. On the basis of these experiments and in view
of the national emergency in wartime, the US authorities decided that about
1 r per day from penetrating radiations to workers was justified in order
to carry out the programme of producing large quantities of radioactive
materials, with the proviso that the exposure of large numbers of
people to higher doses was not acceptable because of the possible adverse
effects on the gonads.
At the time of the Sixth International Congress in 1950, the International Commission on Radiological Protection (hereafter abbreviated to ICRP or the Commission) superseded the International X-Ray and Radium Commission. In preparing its recommendations, the Commission considered the fundamental principles upon which appropriate radiation protection measures could be established, while leaving to national protection bodies the responsibility of formulating specific advice, codes of practice or regulations that were best suited to the needs of their individual countries. Rules governing the selection of members of the ICRP, the appointment of the Commission Chairman,Vice-Chairman and Scientific Secretary can be obtained from published recommendations at about this time .
|The Commission consisted of an English radiologist
as chairman and an English physicist as secretary. In addition there was
1 Swede, 4 Americans, 2 Germans, 1 Dane, 1 Canadian, 1 Frenchman and 1
Englishman, representing a balance between radiologists, physicists and
biologists. Subsequent changes in the Commission mebership have maintained
this balance, with chairmen and secretaries fron various countries including
England, Sweden, Canada and Argentina. In essence, the Commission is composed
of a chairman and not less than six nor more than twelve other members,
chosen on the basis of their recognised activity in the fields of medical
radiology, health physics, and radiation biology, with regard to an appropriate
balance of expertise rather than to nationality. The composition of the
Commissiom is revised every four years and the Executive Committee of the
International Society of Radiology is informed of any changes at the next
meeting of the Congress. The Commission is supported by specialist committees,
initially giving advice on.permissible doses from external and internal
radiations, and on the handling and disposal of radioactive isotopes.
The 1950 Commission reiterated the previously held view, now based on a firmer biological evidence, that radiation-induced cancers and leukaemias, cataract, impaired fertility and the possibility of radiation-induced hereditary diseases in the irradiated person's progeny needed to be taken into account when setting standards. In view of the increasing complexity of radiation spectra to which workers in industry were likely to be exposed, as opposed to the more restricted spectra used in medical applications, it was considered prudent to reduce exposures to all radiations to the lowest possible level. This is the first formal acceptance of the concept of optimisation.
| The maximum permissible occupational
dose following whole body exposure was restricted to 0.5 r per working
week for x- and gamma rays of less than 3 MeV of quantum energy at the
surface of the body, corresponding to about 0.3 r per week in the
blood-forming organs (a recognition of differing tissue radiosensitivity);
and 0.03 r per working week for neutrons (a recognition of the more damaging
effects of high linear energy transfer (LET) radiations). For
beta rays and gamma rays affecting only superficial tissues (the basal
layer of the epidermis), the recommended maximum permissible dose was 1.5
r per working week.
For incorporated radionuclides, values of maximum permissible concentrations of radioactive materials in the air or in drinking water were proposed, taking into account the metabolism and characteristics of the material containing the radioactive isotopes. Maximum permissible concentrations of radioactive isotopes of radium-226, plutonium-239, strontium isotopes, natural uranium, polonium-210, hydrogen-3, carbon-14, sodium-23, phosphorus-32, cobalt-60 and iodine-131 were given, based upon the intake by a “reference man” in which the weights of tissues and organs had been averaged so as to be applicable to all workers.
It had also been accepted that the röntgen was inappropriate as a measure of the tissue doses from external radiation. In 1953, the ICRU recommended that limits of exposure should be based on consideration of the energy absorbed per unit mass of irradiated material at the point of interest. They introduced the rad (radiation absorbed dose) as a unit of absorbed dose in tissue.
In 1954, the Commission introduced the rem (roentgen equivalent man) as a unit of absorbed dose weighted for relative biological effectiveness of the different types of radiation distributed in tissue (called the dose equivalent in 1966). The rem was defined as "the absorbed dose of any kind of radiation which may have the same biological effectiveness as 1 rad of x-irradiation with average specific ionisation of 100 ion pairs per micron of water
|(linear energy transfer), in terms of its air
equivalent, in the same region". For practical purposes, a value of
10 for relative biological effectiveness (a term abandoned later and replaced
by a quality factor) was used for neutrons and protons up to 10 MeV, based
upon occurrence of cancer as the measurable endpoint. A value
of 20 was used for heavy recoil nuclei, based upon evidence of cataract
While no change was recommended in the maximum permissible dose for whole body irradiation, there was a need to reconsider the values for partial body irradiation involving the critical tissues and organs at risk. The recommended weekly permissible doses for whole body exposure to any type of radiation were 0.6 rem in the basal layer of the epidermis in a significant area, and 0.3 rem for irradiation of a significant volume of the blood-forming organs, gonads, and lens of the eye. Less restrictive limits were proposed for radiation with very low penetration into tissues and where a limited portion of the extremities were exposed.
