Radiation Protection
Sensitivity of Reproductive Organs to Radiation
Occupational radiation exposure are usually measured external to human body by monitoring equipment. These measurements, however, cannot be directly used to evaluate potential health effects because various human tissues and organs have different sensitivities to the same amount of radiation. The annual radiation exposure limit of 20 mSv (milli-Sievert) for radiation workers is based on a whole body dose equivalent amount, which is the sum of radiation exposure to individual tissue or organ times the corresponding weighting factor (see Table 1).
| Table 1. Tissue Weighting Factors for Assessing Radiation Exposure | |
| Tissue or Organ | Tissue Weighting Factor |
|---|---|
| Gonads | 0.20 |
| Bone Marrow (Red) | 0.12 |
| Colon | 0.12 |
| Lung | 0.12 |
| Stomach | 0.12 |
| Bladder | 0.05 |
| Breast | 0.05 |
| Liver | 0.05 |
| Oesophagus | 0.05 |
| Thyroid | 0.05 |
| Skin | 0.01 |
| Bone Surface | 0.01 |
| Remainder | 0.05 |
The reproductive organs (gonads) have the highest sensitivity to radiation hazard (represented by the highest weighting factor of 0.20). In other words, a certain amount of radiation exposure to sensitive organs such as the gonads may produce much more serious health effects than the same amount of exposure to, say, the skin.
Health Risk Due to Radiation Exposure
Table 2 summarizes probability of fatal cancer, and aggregated detriment (which includes non-fatal cancers, cancer effects on life quality of survivors, relative reduction in life-span, and relative length of impaired life) in different organs or tissues upon exposure to a certain amount of radiation. The numbers represent additional cases of fatal cancer or aggregated detriment in 1 00 individuals due to an exposure of 1 Sv in one year's time.
| Table 2. Nominal Probability Coefficients for Individual Tissues and Organs to Develop Adverse Health Effects upon Exposure to Radiation | ||
| Tissue or Organ | Probability of Fatal Cancer (10-2 Sv-1) | Aggregated Detriment (10-2 Sv-1) |
|---|---|---|
| Bladder | 0.30 | 0.29 |
| Bone Marrow | 0.50 | 1.04 |
| Bone Surface | 0.05 | 0.07 |
| Breast | 0.20 | 0.36 |
| Colon | 0.85 | 1.03 |
| Liver | 0.15 | 0.16 |
| Lung | 0.85 | 0.80 |
| Oesophagus | 0.30 | 0.24 |
| Ovary | 0.10 | 0.15 |
| Skin | 0.02 | 0.04 |
| Stomach | 1.10 | 1.00 |
| Thyroid | 0.08 | 0.15 |
| Remainder | 0.50 | 1.59 |
| Probability of Severe Hereditary Disorder | ||
| Gonads | 1.00 | 1.33 |
The main effects of radiation exposure to reproductive organs are hereditary disorders, or birth defects. As can been seen from the probability, the reproductive organs are among the most sensitive tissues.
Radiation Protection for Reproductive System
Although International Commission on Radiological Protection (ICRP) did not specify any special protection for the reproductive system for radiation workers, it is good practice to minimize radiation exposure whenever possible, not only to high sensitivity organs but to any part of the body. Since only low doses and relatively low energy radionuclides are currently used for research purposes in HKUST, and most of the experiments involving radioactive materials are equipped with proper shielding, and/or conducted on benches which blocked direct exposure to lower part of the body, the potential radiation hazard to the reproductive system is relatively small.
Radiation Hazard of Causing Birth Defects
Teratogenic(able to cause birth defects)effects of ionizing radiation in the period of pregnancy have long been known. There were studies which indicated an association between radiation exposure to fetus and an increased incidence of childhood cancers, although some investigators maintain that no such relationship exists. Hence the National Council on Radiation Protection and Measurements(NCRP)Report 53 concluded 鄭lthough the reported epidemiological association of excess risk of childhood cancer with prenatal radiation exposure of embryo or foetus is still uncertain as to the cause-effect relationship and magnitude of possible risk, R is conservative radiation protection philosophy to assume that such risk may exist."
