Radiation is fascinating. I have had friends and relatives die of cancers and until fairly recently, radiation therapy was not an option. Now, radiation therapy is getting close to being my preferred treatment should I happen to get cancer. Malignant cells are pretty easy to kill, it is not killing the patient that is the trick. Surgical removal is still the most popular treatment by doctors, but I have seen too many friends have large sections of things cut out only to find that the cancer is still active. Chemotherapy has some effectiveness, but a lot of side effects. Done properly, radiation therapy does the job quite well with not a lot of side effects.
While looking into radiation, both as a therapy and as an energy source, I have picked up on a few interesting things. Since the world is freaked out over Fukushima fall out, I thought I would start putting some facts, as I understand them, down in a post. This post may grow as I learn more or scientists learn more about this fascinating subject. I do recommend that all new Geiger counter owners read this and much more before worrying themselves to death over radiation. Stress related to radiation fear is just as big of a health problem as the radiation itself.
There are three main types of radioactive isotopes released by Fukushima that cause concern, Iodine 131, Cesium 137 and Strontium 90.
Iodine 131 has a half life of about 8 days. The half life, or time it takes for half of the concentration to decay, is short which is good and bad. It is good because after 5 half lives, 40 days, it is pretty much gone. It is bad because it is very active during its biological half life. Biological half life is the time it takes the body to pass half of the concentration.
Each decay is a radioactive pop. In Iodine 131's case the pop is a beta particle or electron with some gamma radiation. Each pop releases a certain amount of energy which is what damages the cells of the body. The beta particle damages cells close to the pop and the gamma rays can travel further. The decay of I131 is in two steps. The first is beta decay to Xenon 131 which then emits gamma radiation. The average total energy of a Iodine 131 pop is 971 KeV 0r 971 thousand electron volts.
The biological half life of I131 is a little tricky. I131 many is absorbed after ingestion by the thyroid gland. If absorbed by the thyroid, its biological half life is longer than it radiative half life. If not absorb by the thyroid the biological half life is shorter. For this reason, the biological half life is not commonly published.
I131 also impacts health oddly. Smaller amounts do more damage than larger in most cases. For these reasons, the relatively short half life, affinity for the thyroid and smaller hazardous dose, Iodine 131 is generally the most dangerous of the radioactive fallout. Large doses of stable Iodine reduce the amount of radioactive iodine that can be absorbed by the thyroid. Because of its short radioactive half life, Iodine 131 is not a long term problem.
Cesium 137 or Caesium 137 has a half life of about 30 years. Unlike Iodine, cesium 137 is chemically similar to potassium and Rubidium. Potassium is a common electrolyte used mainly in the muscles of the body, though some may be absorbed in the bones. Cesium 137 also has a two step decay. Cs137 decay to Barium 137 releasing a beta particle and the barium 137 releases gamma radiation. The total energy released is 1176 KeV of 1176 thousand electron volts. This is a little more energy than the Iodine, but not much. Cesium 137 has a biological half life of 70 days. Prussia Blue is an antidote for Cesium 137. Since the cesium 137 is treated as potassium by the body, maintaining proper electrolyte levels reduces the amount of cesium 137 absorbed.
Strontium 90 has a half life of about 29 years. Strontium 90 is a bone seeker. It is treated like calcium in the body, can cause bone cancer and leukemia. It has a three step decay to Yttrium 90 with a half life of 64 hours and then Zirconium 90 with total beta decay energy of near 2800 KeV or 2800 thousand electron volts. More than twice the decay energy of Iodine 131 or Cesium 137, but second decay to Zirconium 90 has the most energy at 2200 KeV. As a bone seeker, Sr 90's biological half life is indefinite if absorbed by the bones or teeth. 70 to 80 percent of ingested Sr 90 passes quickly through the body without being absorbed. Because of the high energy and absorption in bone and teeth, Sr 90 has a greater probability of causing cancer.
