Radiation is all around us, all the time. But what level does it have to get to before it becomes really dangerous? The World Nuclear Association which represents the 'global nuclear profession' does have a guide. And while there is a touch of Smilin' Joe Fission , it is a good place to start for a useful primer. There are different kinds of radiation - which you can read about in the WNA guide. The problems we're concerned about come from ionising radiation.
Radiation dosages are measured in sieverts - but because these are so big we're talking about millisieverts mSv a thousandth of a sievert. Rather than being an exact unit of size because different types of radiation have different effects an mSv measures the effective radiation dose. According to the WNA, each mSv of radiation "produces the same biological effect". We're exposed to radiation when we fly and when we get medical treatment - and whenever we leave the house.
But the large dosages can have dramatic effects. It has been known for many years that large doses of ionising radiation, very much larger than background levels, can cause a measurable increase in cancers and leukemias 'cancer of the blood' after some years delay.
It must also be assumed, because of experiments on plants and animals, that ionising radiation can also cause genetic mutations that affect future generations, although there has been no evidence of radiation-induced mutation in humans. At very high levels, radiation can cause sickness and death within weeks of exposure. What is a lethal dose from a single instance of radiation? According to studies made after the atomic bomb explosions in at Hiroshima and Nagasaki, half of the people died whose entire bodies were exposed to , millirems of radiation from the atomic bomb.
All persons died whose bodies were exposed to , millirems of radiation. This also was the annual occupational limit for adults from World War II through Limit changed in to 5, millirems. It is "as low as reasonably achievable; however, not to exceed 5, millirems. An embryo or a fetus of a pregnant worker exposed to radiation a new regulation as of Jan. Potassium is an essential dietary element that is present mostly in the muscles.
A localized dose delivers 10,, millirems to the thyroid and about 20, millirems to the rest of the body. A radiation dose to kill a cancerous tumor often sends a beam delivering 6,, millirems to the cancerous tissue, but the whole body equivalent dose is much less, as in the thyroid case.
The research showed iron supplements are more effective if not be taken with meals. The study used radioactive calcium 45 to track calcium absorption. One adult age 21 was also in the study and received a higher dose, resulting in an exposure equal to 11 millirems for the whole body. A typical X-ray exposes the chest to a dose equal to 20 millirems at the entrance and 1 milliRem at the exit. Averaging this exposure over the whole body yields a whole body equivalent of about 2 millirems. The US Food and Drug Administration's current regulations state, "The amount of radioactive material to be administered shall be such that the subject receives the smallest radiation dose with which it is practical to perform the study without jeopardizing the benefits to be obtained by the study.
Under no circumstances may the radiation dose to any adult research subject from a single study, or cumulatively from a number of studies conducted within one year, be generally recognized as safe if such doses exceed the following:.
For a research subject under 18 years of age at the last birthday, the radiation dose shall not exceed 10 percent of that set forth above. Therefore, the single exposure limit for a child is millirems whole body equivalent and the annual total exposure cannot exceed millirems.
Since , medical researchers at institutions doing the research have been required to follow informed-consent procedures. These procedures require the assent if feasible of a child 7 years of age or older, and the consent of both parents if there is any perceived risk involved in the research. For research involving any perceived risk, there also has to be a relationship between the study and a child's disorder or disease.
If there is direct benefit that is likely to accrue from participation in the study, then a researcher needs the assent of the child age 7 or older and the consent of at least one of the parents.
In such direct benefit research situations, the permitted levels of radiation can be exceeded. How ionising radiation is measured and defined has changed over the decades as we learn more about this relatively young science. Dose was originally measured in air by the unit of Roentgens R, named after the discoverer of X-rays, Wilhelm Roentgens. With the introduction of metric units the basic unit of absorbed dose became the Gray Gy , which represents an absorbed dose of 1 Joule of energy per kilogramme.
Unfortunately absorbed dose is not very convenient for radiological protection purposes because 1Gy of the different radiations - gamma and X-rays , beta particles , neutrons and alpha particles - is not equally damaging to tissue.
Hybrid, because it is really not a unit of radiation dose but a unit of risk. Thus, we talk of the equivalent dose of 1Sv as carrying the same risk, for example, as 1Gy for X and gamma rays, or 0. But there is a further complication, as not all tissues in the body are equally sensitive.
So the term effective dose which incorporates the correction for equivalent dose and is also measured in Sv, is used. This way, if only part of the body is irradiated the risk can be presented in terms of an effective risk to the person.
This allows risks from different exposures to be added together. The unit Sv should not be used for large doses greater than 1Sv to the whole body.
Typically, everybody is exposed to two milliseiverts mSv per year throughout our lives from natural background radiation. We might receive a dose of up mSv from diagnostic radiology - say 10mSv for a CT chest scan. The firefighters and plant workers at the Chernobyl accident received doses of several Gy and these doses led to deaths from acute radiation sickness within about 60 days.
Typically Gy received over a short period of hours will be lethal, but can be tolerated if delivered over a much longer period.
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