In time, for survivors, diseases such as leukaemia (cancer of the blood), lung cancer, thyroid cancer, breast cancer, and cancers of other organs can appear due to the radiation received.
Tissue Sensitivity
In general, the radiation sensitivity of a tissue is: Proportional to the rate of proliferation of its cells, Inversely proportional to the degree of cell differentiation. For example, the following tissues and organs are listed from most radiosensitive to least radiosensitive:
This also means that a developing embryo is most sensitive to radiation during the early stages of differentiation, and an embryo/foetus is more sensitive to radiation exposure in the first trimester than in later trimesters.
Prompt and Delayed Effects
Radiation effects can be categorized by when they appear.
- Prompt effects: effects, including radiation sickness and radiation burns, seen immediately after large doses of radiation delivered over short periods of time.
High doses delivered to the whole body of healthy adults within short periods of time can produce effects such as blood component changes, fatigue, diarrhea, nausea and death. These effects will develop within hours, days or weeks, depending on the size of the dose. The larger the dose, the sooner a given effect will occur.
- Delayed effects: effects such as cataract formation and cancer induction that may appear months or years after a radiation exposure, such as :
Cataracts
Cataracts are induced when a dose exceeding approximately 200-300 rem is delivered to the lens of the eye. Radiation-induced cataracts may take many months to years to appear.
Cancer
- Studies of people exposed to high doses of radiation have shown that there is a risk of cancer induction associated with high doses.
- The specific types of cancers associated with radiation exposure include leukaemia, multiple myeloma, breast cancer, lung cancer, and skin cancer.
- Radiation-induced cancers may take 10 - 15 years or more to appear.
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There may be a risk of cancer at low doses as well.
The need for legislative requirements and dose limits
It is important to protect the health of the population and the workers against the dangers of ionizing radiations, because the exposure of radiation can cause damages as cited above.
Employees in the medical industry working with ionising radiation must wear sensitive radiation-detecting equipment and follow strict operational guidelines for the own safety and that of others. This equipment must be visibly worn on the body and checked regularly. It is available in two general forms although there are man variations in use:
Film badges – these hold a thin strip of aluminium oxide, which is sensitive to radiation and traps excited electrons, and are measured by a device after every working shift. They are worn on the body between the neck and waist.
Ring badges - these contain lithium fluoride crystals, which also trap excited electrons within them until the badge is heated to very high temperatures. The energy given off as visible light is proportional to the radiation dose. This is called thermoluminescence; they are worn on the hand.
Reduce the risk of radiation exposure:
- Keep a close check on the time in an area of radiation materials.
- Make sure that any breaks are not taken in the areas where radioactive materials are stored.
- Keep distance from sources and check the dosimeters periodically.
- Shield yourself from exposure using the correct procedures.
- Notify your supervisor if you are pregnant.
Procedures for reducing radiation hazards
Because any amount of radiation is potentially harmful every effort should be made to reduce doses to a level that is as low as reasonably achievable. Time, distance, shielding, and substitution represent four practical methods laboratory personnel can use to minimize external radiation exposure.
Time
The dose of radiation received is directly proportional to the amount of time spent in a radiation field. Thus, reducing the time by one-half will reduce the radiation dose by one-half. Users should always work quickly, efficiently and spend as little time as possible around radioisotopes. Personnel can reduce time by ensuring that all tools and materials are in place before the radioactive material is brought out of storage. The use of practice runs without radioactive materials is also recommended to increase familiarization with the technique and increase operator speed.
Distance
Radiation exposure decreases rapidly as the distance between the worker and the radiation source increases. Maximizing distance represents one the simplest and most effective methods for reducing radiation exposure to workers. For example, distance can be maximized by using long handled tools to keep radioactive materials well away from the body and storing radioactive materials as far from workers as possible.
The decrease in exposure from a point source of x or gamma radiation can be calculated by using the inverse square law. This law states that the amount of radiation at a given distance from a point source varies inversely with the square of the distance. For example, doubling the distance from a radiation source will reduce the dose to one-fourth of its original value, and increasing the distance by a factor of three will reduce the dose to one-ninth of its original value.
In contrast to x or gamma radiation, beta particles have a finite range in air. Low energy beta emitters such as H-3, C-14, S-35 do not pose an external radiation exposure problem when the material is handled in containers. Higher energy beta emitters such as P-32 do pose an external hazard. Since the energy distribution of betas is from zero to some maximum (dependent upon the isotope), the average energy is approximately one-third of the maximum. Once the distance from a beta source exceeds 4 inches, dose rate reduction follows the inverse square law as the separation distance increases.
Shielding
Radiation exposure can also be decreased by placing a shielding material between a worker and the source of radiation. Materials with high densities and high atomic numbers are the most effective shielding choice for protection from x and gamma rays. The energy of the photons is reduced by Compton and photoelectric interactions in the shielding material. Thus, substances such as lead, concrete, and steel are very practical shielding materials because of the abundance of atoms and electrons that can interact with the photon. As the energy of the gammas increase, the thickness of shielding must increase to provide comparable stopping power. The amount of shielding necessary to reduce the radiation intensity to a desired level can be calculated from the half-value layer of the material. The half-value layer is the thickness of the material necessary to reduce the radiation intensity to one-half of its original value. Seven half-value layers will reduce the radiation level approximately 100 times, and ten half-value layers will reduce the radiation level by a factor of 1000.
Substitution
The isotope selection process is another effective method to reduce potential radiation exposures. The areas to be considered are: the radioactive half-life, the energy and type of emissions, the quantity of isotope, and the chemical form of the isotope. The half-life of the isotope selected can affect waste management. Generally, shorter lived isotopes are preferred over longer lived.
The energy and type of emissions from the perspective isotopes must be considered. Selection of low energy beta or gamma emitters is preferred because radiation hazards are proportionally related to the energy. Beta emitters are preferred over gamma emitters because betas require less shielding. The radiation hazard is also proportionally related to the quantity (radioactivity) of the isotope to be used. The use of small activities is preferred. The chemical form selected for the experiment can also affect the radiation hazards associated with the work. It is preferred to avoid the use of compounds that are or produce volatile or gaseous compounds.