External vs internal exposure
External exposure is exposure which occurs when the radioactive source (or other radiation source) is outside (and remains outside) the organism which is exposed.
Below are a series of three examples of external exposure.
- A person who places a sealed radioactive source in his pocket
- A space traveller who is irradiated by cosmic rays
- A person who is treated for cancer by either teletherapy or brachytherapy. While in brachytherapy the source is inside the person it is still external exposure because the active part of the source never comes into direct contact with the biological tissues of the person.
One of the key points is that external exposure is often relatively ''easy'' to estimate, and the irradiated objects do not become radioactive (''except for a case where the radiation is an intense neutron beam which causes activation of the object'').
It is possible for an object to be contaminated on the outer surfaces; assuming that no radioactivity enters the object it is still a case of external exposure and it is normally the case that decontamination is relatively easy.
Internal exposure occurs when the radioactive material enters the organism, and the radioactive atoms become incorporated into the organism. Below are a series of examples of internal exposure.
- The exposure caused by 40K present within a ''normal'' person.
- The exposure to the ingestion of a soluble radioactive substance, such as 89Sr in cows' milk.
- A person who is being treated for cancer by means of an ''open source'' radiotherapy method where a radioisotope is used as a drug. A review of this topic was published in 1999. Because the radioactive material becomes intimately mixed with the affected object it is often difficult to decontaminate the object or person in a case where internal exposure is occurring. While some very insoluble materials such as fission products within a uranium dioxide matrix might never be able to truly become part of an organism, it is normal to consider such particles in the lungs as a form of internal contamination which results in internal exposure. The reasoning is that the particles have entered ''via'' an orifice and can not be removed with ease from ''what the lay person (non biologist)'' would regard as within the animal. It is important to note that in a strictly topological sense, the contents of the digestive tract and the air within the lungs are outside the body of a mammal (whereas, for instance, the abdominal cavity is topologically inside the mammalian body).
- Boron neutron capture therapy (BNCT) involves injecting a boron-10 tagged chemical that preferentially binds to tumor cells. Neutrons from a nuclear reactor are shaped by a neutron moderator to the neutron energy spectrum suitable for BNCT treatment. The tumor is selectively bombarded with these neutrons. The neutrons quickly slow down in the body to become low energy ''thermal neutrons''. These ''thermal neutrons'' are captured by the injected boron-10, forming excited (boron-11) which breaks down into lithium-7 and a helium-4 alpha particle both of these produce closely spaced ionizing radiation.This concept is described as a binary system using two separate components for the therapy of cancer. Each component in itself is relatively harmless to the cells, but when combined together for treatment they produce a highly cytocidal (cytotoxic) effect which is lethal (within a limited range of 5-9 micrometers or approximately one cell diameter). This system was in clinical trials in Norway.
Nuclear warfare and bomb tests
Nuclear warfare and bomb tests are more complex because a person can be irradiated by at least three processes. The first (the major cause of burns) is not caused by ionizing radiation.
- Thermal burns from infrared heat radiation
- Beta burns from shallow ionizing beta radiation (this would be from fallout particles; the largest particles in local fallout would be likely to have very high activities because they would be deposited so soon after detonation and it is likely that one such particle upon the skin would be able to cause a localised burn); however, these particles are very weakly penetrating and have a short range.
- Gamma burns from highly penetrating gamma radiation. This would likely cause deep gamma penetration within the body, which would result in uniform whole body irradiation rather than only a surface burn. In cases of whole body gamma irradiation (''circa'' 10 Sv) caused by accidents involving medical product irradiators, some of the human subjects have developed injuries to their skin between the time of irradiation and death.
In the picture to the left, the normal clothing that the woman was wearing would have been unable to attenuate the gamma radiation and it is likely that any such effect was evenly applied to her entire body.
Beta burns would be likely all over the body caused by contact with fallout, but thermal burns are often on one side of the body as heat radiation does not penetrate the human body. In addition, the pattern on her clothing has been burnt into the skin. This is because white fabric reflects more infrared light than dark fabric. As a result, the skin close to dark fabric is burned more than the skin covered by white clothing.
There is also the risk of internal radiation poisoning by ingestion of fallout particles.
Nuclear reactor accidents
The first known incident of a reactor meltdown occurred in Canada in the NRX Reactor. Radiation poisoning was a major concern after the Chernobyl reactor accident. Thirty-one people died as an immediate result.
Of the 100 million curies (4 exabecquerels) of radioactive material, the short lived radioactive isotopes such as 131I Chernobyl released were initially the most dangerous. Due to their short half-lives of 5 and 8 days they have now decayed, leaving the more long-lived 137Cs (with a half-life of 30.07 years) and 90Sr (with a half-life of 28.78 years) as main dangers.
Improper handling and care of radioactive and nuclear materials lead to radiation release and radiation poisoning. The most serious of these, caused by improper disposal of a medical device containing a radioactive source (teletherapy), occurred in Goiânia, Brazil in 1987.
During human spaceflights, particularly flights beyond low Earth orbit, astronauts are exposed to both galactic cosmic radiation (GCR) and possibly solar particle event (SPE) radiation. Evidence indicates past SPE radiation levels which would have been lethal for unprotected astronauts. GCR levels which might lead to acute radiation poisoning are less well understood.
Air travel exposes people on aircraft to increased radiation from space as compared to sea level, including cosmic rays and from solar flare events. Software programs such as Epcard, CARI, SIEVERT, PCAIRE are attempts to simulate exposure by aircrews and passengers. His is the first case of confirmed death by such a cause, although it is also known that there have been other cases of attempted assassination such as in the cases of KGB defector Nikolay Khokhlov and journalist Yuri Shchekochikhin where radioactive thallium was used.
In addition, an incident occurred in 1990 at Point Lepreau Nuclear Generating Station where several employees acquired small doses of radiation because of the contamination of water in the office watercooler with tritium-contaminated heavy water.
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