Environmental Health and Safety

Radiation Safety Training Manual

Regulatory Issues and Risk

1. Dose Limits

Occupational dose limits are set by the federal government and are cited in Title 10 of the Code of Federal Regulations. The following is a list of the limits for external occupational dose to adult workers.

Occupational dose limits for minors are limited to 10‰ of the limits for adult workers.

Dose limits to an embryo or fetus from occupational exposure shall be restricted to 0.5 rems DDE over the term of pregnancy. Fetal or embryonic cells are rapidly dividing and are therefore more radiosensitive and warrant a lower dose limit.

Note: It is strongly recommended that pregnant radiation workers advise the Radiation Safety Officer of their condition when they become aware of it. A workplace evaluation and possible changes in personnel monitoring may be warranted.

Facilities or institutions in which it is likely that persons would receive 10‰ of the prescribed limits shall provide individual monitoring devices for those persons.

Dose limits are determined partially by the varying radiosensitivity of particular organs or tissues. Tissues which are less differentiated or cells which are rapidly dividing are the most radiosensitive. Other parts of the body which are less radiosensitive, such as the skin or extremities do not warrant as much dose restriction as shown in the limits for shallow dose equivalent.

Radioactive material taken up by the body contributes to the dose received and is directly proportional to the amount of uptake. The amount of radioactive material taken up which results in 5 rems committed effective dose equivalent (CEDE)* or 50 rems to an organ is called the annual limit on intake (ALI). Due to the differences in the emissions and affinity for particular organs, the ALI is varies widely for different radioactive elements. For most radioactive materials used at SDSU the internal hazard far outweighs the hazard outside the body. Therefore, it is very important to avoid practices which may result in an ingestion, inhalation or dermal absorption of radioactive materials.

* Committed Effective Dose Equivalent is that dose to organs or tissues resulting from a single intake for 50 years following the intake multiplied times a weighting factor which proportionalizes the stochastic risk to the total risk of stochastic effects if the whole body were irradiated uniformly. (See the definition of stochastic effects in the following section on risks.)

2. Risks

Most of the data used to evaluate risks from exposure to ionizing radiation comes from bomb survivors in Japan and radiation therapy patients.

The following are committees or other organizations involved in the study of biological effects and risks associated with exposure to ionizing radiation. Based on risk estimates, recommendations are made regarding dose limits to members of the general public and those occupationally exposed.

• United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)
• The Committee on the Biological Effects of Ionizing Radiation (BEIR)
• National Council on Radiation Protection and Measurements (NCRP)
• International Commission on Radiological Protection (ICRP)

The Law of Bergonie and Tribideau states that cells which are less differentiated or are rapidly dividing are the most radiosensitive.

Although it is not known precisely what effects there are to biological organisms at low chronic doses of radiation, a conservative perspective should be maintained in an attempt to keep doses “As Low As Reasonably Achievable” (ALARA), see Radiation Safety Manual. Until the effects from low doses of radiation are more defined, the current viewpoint is that there is no threshold dose under which the incidence of cancer cannot be attributable. This perspective is also known as the linear-no-threshold dose model (curve 1). Some scientists believe that the risk drops off to zero at some low dose (curve 3) the threshold effect. The ICRP and NCRP endorse the linear quadratic model as a conservative means of assuring safety (curve 2).

A common sense approach should be to reduce the potential for exposure whenever practical. At higher doses of radiation (25-50 rems), detrimental effects are known to occur. There is a linear dose response at high doses of radiation indicating increases in the occurrence or the severity of induced effects as the dose increases.

There are two types of effects which could result from exposure to ionizing radiation: stochastic and non-stochastic. Stochastic effects are those which are not categorized by their severity but by their incidence. Based on the probability of occurrence, an example of a stochastic effect would be cancer. Non-stochastic effects (also referred to as deterministic) are those effects for which their severity is based on increasing exposure rather than the occurrence. For deterministic effects, a minimum threshold dose must be received before the effect will occur. An example of a non-stochastic effect would be erythema or skin reddening. There is a threshold amount of exposure before non-stochastic effects would occur. For stochastic effects, the conservative perspective assumes no threshold and that any amount of exposure may cause the effect. The goal in radiation safety is to eliminate non-stochastic effects and reduce the incidence of stochastic effects.

• Somatic effects are those which are observable in the exposed individual. An example of a somatic effect would be the formation of cataracts in the lens of the eye or blood pH changes.
  
  
• Teratogenic effects are those which occur to the developing fetus during gestation. Since the fetus is rapidly developing, its radiosensitivity is high.
  
  
• Mutagenic effects are those which involve the disruption of the base pair sequence in DNA molecules which may result in a genetic defect or mutation. Although the current regulatory position is based on the conservative viewpoint that there is no threshold dose required for such biological aberrations, experimental evidence suggests a repair mechanism corrects what could be point mutations. At very high doses of radiation the repair mechanism becomes inadequate to prevent mutations.
  
  
• Genetic effects are similar to mutagenic effects but the genetic aberration occurs in one or both of the sex cells which may effect the progeny.
  
  

It is implied that by becoming a radiation worker, an acknowledgment exists of the presence of risk. A balance exists between the risks to be accepted versus the benefit of the work. Although limits are in place which should never be exceeded, no individual can determine the acceptability of risk for another. In the evaluation of risk, you should compare the relative risks which are incurred in everyday life to your anticipated use of radioactivity. If you understand what is known about radiation and are willing to use common sense and good judgment, you are likely to conclude your risk from occupational exposure at SDSU to be one of the smallest factors to consider.

A number of studies have been performed in an attempt to quantify the risk of cancer to exposed populations. Numerical estimations of the risk are dependent on differences within the exposed population such as age and sex. Differences in the relative risk are also dependent on the type of radiation, the manner in which it was received and the probability of the particular induced effect (e.g., leukemia vs. tumor formation, etc.). Reasonable estimates of mortality due to cancer from exposure to ionizing radiation range from 3.5 to 8.0 E-4/rem.