Radiation Exposure Limit for Pregnant Women
Pregnant women are, therefore, not recommended to receive any radiation in excess of the natural background. In situations where exposure to ionizing radiation cannot be avoided, such as in emergency medical diagnosis or life-saving treatment, the ICRP principles of justification, and of applying a radiation dose "as low as reasonably achievable", shall be strictly adhered to.
Occupationally, while it is preferable not to receive any radiation other than that from the environment, pregnant radiation workers can and do receive radiation. ICRP recommends that after pregnancy has been diagnosed a woman can continue to work only in Radiation Working Condition l3 which is defined as conditions where it is most unlikely that annual exposure will exceed 3/10th of the internationally accepted annual exposure limit of 20 mSv (whole body dose equivalent)for radiation workers. In other words, pregnant women should not be exposed to annual radiation dose higher than 6 mSv.
Estimated Exposure to Foetus
ICRP believes that any pregnancy of more than two month's duration would have been recognized by the woman herself or by a physician, and hence, according to the ICRP recommendation, the maximum dose a foetus may receive is estimated to be:
2/12 x 20 mSv + 7/12 x 3/1 0 x 20 mSv = 6.8 mSv
The first term is the maximum dose received in the first two months in the absence of knowledge of pregnancy. The second term is the maximum dose expected in the rest of the gestation period when the pregnancy has been known and ICRP recommendation is followed. As a reference, a diagnostic X-ray examination can give 0.5-2 mSv, while two hours of television-watching per day for a year is 0.01 mSv.
The Council of the European Communities has endorsed the ICRP recommendation for pregnant radiation workers, and in addition said the dose to the foetus should in no case exceed 1 0 mSv after pregnancy has been diagnosed. The NCRP of the United States recommends a monthly limit of 0.5 mSv to the embryo-foetus (excluding medical and natural background radiation) once the pregnancy is known.
Potential Effects to Foetus in Worst Case
NCRP further states that " Under the highly unlikely maximum exposure scenario (50 mSv before pregnancy is known), the potential impact on intelligence quotient (IQ) would be expected to be negligible since the period of enhanced sensitivity is 8 to 15 weeks and beyond. In addition, the [increase in] lifetime cancer risk would be expected to be less than 0.005, i.e., 5 in 1,000. " Since it is extremely unlikely that any radiation worker would receive more than 50 mSv under normal circumstances, especially in an academic institution, there is no special recommendation regarding the occupational exposure of female radiation workers not known to be pregnant.
Radiation Workers vs the General Public
The above discussions are limited to reproductive hazards of occupational exposure to radiation workers. Reproductive hazard due to medical radiation exposure has only been briefly touched on.
A separate article on potential health (including reproductive) hazards of medical radiation exposure, especially from X-ray, to the general public is planned for a coming issue of Safetywise.
Radiation Safety Program
The mission of the Radiation Safety Section of HSEO is to protect all campus occupants, both genders, pregnant or not, from radiation hazards; and to serve HKUST radiation workers. Radiation safety equipment, and whole body radiation monitoring dosimeters are provided to radiation workers. Facility contamination is monitored by swipe tests and/or portable radiation detectors. Appropriate medical surveillance for specific radionuclides is also available. Please contact us for assistance or further information.
Ionizing radiation presents an invisible form of health hazard to users of radioactive substances and irradiating apparatus. Although the common radiation quantity encountered in an academic setting is usually relatively low, the cumulative effects could be significant. Therefore personal protective equipment must be used, and exposure monitoring performed, to ensure the safety of radiation workers.
Thermoluminescent Dosimetry (TLD) is the primary form of personnel radiation exposure monitoring used at HKUST. There are almost two hundred radiation workers on campus wearing these blue plastic badges.
What is TLD
Thermoluminescent (TL) means emitting light when heated. We can briefly describe the mechanism of TL as follow: When a strong energy source (such as ionizing radiation) hits a TL material, electrons are freed from some atoms and moved to other parts of the material, leaving behind "holes" of positive charge. Subsequently when the TL material is heated, the electrons and the "holes" re-combine, and release the extra energy in the form of light. The light intensity can be measured, and related to the amount of energy initially absorbed through exposure to the energy source.
