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What Does mSv Dose of Radiation Exposure Mean?

March 21, 2011

In the wake of looming nuclear crisis, we keep hearing or reading about the units Sievert or Sv for the amount of radiation exposure. So what is Sievert and what to make of it when media reports that 0.6 μSv radiation per hour has been detected (as seen in the picture above).

Scientists use different terms to report the radiation measurement depending upon the context. For eg, unit Curie (Ci) or Becquerel (Bq) is used to measure the amount of radiation which is being released or emitted by a radioactive material, while unit rad or Gray (Gy) is used to measure the amount of radiations absorbed and deposited in the human tissue. A person’s biological risk which means the risk that person will suffer health effects due to radiation exposure is measured and reported in units  rem (roentgen equivalent man) or Seivert (Sv). Biological risk depends upon the type of ionizing particles emitted (alpha, beta, gamma, X-rays) and their ability to transfer energy to the cells. Based on this energy transfer function, each particle has been assigned a Quality factor (Q). In order to asses the biological risk this factor is multiplied by the radiation dose measured.

So, 1 Sv = 100  rem = Q x rad

Are these nuclear disasters the only source of radiation dose to humans? No. we constantly get slight dosage of radiation from variety of sources including cosmic rays (more when we are flying), naturally occurring radioactive minerals in soil, medical X-rays etc. In order to get some sense of what these numbers mean, here are few typical numbers from various sources (CDC):

Hopefully these numbers will give us some idea the radiation exposures being reported in the media. So on average, we receive about 300-350 mrems (3-3.5 mSv) of radiation dose per year, sometimes even higher if we are undergoing medical treatments. International law permits exposure of about 2000 mrems per year for those who work with or around radioactive materials. Here is an excellent chart by XKCD  and another by BoingBoing which can be helpful in understanding radiation exposure and health effects.

So how much are the radiations from the failed Japanese plants. It was reported that radiation level shot to about 0.8 rem per hour on March 15th. There have been conflicting reports on the correctness of amount of radiation exposure reported by various media as well as by Govt agencies. According to NRC, if a person is exposed to 500 rems of radiation at once, person will likely die without any medical treatment. Single dose of 100 rem will cause nausea and skin reddening and 25 rem of single dose can cause  temporary sterility in men. However, NRC also says that if the radiation is spread out over time instead of being delivered at once, the affects are less severe.

More about how radiations are measured and the current status in Japan in later post.

Image credit:| AP photos|Kyodo News: A radiation detector measures 0.6µSv, exceeding normal day data on Mar 15 near Tokyo station.

Data credit: CDC, NRC, NRC

Filed under: News,Science

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