The Application of External and Internal Radiation Exposure Limits (American Industrial Hygiene Association Quarterly Vol 16 No. 4 Dec 1955)

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lone of them action of the the gas. The om the kilns AMERICAN JDUSTRUU. HYGIENE ASSOCIATION QUARTERLY Vol. 16, No. 4 , December 1955 The Application of External and lntevnal Radiation Exposure Limits Oak Ridge Nafonal Laboratory, Oak Ridge, Tennessee ROM THE EARLY beginning of work with Fionizing radiation there has been the problem of coining useful units and making certain that they are used properly. Some of the early units for ionizing radiation isfactory and unreliable as the early electrical unit which purported to measure current by the motion it induced from a voltage a p plied to the nerve in a frog’s leg. The erythema unit was not a good choice because of the large variation in human susceptibility to eiythema and because of the energy dependence-the roentgen dose required to produce an erythema is much less for 60 KV x-rays than for 1 Mev x-rays. These two complicating factors, i.e., human variability and energy dependence, continue even today to present difficulties in radiation dosimetry. One of the earliest and more commonly used units of ionizing radiation is the roentgen. This unit is intended t o measure the damage produced in soft tissue by ionizing radiation and is a compromise between theoretical requirements and a property of radiation that is reproducible and can be measured conveniently. From the definition of the roentgen and as a result of the behavior” of radiation in its passage through air, it may be stated that the roentgen corresponds to 1 esu of charge produced in 1 cc of -- R rented at the Sixteenth Annual YAng of the New York. April 28, 1D66. *mi. practical appUution of the roentgen Is mib le because tbe a u produced in air by meondarr ehctroru orialnating In 0.001298 g of air (1 ee a h m t 0°C and 760 m m Re) t ram1 to the a u eolkeM In 0.001298 r of air which are p duce d by Heondars electrons or&!- mating In the surrounding d r wall of quilibrium thicksecondary electrons originating from the (energy from a few Kev to about 3 MeV) in a surrounding air wall of equilibrium thickness. Some of the limitations of this unit may be summarized as follows: (1) it apcient to produce ionization in a i r but low enough that the secondary electrons dissipate their energy in the medium in the neighborhood of their origin; (2) it refers to esu produced in air rather than in tissue; and (3) it refers to esu per cc of air rather than ergs!g of tissue. Some of these difficulties are avoided by the use of the rep and the rad which are units of absorbed energy. The rep corresponds to an absorbed dose of 93 ergs g of tissue and the rad corresponds to an absorbed dose of 100 ergslg of any medium. Both units apply to any type of radiation which directly or indirectly proproduce ionization directly while y-rays and fast neutrons produce ionization indirectly by way of secondary electrons and the recoil atom respectively. Difficulties arise in the use of these units; for example, there is no certainty that the damage is proportional to the energy absorbed in tissue and even if it were, in present-day dosimetry perhaps the best that can be accomplished is t o measure the ion pairs produced in a tissue equivalent chamber. In converting this measurement to rep or rad units the BraggGray principle must be applied in which case a knowledge of the relative mass stopping power (P) and the electron volts to produce an ion pair (w) is required. For the Bragg-Gray principle to apply not only