Research by the Radiation and Public Health Project has indicated that Strontium 90 released during nuclear tests and near nuclear reactors has caused elevated concentrations in the public and increased cancer rates. The reports have been criticized by the Nuclear Regulatory Commission and the results available online appear to be inconclusive and poorly compiled. Per the study, approximately 1.6% of the respondents had some form of cancer and the majority of the cases were from 1962 to 1964, dramatically decreasing after 1964. The Wikipedia editor that referenced the report included a link to a New York Times article and not the actual report. Comparing the reported results to the Center for Disease Control National Vital Statistics Summary , there does not appear to be any verification of the reported results.
While Strontium 90 is clearly likely to cause cancer in sufficient dosage, there is no credible evidence that dosage under the regulatory limits of NRC pose any significant increase in cancer risk.
For example, ingesting 100 becquerel equivalent of Sr 90, the body may retain 30 becquerel equivalent or 30 pops per second. The average energy per pop would be 1400 KeV, about 40% greater than the average, so the damage would be roughly equal to 42 pops or counts per second. For an average adult, this is less than 1/1000 of the normal background radiation produced by your own body and a much smaller fraction compared to the total normal background radiation. Since Strontium 90 is a small percentage of the total radiation released at Fukushima, the 100 becquerel example is probably quite high. I will review the reports, but Sr 90 was less than 1 percent of the radioactive isotopes released with 3.2 to 32 becquerel per kilogram found in only a few soil samples.
I will continue after further research to find a more compelling peer reviewed report if one exists.
With nuclear radiation there is a lot of contradictory information available. Like the Strontium study, statistics can be very misleading because of the natural occurrence of cancers that may be linked to man made radiation, but existed naturally before the start of man made atomic fission and increased not because of radiation but improvements in health care, changes in environment and exposure to other non radioactive carcinogens. One major factor that is missing from the more controversial studies is with increased life expectancy, the causes of death change. In the fifth and sixth decade cancer is the more prominent cause of death which is more likely due to hereditary and environmental factors, than man made radiation exposure.
These studies are also complicated by the paradoxical "Vaccine" effect of some low levels of certain types of radiation exposure. Lower levels of Iodine 131 tend to increase thyroid cancers while very low levels of tritium, (the radioactive isotope of hydrogen) tend to reduce thyroid cancers. To attempt to simplify the risk of adverse health impact by exposure to man made radiation, the increase in radioactive decay energy may be a useful tool.
As a general rule of thumb, more rapid decay energy absorption is more detrimental to health. This is far from a perfect rule of thumb as parts of the body respond differently to radiation levels.
Counts or pops per second are the simplest measure decay energy. Isotopes with shorter half lives pop more quickly. Isotopes with half lives shorter than hundreds of thousands of years are generally man made. On average, the human body contains radioactive isotopes that result in 4,400 pops per second per 75 kilograms of mass. The 75 kilograms is considered the average mass of the average human. Since few people are truly average, 60 pops per kilogram or 30 pops per pound are good baseline numbers to use for radioactive health purposes. Depending on environment and lifestyle there can be a significant variation from this baseline per individual. Since hundreds of thousands of people have or plan to buy Geiger counters, don't freak out if your baseline is higher, because the normal background radiation where you live can be much higher than your body mass baseline.
The Becquerel is defined as the decay of one nucleus per second. Because of radioactive potassium 40 producing the pops, the average kilogram of human mass has a Becquerel count of 60 Bq/kg. Animal testing on dogs primarily, have been used to determine the lethal dose for 50% of the population or LD50. A test on Beagles with Cesium Cloride did not publish the LD50, (the paper is behind a pay wall) in its abstract, but the LD50 from the information available is greater than 1900 micro Curries per kilogram. 1900 micro Curries per kilogram is equal to 70.3 million Becquerel per kilogram which is considerably larger than 60 becquerel per kilogram. As an estimate, 1/1000 of 70.3MBq would be 70.3 KBq or 70,300 Becquerel per kilogram which is where possibly a statistically significant adverse health impact could easily be determined.
Beagles are not people and the test was not designed to determine increased incidence of cancers due to radiation. Still, the 70,300 Becquerel per kilogram implies a maximum exposure limit that may possibly not cause adverse health impact, but probably a measurable health impact. Definitely a when to be worried point.