The blue box issued to radiation workers is a plastic casing that holds a sheet of TL material used as a dosimeter (i.e. TLD), and some metal foil to filter the radiation to be recorded. After the designated monitoring period, the TLD is collected and read by a TLD Reader which can heat up the TLD, detect the resulting light emission, and calculate the radiation exposure to the person wearing that particular TLD. The TLD may be reused after a controlled heating procedure which completely recombines all "electron-hole" pairs, and restores the TLD to the original condition.
Keep in mind that, these TLD in blue boxes:
- ARE NOT radioactive themselves
- DO NOT protect wearer against radiation
- DO measure the amount of strong B, alpha(?), and X-, radiation exposure of the wearer, if used properly.
The TLD services at HKUST
HSEO administers the HK Government Department of Health TLD service for the HKUST campus. Laboratory workers registered with HSEO as radiation workers receive their TLD at the beginning of each month through HSEO, at which time they are to return the TLD for the previous month, again through HSEO, for reading. Monitoring reports for the month before last is provided with the replacement TLDS.
Finger TLD
Finger TLD is one form of extremity dosimetry, in contrast with the common whole body TLD described above. Finger TLD measures the radiation exposure of worker's hands by a special ring or strip containing a small TLD.
Many times when radioactive materials are used in bench-top experimerits, the body trunk of the experimenter is protected by the bench and/or specialized shields, while the hands are directly exposed to radiation during experiments. In these cases, the whole body dosimetry does not reflect the exposure of the worker's hands, which are presumably most exposed to radiation. Finger TLD will give a true measurement of exposure for this part of a workers' body.
This particular type of dosimetry is not available from the Government Department of Health, but is widely employed in foreign countries. Therefore, SEPO has retained the service of an outside consultant to develop a finger TLD program tailored for HKUST radiation workers. Details of this supplementary program will be announced in the near future.
Reminder for TLD Users
A user must wear a dosimeter issued to him as indicated on the dosimeter label. Under no circumstances shall a dosimeter issued to a specific wearer be worn by another person.
- The user must wear the dosimeter all the time when performing radiation work.
- The dosimeter must be worn near the centre of the wearer's trunk, with the front window of the holder facing outward.
- The user must keep the dosimeter away from the radiation work area when not performing radiation work.
- Never remove the dosimeter from its Polythene pouch.
- Never wear a dosimeter without a dosimeter holder.
- Never use a dosimeter holder which has an incomplete set of filters.
- Never lag behind the schedule on the return of dosimeters for analysis.
(Adopted from "Instruction on the use of TLD personnel radiation monitoring dosimeters" issued by Radiation Monitoring Service, Department of Health, Hong Kong Government, and appeared on the August/1995 issue of SafetyWise.)
Introduction
Radon is a colorless, odorless, tasteless and almost chemically inert radioactive gas that occurs naturally from the decay of uranium and radium soils. Among the various rock, granite has the highest amount of radium. More than one third of the local territory is made up of granitic bed rocks resulting in a high gamma dose rate in the air. Due to the air pressure gradient between outdoor and indoor, radon is sucked into confined space and dwellings through gaps in the walls and floor. Radon itself further decays to short lived metallic daughters. Exposure to the alpha particles emitting from these daughters will increase the incidence of lifetime lung cancers. Studies demonstrate a linear exposure risk relatioship without a threshold. The incidence of lifetime risk of lung cancer due to radon exposure is tabulated in table 1. The World Health Organization (WHO) recommended a safety guideline level of 200 Bq/m3 in 1993 to minimize the adverse effect from radon exposure.
|
**Table 1: Lifetime Risk of Lung Cancer |
|||
|
Avg Concentration |
Lifetime Risk % |
||
|
Bq/m3 |
Whole | Smoker | Non-smoker |
|
20 |
0.3 |
1 |
0.1 |
|
100 |
1.5 |
5 |
0.5 |
|
200 |
3.0 |
10 |
1.0 |
|
400 |
6.0 |
20 |
2.0 |
**Source: National Radiological Protection Board, GS-6, 1993
Radon Survey at HKUST
A campus radon survey was conducted for 6 months starting from October 1996 to March 1997. Passive charcoal canisters as well as an active real-time radon gas monitor were used to define the radon levels in the main buildings, senior and junior staff quarters, utility and sewage tunnels. The results are presented graphically in Figures 1 to 5.
