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Radiation Training Module 4

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Open Source Radiation Training - Module 4

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External and Internal Dose Limits

This module provides information about the following topics:

  • Annual Radiation Dose Limits
  • As Low As Reasonably Achievable (ALARA)
  • Annual Limits of Intake
  • Declared Pregnant Worker Program

Annual Radiation Dose Limits

The Nuclear Regulatory Commission (NRC) and the university have established dose limits which are based on recommendations from national and international commissions.  The table below lists the limits set by the NRC:
 

Organ

NRC Limit
(mrem/year)

(mrem)

Comments

Whole Body

5000

100

Includes dose from both internal and external sources.  The Whole Body limit applies to exposure of the torso and head when the radiation is penetrating enough to irradiate tissues at a depth of  1 cm where the deeper blood-forming tissues are located. 

Lens of the Eye

15,000

300

The Lens of the Eye limit applies to exposure of the eye to radiation penetrating enough to irradiate the lens, at a depth of  0.3 cm. 

Extremities

50,000

1000

The extremities include the arm or leg below the elbow or knee.  The Extremities limit applies to exposure of the extremities when the radiation is penetrating enough to irradiate tissues at a depth of  1 cm. 

Skin

50,000

1000

The Skin limit applies to dose deposited in the skin when the radiation is penetrating enough to irradiate tissues at a depth of  0.007 cm. 

Embryo/Fetus

500 (for the entire pregnancy)

50

Applies only when a Declaration of Pregnancy has been submitted 

Occupational exposure of a minor

10% of the limits above

50

Applies to anyone under 18
 years of age

Member of the general public

100

50

 

Investigational Levels

The University has established investigational levels at doses considerably less than the dose limits. When a worker accumulates during any part of a year a dose at or above the investigational level, ESH will investigate to determine causes of the dose and to recommend practices to minimize radiation exposure in the future.

As Low As Reasonably Achievable (ALARA)

Since the current model of radiation-induced cancer risk assumes that there is a risk no matter how low the radiation dose, it makes good sense to minimize radiation exposure. In fact, the University is required by NRC regulation to keep doses As Low As Reasonably Achievable (ALARA). This means that the University must work to keep doses as far below the dose limits as can reasonably be achieved.

To keep doses ALARA, the University has established investigational dose levels and will investigate any dose exceeding these levels in an effort to address causes of unnecessary radiation exposure.

In addition there are a variety of practical steps that you can take while working in the lab to minimize your radiation exposure. 

Annual Limits of Intake

Internal Exposure
Radiation exposure can occur both from radiation sources outside the body (external exposure) and from radiation sources within the body (internal exposure).  For example, internal exposure may occur through absorption through the skin when the skin becomes contaminated or when a volatile radioisotope, such as I-125, is inhaled.

Internal Dose Information for Common Radioisotopes
The NRC has defined a quantity, the Annual Limit of Intake (ALI), which is the amount of a specific radioisotope taken internally which will produce an annual Whole Body dose of 5000 millirems.  The following table below lists the ALIs for radioisotopes commonly used at Murray State University :
 

Radioisotope

ALI for Ingestion 
(millicuries)

ALI for Inhalation
(millicuries)

H-3

80

80

C-14

2

2

P-32

0.6

0.9

S-35

10

20

I-125

0.04

0.06

Declared Pregnant Worker Program

This section provides a general overview of the Declared Pregnant Worker Program.  The NRC’s Fetal Dose Regulations apply only to a woman who has

voluntarily informed her employer, in writing , of her pregnancy and the estimated date of conception.  The dose to the fetus resulting from occupational exposure of a declared pregnant woman may not exceed 500 mrem for the entire pregnancy.

Submitting a Declaration of Pregnancy
Any radiation worker who is pregnant or believes that she may be pregnant should contact ESH. All inquiries will be kept in confidence. ESH will take the following steps:

  • Provide an opportunity to submit a Declaration of Pregnancy.  (A Declaration of Pregnancy form is included in the Radiation Safety Manual or may be obtained from ESH.
  • Provide information concerning risk of fetal radiation exposure.
  • Evaluate the worker’s dose history and exposure potential.
  • Make recommendations for reducing radiation exposure.
  • Monitor the worker’s radiation dose with regard to worker and fetal dose limits.

For the type of radiation work performed at Murray State University, it is rarely necessary to recommend reassignment or changes to job duties.

If a written declaration of pregnancy is not submitted to ESH, then the worker’s dose continues to be controlled under the normal dose limits for radiation workers.

 

This is the fourth module of the Radiation Safety Training Modules.

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