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be known, but they should be relatively independent of energy and of the physical state of the medium. Our knowledge of P and w is sufficient for approximate dosimetry in the common energy range, but unfortunately both P and ware somewhat dependent on the energy as well as upon the type of radiation and the atomic number of the medium and P for heavy particles seems to be a function of the physical state of the medium.’ Also there is good reason to suppose that radiation damage depend3 on the differential stopping power or linear energy transfer (LET)of the radiation in tissue and information relative to this relationship is not readily available and dimcult to obtaln experimentally with practical dosimetry since the differential values of LET may be changing constantly along the path of each ionizing particle. Usually one must be satisfied with a measurement of the rep or rad in tissue for each type of radiation, Le., a, 8, n, etc., and obtain the total dose in rem units by applying a single relative biological effectiveness (RBE) for simetry using threshold detectors which enables one to measure the LET associated with the neutron absorbed dose and perhaps similar techniques can be applied to other types of radiation. In the use of the rem unit i t must be kept in mind that RBE differs for the various types and degrees of radiation damage. and in some cases is a function of the dose rate.3 Although it has not been proved, there is good reason to believe that there is an LET that corresponds to a maximum RBE for each type of radiation damage. For example, if it requires 200 ion pairs per fJ of tissue in the region of interest to produce a certain biological damage, it follows that the RBE of a high energy ionizing particle would increase as its ion pairs/p of tissue and would decrease at lower energy where S is greater than 200 ion pairs’p of tissue. It is sometimes assumed or implied that the ionization per se is responsible for all the damage from ionizing radiation. This, however, may be f a r from the truth since nuclear collisions, molecular and atomic excitations. and various other forms of energy transitions may make an important contribution to the damage. There has been considerable confusion in the use of the terms “dose” and “absorbed dose.” The International Commission on Radiological Units has made a distinction between these two terms by defining the dose as a quantity of radiation passing through unit area perpendicular to the beam or the time integral of intensity, Le., ergs‘ cm2, and absorbed dose as the amount of energy imparted to matter by ionizing particles per unit mass of irradiated material at the place of interest. It should be pointed out that most of the literature uses the word “dose” to refer to what is now officially defined as “absorbed dose” and the confusion is by no means removed by having an official definition of “absorbed dose” since even today most writers continue to use “dose“ when they mean “absorbed dose.” I n any case, only the present official definitions are used in this discussion and absorbed dose is given in rep and rad units or in terms of ergslp. On this basis the roentgen is unit, however, if the average value of w is known i t is related to the “absorbed dose” by the equation It should be noted that the rad is a unit of absorbed dose in any medium and, therefore, the medium in which the rad is measured should be specified, and that the roentgen relates to the “absorbed dose” in air while the rep is a unit of “absorbed dose” Another unit that is sometimes carelessls used in defining radiation exposure limits and that deserves discussion here is the curie. This unit was originally defined as that amount of radon in equilibrium with 1 g of Ra2Za. A t present i t is defined as that amount of any radionuclide that disintegrates at the tions per sect one or more in these cas body in air of the paren. and q as lis, long half li

