srijeda, 16. ožujka 2011.
How Does Nuclear Radiation Harm CCRES
CROATIAN CENTER of RENEWABLE ENERGY SOURCES
How Does Nuclear Radiation Harm the Body?
The amount of radioactive material being released from the damaged nuclear reactors in Japan, and the eventual impact it will have on human health, are still being determined.
How does nuclear radiation harm the body, and what are the risks from long-term exposure to low levels after an accident? MyHealthNewsDaily spoke with experts about these questions.
How does radiation harm the body?
About 150 people living or working around Japan's damaged nuclear facilities have been monitored for potential radiation exposure, and 23 have been found to be in need of treatment. How is the extent of their exposure measured?
According to the United States Nuclear Regulatory Commission (NRC), "exposure" refers to the amount of radiation, such as X-rays, gamma rays, neutrons, alpha and beta particles, present in the air. Exposure, usually expressed in units of roentgens, is measured by Geiger counters and similar devices. A Geiger counter registers how much the gas it contains gets ionized by incoming particles of radiation, and converts that information into an electronic signal.
People don't absorb all the radiation they're exposed to, however; most of it passes straight through their bodies. A small amount of the energy carried by radiation gets absorbed by bodily tissues, and that absorbed amount is measured in units of "radiation absorbed dose" (rad). Radiation affects different people in different ways, but a rule of thumb used by safety crews is that a single roentgen of gamma- or x-ray exposure typically produces an absorbed dose of approximately 1 rad. By measuring the radiation level around a person's body using a Geiger counter, a safety officer can approximate that person's absorbed dose.
There's been some reported evidence that radioactive iodine and cesium are being released into the environment from the malfunctioning nuclear reactors in Japan, said Kathryn Higley, director of the Oregon State University department of nuclear engineering and radiation health physics.
As radioactive material decays, or breaks down, the energy released into the environment has two ways of harming a body that is exposed to it, Higley said. It can directly kill cells, or it can cause mutations to DNA. If those mutations are not repaired, the cell may turn cancerous.
Radioactive iodine tends to be absorbed by the thyroid gland and can cause thyroid cancer, said Dr. Lydia Zablotska, an assistant professor in the department of epidemiology and biostatistics at the University of California, San Francisco.
But radioactive iodine is short-lived and will be around for only about two months after an accident, said Andre Bouville of the National Cancer Institute, who has studied radiation doses from the fallout of the 1986 Chernobyl explosion in Ukraine. So, if the exposure to the air comes after that time, radioactive iodine does not pose a health risk, Bouville said.
Children are most at risk for thyroid cancer, since their thyroid glands are 10 times smaller than those of adults, he said. The radioactive iodine would be more concentrated in them.
Radioactive cesium, on the other hand, can stay in the environment for more than a century. But it does not concentrate in one part of the body the way radioactive iodine does.
The Chernobyl accident released a plume of radioactive materials into the atmosphere in a fraction of a second. In the following years, the incidence of thyroid cancer among those exposed as children increased in Ukraine and nearby countries, Zablotska said. The cancer showed up between four and 10 years after the accident, Bouville said.
Children were exposed to radioactive material mainly from eating contaminated leafy vegetables and dairy. There have been no detectable health effects from exposure to radioactive cesium after the accident.
In general, it takes a pretty high dose of radiation to increase cancer risk, Higley said. For instance, there were reports that one Japanese worker was exposed to 10 rem (100 millisievert, mSV), a measurement of radiation dose. From that exposure, his lifetime cancer risk would go up about half a percent, Higley said. According to Higley, the dose is the equivalent of about five CT scans. Americans are exposed to about 0.3 rem (3 mSv) each year from natural sources, such as the sun.
Potentially, exposure to any type of radiation can increase cancer risk, with higher exposure increasing the risk, Bouville said.
No increases in cancer rates were observed after the release of radioactive from a power plant on Three Mile Island, Pa., in 1979, Zablotska said.
A person's risk of getting sick depends on how much radiation the body absorbs. Those exposed to high levels of radiation, about 200 rem, (2000 millisievert ) could develop radiation sickness, Bouville said. A chest X-ray is about 0.02 rem, (0.2 millisieverts mSv), according to the Interational Atomic Energy Agency.
People are exposed to about 0.24 rem (2.4 mSv) per year from natural background radiation in the environment, the IAEA says.
Radiation sickness is often fatal and can produce such symptoms as bleeding and shedding of the lining on the gastrointestinal tract, Zablotska said. About 140 people suffered from it as a result of the Chernobyl accident, Zablotska said.
A radiation dose of 40 rem, (400 mSv) per hour was reported at one of the Japanese power plants at one point following the March 11 earthquakes and tsunami that damaged their cooling systems, according to the IAEA. This is a high dose but was isolated to a single location, the IAEA says.
"That is definitely an area where you do not want to stay for prolonged period," Higley said. She notes that a total dose of 400 to 600 rem can be lethal. But the radiation levels have been decreasing after the observed spike, she said. She speculates the spike may have been due to the release of a puff of radioactive material when pressure dropped at the facility.
Follow MyHealthNewsDaily staff writer Rachael Rettner on Twitter @RachaelRettner.
This story was provided by MyHealthNewsDaily, a sister site to LiveScience.
How Is Radiation Exposure Measured?
A more sophisticated measure of radiation exposure, called the effective dose, accounts for the harmfulness of the specific type of radiation present. While the effective and absorbed doses are the same for beta and gamma radiation, for alpha and neutron radiation – types that are especially dangerous for the human body – the effective dose has a larger value than the absorbed dose. A measure of the effective dose therefore gives a concrete scale for determining how dangerous an incident of exposure actually is. Units of effective dose are the "roentgen equivalent man" (rem) and the sievert (Sv), where one Sv equals 100 rem.
An average person receives an effective dose of 0.36 rem every year, 80 percent of which comes from natural sources of radiation, such as radioactive materials in the Earth's crust and mantle and sources in outer space. The remaining 20 percent of an average person's effective dose results from exposure to artificial radiation sources, such as X-ray machines, industrial smoke detectors, and continuing fallout from nuclear weapons tests.
In the United States, the NRC limits occupational radiation exposure to adults working with radioactive material to 5 rem per year. The limit can be raised to 25 rem when there's an emergency; that level is still not considered dangerous.
Radiation levels at Fukushima shot up to 0.8 rem per hour after an explosion at one of the nuclear reactors earlier today (March 15). If emergency workers had not been evacuated shortly afterward, they would have gotten their yearly occupational dosage in just over 6 hours.
Though potentially dangerous, that amount still would not have been lethal. According to the NRC, "[It] is generally believed that humans exposed to about 500 rem of radiation all at once will likely die without medical treatment. Similarly, a single dose of 100 rem may cause a person to experience nausea or skin reddening (although recovery is likely), and about 25 rem can cause temporary sterility in men. However, if these doses are spread out over time, instead of being delivered all at once, their effects tend to be less severe."
CROATIAN CENTER of RENEWABLE ENERGY SOURCES ( CCRES )