March 21, 2011
On March 11th, Japan was devastated by a 9.0 earthquake and tsunami, which has resulted in tremendous loss of life and a serious nuclear issue that people around the world are watching carefully. As of Friday, March 18th, the disaster now is classified as Level 5 on the 7-point International Nuclear Event Scale. As a comparison, the partial meltdown at Three Mile Island in 1979 was also rated as a 5, while Chernobyl was a 7. Data released by the Tokyo Electric Power Company (Tepco) indicate that the radiation level at a sampling location at the west gate to the quake-damaged Fukushima No. 1 nuclear plant had decreased steadily to 265.0 microsieverts per hour at 11 a.m. Friday, from a previous level of 351.4 microsieverts per hour at 12:30 a.m. Thursday. The Wall Street Journal reported that levels in the Tokyo area were around typical background levels on Friday afternoon, while in areas close to the Fukushima plant, they remained high but still below levels that would pose a threat to human health.
When a person is exposed to radiation, energy is deposited in the tissues of the body. The amount of energy deposited per unit of weight of human tissue is called the absorbed dose. A rem (or the SI unit Sv) is a measure of radiation dose.
Comparison of Exposure Dose in Microsieverts (1 Sv = 1,000,000 microsieverts = 100 rem)
Exposure to cosmic rays during a roundtrip airplane flight from New York to Tokyo: 150 microsieverts (µSv)
One chest x-ray: 100 µSv
One mammogram: 700 µSv
One abdomen CT scan: 10,000 µSv
One year of exposure to natural radiation (from soil, cosmic rays, etc.): 3,000 µSv
About half of the total annual average U.S. individual’s radiation exposure comes from natural sources. The other half is mostly from diagnostic medical procedures.
The U.S. Nuclear Regulatory Commission sets the annual occupational dosage limit for workers who deal with radiation at 50,000 µSv per single year, with a 20,000-µSv/year average, and the limit per nuclear event at 10,000 µSv. The U.S. Environmental Protection Agency (EPA) advises that a dose of 500,000 µSv can cause nausea, while 4 million µSv can be fatal without major medical care.
Read more about Radiation Monitoring, Exposure Assessment, Direct Risk to Individuals from the Fukishima Release, Food Issues, and Risk to Residents in the U.S.