Studies of Chernobyl offer more information on direct human impacts of radioactive Cesium. Studies by the former Soviet Union Government are appropriately questionable. Studies by international agencies are much more likely to be trust worthy, though they may tend to be overly critical in some cases due to legal and political issues. The most likely studies to be overly critical are studies using linear no thresh hold models. These models easily confuse normal mortality rates with possible radiation impacts. Political influence may have caused lower impact estimates by international agencies with interests in nuclear power. It is a tangled mess, but there are still some reasonable conclusions that can be drawn, if you wish to be unbiased.
First, studies of the individuals actively involved in the containment of the incident provide a reasonable maximum impact. While criticized by anti-nuclear advocates, The WHO reports indicates that approximately 9000 excess cancer deaths for a population 600,000 exposed to the highest radiation levels of approximately 40,000 Becquerel per meter squared. This estimate includes iodine 131 death rates which are extremely high due to poor emergency procedures employed by the former Soviet Union and somewhat surprisingly France. It also includes emergency workers exposed to much higher direct radiation levels. It should be noted that direct exposure caused fewer deaths than ingestion of radioactive isotopes.
Second, countries near the Chernobyl site conducted individual studies and established maximum safe radiation levels for food products. The EU for example has a 600 Becquerel per kilogram limit on food products. The UK placed a limit of 1000 Becquerel per kilogram on sheep meat. Wild game, boar in particular, were found to have levels up to 40,000 Becquerel per kilogram with an average of 6800 becquerel per kilogram. The EU and UK limits were established where there is no statistically significant cancer risk. There is a gray area between the safe limit for humans at ~1000 Becquerel per kilogram and animal limits of approximately 40,000 to 70,300 Becquerel per kilogram.
While animals are not people, the food limits that apply to meats is an indication the human radiation levels can be considerably higher than the average 60 pops per kilogram and still be safe.
Where to draw the concern line is a personal decision. Twice average should not be a level of concern, but ten times average may be. The type of isotope needs to be considered, Iodine 131 because it is thyroid specific. Strontium 90 because it is bone specific and higher energy. There is no indication though that limits set by nations for food stuffs and water supplies pose any significant health risk.
If you happen to be the proud new owner of a Geiger counter, you may wish to establish your own body mass baseline. If you properly allow for background radiation, 60 pops per second/kg is perfectly normal. 120 pops per second/kg is very likely to be safe. 360 pops per second/kg may be your concern thresh hold. With 600 to 1000 pops per second/kg a definite level of concern. Remember that background levels are not only higher, they can be variable. Your fancy new Geiger counter may not have the sensitivity to do anything but scare you or give you a false sense of security. If it provides you some piece of mind, read up on the limitations of your Geiger counter to avoid freaking yourself and others out with erroneous readings and interpretations. If you are confident in your ability to properly use your new Geiger counter, think about publishing your results, including methods, online. Note: I dropped a sentence, Trying to measure your body mass radiation level with a Geiger counter will not give you a number to be confident in, properly calibrated it can give you an indication of change in radiation level, like testing food.
After Thoughts:
"The report said cancer risk from exposure to between 100 and 200 millisieverts is 1.08 times higher when compared with people who weren't exposed, while the cancer risk of people whose body mass index was between 30 to 39.9 was 1.22 times higher than a group of people whose body mass index was between 23 and 24.9. The cancer risk was 1.6 times higher for a group of people who smoke, when compared with nonsmokers, it said.
"The risk of cancer incidence is not zero even at low doses. . . . But the levels we are now exposed to are not something people have to worry deeply about," said Ikuro Anzai, a professor emeritus at Ritsumeikan University who has criticized the safety of nuclear power plants for decades.
"Many people get scared simply by hearing the word radioactivity. But we have to base our worries on reality. It is very difficult, but we need to have rational fears," said Anzai, an expert on radiation protection." I found this published in the Japan Times after I posted this. It is excellent and by someone with anti-nuclear leanings.
Still, the units used to describe radiation levels are confusing. That is the main reason I put this together using the Becquerel/kilogram or counts per second which most of the Geiger counter buyers will have as a reading. As I have mentioned in other posts, there is no direct conversion from Becquerels to millisieverts, though ingested radiation allows a somewhat reasonable comparison. A millisievert is a small unit and a becquerel is a very small unit, so don't confuse them, stress can be pretty bad for your health too.
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