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ies the word alue of w is 3 is a unit of lly defined as Industrial Hygiene Quarterly grates a t the rate of 3.700 x1Olo disintegrations per second. One gram of Ra226 in equilibrium with its radon daughter corresponds, according to the new definition, to about 3.6,‘ 3.7 curies. In referring to a curie, or a fraction thereof, of a radionuclide that has one or more radioactive daughters it is usually necessary to be specific and indicate the fraction of equilibrium of each of the daughter products. In the most recent published tables of maximum permissible consible body burden, q, of the radionuclides,’ values are given for parent-daughter pairs in these cases it is stated specifically that the values of MPC refer to pc!cc of the body in air or water and the q refers to pc of the parent deposited in the total body following an exposure for 70 years. It is assumed that the daughter products accumulate while the parent isotope is in t h e body and reach transient equilibrium with the parent unless the radioactive half life of the daughter is a large fraction of 70 years. Therefore, although it is assumed none of the daughter products are present during inhalation or ingestion of the parent, the growth of the daughters in the body is responsible for reducing the values of MPC and q as listed in the ICRP Handbook4 and the NCRP Handbook-52.5 If the daughter is present initially in the air or water, the resulting (MPC), of the parent and daughter mixture is given by the equation: fractions of the respective (MPC’s) for the parent and daughter based on the same body organ. In the case of Rag’s values a r e given in the ICRP Handbookg f o r Ra*?6 + 55% of daughters. This may be defined as meaning that MPC and q are given for pure Razz‘; taken into the body where a portion is deposited in the bone and the daughter products (down to RaD) reach 65% of equilibrium contributing 55% of their energy to the absorbed dose. Because of the long half life of RaD, i t together with its 1.5% of the total dose to the bone over a 70year exposure period. The values in the ural thorium, and Rn??‘ are exceptions to the above general rules in that, when referring to MPC and q, one curie of natural uranium means 2.03 curies, i.e., 3.7 x 1 O l o and 9 x 10‘ dis 1 sec of U23.7, and one curie of natural thorium means 2 curies, i.e., 3.7 x of Th”“.One curie of Rn?’? means 5 curies, Baric Criteria of Permissible Exposure THERE are two basic criteria of permissible exposure to ionizing radiation: (1) an absorbed dose rate of not more than 0.3 rem’wk to the critical body tissue, and (2) a body burden of radioisotopes that delivers an absorbed dose to the bone which is not more than that delivered by 0.1 pg of Ra2*8. Both of these criteria are applied to the calculation of the maximum permissible concentration in air and water, (MPC), and (MPC),, and the permissible body burden, q, of radioactive material deposited inside the body, but usually only the first criterion is related to permissible external exposure. Fig. 1 indicates the maximum permissible exposure of various portions of the body to ionizing radiation from an external source a s discussed in detail in the National basic absorbed dose rate has undergone two major reductions during the past two decades. First, it was 0.2 r/day (or 1 0.5 r;wkjs and recently‘J~0 it was reduced to 0.3 rem.’wk with the provision that in certain cases the absorbed dose rate could be increased to 0.9 rem/wk provided the total absorbed dose in 13 weeks does not exceed 3.0 rem. This 25% penalty in the 13week exposure when the 0.3 rem/wk rate is exceeded has served the useful function in our Laboratory (ORNL) of discouraging the practice of exceeding the basic exposure rate. There are no human experiences or animal experiments that establish strong support of the 0.3 rem/wk value. It is believed that the average exposure of radiologists during the past two decades has exceeded this value considerably-perhaps by a factor of 10-and there is irrefutable evidence that many of them have been damaged in that the incidence of leukemia in

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this group is about nine times that among medical men that are not radiologists.* Although there is no way of knowing how safe the present basic permissible rate of 0.3 rem'wk really is, it is certain that it should not be averaged over a large fraction of a life span. I t should be noted that if the basic exposure rate as outlined in Fig. 1 were permitted for the lifetime of an individual, the critical body organs would accumulate an absorbed dose of 1200 rem, the skin 2400 rem and the appendages of the body 4200 rem. I t is impossible a t present to assess accurately the damage to an individual who accumulates such a large absorbed dose. I t seems likely that such exposure would increase the probability of leukemia and other forms of cancer, that it might result in an increased incidence of cataracts, that it would shorten the average expected life span by about 10% ; and, if a large fraction of the population received this exposure, it could increase the number of mutations by several orders of magnitude, perhaps resulting in serious consequences to the human race. Certainly we, as health physicists, agree that such accumulation of absorbed dose is undesirable, that probably all radiation exposure is harmful to some degree, and that our goal should be difficulty -we do not mit us to

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mcer, that it tainly we. as roal should be tensibly the purpose of this meeting was to review the more recent surveys of per3 sons having large 1 body burdens of Ra??O 8 whether or not the 0.1 ug value could be increased in light of the lack of body damage E at this level and in view of the fact that cated that many perradium dial painters -who now have large body burdens of Ra2?* to permit no unnecessary exposure where a may have carried initially large burdens reasonable expenditure of time and money of mesothorium (Ra?"). If a person will prevent it. initially carried 100 pc of radium comOne may ask what is a reasonable ex- posed of 10 /IC of RaZZGand 90 pc of Ra2Z8 penditure to reduce radiation exposure to and if 10 years later measurements were the lowest possible value? The answer can made only of the RaZZGremaining in the be more easily given than interpreted ; body the integrated absorbed dose (neglectnamely, "A reasonable effort is one that re- ing the contribution of Ra"* and its daughduces the hazard from ionizing radiation to ters) would be underestimated by a factor the same order of magnitude as other haz- of 10. A t first thought one might suppose ards with which we are accustomed-over- that the underestimation from the neglect eating, lack of rest, dusty homes, etc." The …