With increasing availability of data on the metabolism and excretion of incorporated radionuclides, the Commission recommended values of maximum permissible body burden and maximum permissible concentrations in air and water for a large number of radioactive isotopes encountered in occupational exposure, in addition to those defined in 1950. The values recommended were based on knowledge of the chemical and physical characteristics of the materials. In choosing the maximum permissible body burden of a radionuclide, it was necessary to define the tissue or organ where the greatest damage from radiation could be expected. The maximum permissible total body burden expressed in microcuries, or the maximum permissible concentration in air or water expressed in microcuries per cubic centimetre, were intended to indicate the amount of radioactive isotope accumulated in the most heavily irradiated (critical) tissue or organ of a worker, such that the tissue or organ would receive an average dose of not more than 0.3 rem per week.
| The Commission carried out an
extensive revision of the recommendations dealing with protection against
x rays at potentials of 5 kV up to 2 million volts and introduced numerical
data regarding the permissible exposure levels for high energy x rays and
neutrons. In view of biological evidence of genetic damage at low
doses, it was recommended that every effort should be made to ensure that
the use of diagnostic x rays should be kept to a minimum and that extraneous
exposure of members of the public should be carefully monitored.
For example, the wide use of x rays associated with shoe-fitting machines
should be restricted to strictly medical procedures under the supervision
of qualified experts.
Amendments to the 1954 recommendations were made in 1956, reinforcing the concept of a controlled area for workers under the supervision of a radiation safety officer. For workers outside this controlled area, the maximum permissible levels of exposure should be one-tenth of the recommended occupational level.
The reality of occasionally exceeding the recommended weekly maximum permissible dose was recognised. To overcome this, a relaxation was introduced so that the time interval could be extended, provided that the total dose accumulated in a period of 13 consecutive weeks did not exceed ten times the recommended weekly dose. Special care was needed to ensure that pregnant women were not inadvertently exposed to a dose which exceeded the maximum permissible weekly dose.
Several significant developments occurred in the 1950s which influenced the Commission’s attitude to protection. These included a rapidly expanding civil nuclear power prgramme where many more people outside medical radiology would be exposed to low doses of radiation; another was the establishment of the Atomic Bomb Casualty Commission (later to become the Radiation Effects Research Foundation) in Japan to make a long-term epidemiological study of the effects of atomic bomb radiations on the surviving populations of Hiroshima and Nagasaki. This was referred to as the Life Span Study.
|(These studies are expected to continue for many
years yet to come since a substantial numbers of the populations are still
alive). And another was the establishment of the United Nations
Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) to provide
a means of correlating and updating information on genetic and somatic
effects, on natural radiation levels, the levels of environmental contamination
from man-made sources of radiation, and on levels of exposures during medical
procedures and occupational exposures.
Reviews were carried out by the British Medical Research Council (MRC) and by the Biological Effects of Ionizing Radiations (BEIR) Committee of the USA National Academy of Sciences (NAS) on the available radiobiological evidence of genetic effects. These studies were a response to thermonuclear weapons testing programmes undertaken by various countries and the need to consider the consequences on the world population from atmospheric radioactive fallout. The conclusion of the reviews were that the principal hazard at low doses was an increase in the natural incidence of hereditary diseases; that repair did not occur in germline cells (later disproved); and that genetic radiation effects at low doses could be adequately expressed by a dose-response relationship which was linear down to zero dose.
The Commission concluded from these various reviews that the dose accumulated over a long period was the deciding factor in inducing hereditary effects, provided the intermittent doses were sufficiently small. A weekly dose rate of 0.3 rem translated into a cumulative whole body dose of 750 rems in a working lifetime of 50 years. In considering that this was too high a cumulative dose on the basis of hereditary effects, it was recommended that no individual worker should receive more than a cumulative dose to the gonads of 50 rems up to the age of 30 years and not more than an additional dose of 50 rems up to the age of 40 years. At these ages, it was assumed that over half and over nine-tenths of the irradiated persons' children would have been born respectively.
|Taking these combined reviews into consideration, the Commission recommended in 1958 that maximum limits of occupational exposure should be set in terms of dose accumulation at various ages according to an age-related formula, (D = 5(N - 18), where D is the whole body dose in rems and N is the age in years above 18 years.|| For a whole population, it was recommended
that the genetic dose could be assessed as the annual genetically significant
dose multiplied by the mean age of child-bearing, which for the purpose
of these recommendations, was taken to be 30 years; thus, the genetic dose
to the population should be kept to the minimum consistent with necessity
and should certainly not exceed a total of 5 rems, additional to natural
background radiation and the lowest practicable contribution from medical