EXPOSURE CONTROL

Introduction

Dose Limits

Relative Risk

Personnel Monitoring

Dosimetry

Whole Body

Extremity

Embryo / Fetus

Bioassay

Environmental Monitoring

1. INTRODUCTION

Dose Limits

The United States Nuclear Regulatory Commission has established an annual limit on the radiation dose individuals may receive.  The limits were developed using international recommendations and risk estimates.  The Colorado Department of Public Health and Environment (CDPHE) is responsible for enforcing compliance with these limits in the State of Colorado.  Notice that the limits are significantly lower for children and members of the general public than for radiation workers in laboratories.

Exposure to radiation does not automatically determine the dose, because of mitigating factors such as:

• The amount of time spent near the radioactive source;
• The energy and type of radiation emitted from the source;
• The distance from the source; and
• Any shielding used by the individual.

Dose is a measurement of the radiation energy which is absorbed by tissue. Please refer to the Introductory chapter for more information regarding time, distance, and shielding and fundamental principles of radiation safety.

Relative Risk

How will radiation exposure increase the chance (RISK) of cancer death?

The National Research Council established committees on the Biological Effects of Ionizing Radiations (BIER) to prepare a series of reports to advise the U. S. government on the health consequences of radiation exposures.  One of these committees, BIER V, published a report in 1990 titled, Health Effects of Exposure to Low Levels of Ionizing Radiation.  In this report, the BIER V committee indicated that the risk of cancer death is 0.08% per rem for doses received rapidly (acute exposure).  The risk from doses received over a long period of time (chronic exposure) might be as little as 0.04% per rem or 2-4 times lower.  These risk estimates are averages considering males and females, all age groups, and all forms of cancer.  Significant uncertainty is associated with the estimates.  There was little change in the risk estimates provided by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 2000).

In the United States, the current death rate from cancer is approximately 20-25%, therefore out of any group of 10,000 United States citizens, about 2,000 will die of cancer.  Although about 20% of the population will die from cancer, it is impossible to say which specific individuals will die. 

Most scientists believe that a conservative estimate or model of risk from low doses of radiation is that risk is linear with dose and does not have a threshold, the linear non-threshold model.  In this model, the risk increases with an increase in dose. 

Based on these assumptions, in a population of 10,000 people exposed to one rem (per person to the whole body), approximately eight additional deaths (0.0008*10,000*1 rem) would be due to the radiation exposure.  So, instead of the 2,000 people expected to die from cancer naturally, now there are 2,008.  This small increase in the expected number of deaths would not be seen in this group, due to natural fluctuations in the rate of cancer.

It is not certain that 8 people will die, but there is a risk of 8 additional deaths in a group of 10,000 people if they all receive one rem instantaneously (acute exposure).  If the exposure is received over a long period of time (chronic exposure), the risk would be reduced to less than 4 additional expected fatal cancers.

Personnel Monitoring

Federal and State regulations require that individuals expected to receive more than 10% of the annual dose limits be monitored for occupational dose.  At the University of Colorado, very few individuals are likely to exceed 10% of the limits.  However, most technicians working in laboratories using radioactivity are monitored with a dosimeter in order to maintain a permanent record of the dose they may receive.  Any dose received is recorded in units of mrem, and a record of the dose is maintained for the “life of the institution.” Individuals using significant amounts of radionuclides that are likely to cause internal contamination are monitored periodically using bioassay techniques.

2. DOSIMETRY

Dosimeters are issued after proper training has been completed and a completed dosimetry application is received by Health Physics.  Refer to Appendix L or our website http://www.colorado.edu/radsafety for a blank dosimetry application.  By regulation, the University of Colorado is required to request exposure histories from all institutions at which individuals have worked in order to build a lifetime exposure history.  Extremity rings may be requested by an individual.  Extremity rings are issued to individuals working with greater than 1 mCi of high energy radionuclides and at the discretion of the Radiation Safety Officer (RSO), following evaluation of experimental protocol.  A dosimeter may be canceled by returning it along with its holder to Health Physics with a note indicating cancellation is needed.

Whole Body

Individuals in laboratories using radionuclides other than ³H or ¹⁴C should apply for a whole body dosimeter.  This includes individuals using sealed sources and radiation-producing machines.  Individuals working in areas where neutron sources are used are also monitored for neutron dose.

Extremity

Those individuals using 1 mCi or more of ³²P or other high-energy radionuclides or radiation-producing machines are advised to be monitored with an extremity dosimeter, also known as a ring badge.

Embryo/Fetus

If a woman becomes pregnant, it is her choice whether or not to “declare” her pregnancy.  A Declared Pregnant Woman has a lower dose limit than a non-pregnant radiation worker due to the embryo/fetus limit.   If the pregnancy is not “declared,” the woman continues working under the radiation worker dose limit.  She may notify her supervisor and Health Physics that she is pregnant by completing a Fetal dosimeter application.  The notification must be in writing in order to implement the lower dose limit.  Refer to Appendix M for a blank Fetal dosimeter application.  This application includes the signature of the worker affirming that she wishes to fall under the lower dose limit, as well as the estimated conception date.  Individuals in the fetal dosimetry program will receive dose reports from Health Physics during the pregnancy.

3. BIOASSAY

Some radionuclides used at the University of Colorado present an increased risk of internal contamination.  Use of unbound forms of radioiodine and ³100 mCi of ³H (tritium) requires bioassays to monitor internal dose.  Prior to working with these radionuclides, each researcher must have “baseline” bioassays to ensure appropriate measurements following the experiments.  Any internal dose received will be included in the individual’s dosimetry record.  The Total Effective Dose Equivalent (sum of the internal and external doses to an individual) will be determined.  If the Total Effective Dose Equivalent (TEDE) exceeds 5 rem, CDPHE must be notified within 24 hours.  If the TEDE exceeds 25 rem, CDPHE must be notified immediately.

Anyone planning to work with unbound radioiodine or large amounts of ³H (tritium) should contact Health Physics to schedule a baseline bioassay.  The RSO may require that the experimental work be performed in the Health Physics facility.  Follow-up bioassays will be scheduled, taking into account the biological half-life and anatomical target of the radionuclide involved.

Because most iodine in the body accumulates in the thyroid, internal contamination from radioiodine is measured by a non-invasive bioassay of the thyroid.  The bioassay requires about 10 minutes.

Tritium (³H) replaces stable hydrogen in the body water of the researcher; therefore urinalysis is necessary to measure the amount of internal (³H) contamination.

Bioassays will be performed at the following intervals:

For Radioiodines – Within one week of using more than 50 mCi of I-125 or I-131 in a single operation (or within one week of using 10 mCi in a non-contained form).

4. ENVIRONMENTAL MONITORING

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