News agencies have reported radiation measurements from nearby the Fukushima nuclear reactor, as well as in other locations in Japan, such as Tokyo. The following is an attempt to explain those measurements and put them in context.

What is a Sievert? What is a rem?
There are several ways of measuring radiation. The Sievert is the metric measurement of the biological risk of exposure to radiation. The rem is the equivalent measure in conventional units. The Sievert is a cumulative measure of risk – therefore, media agencies often report exposure in units of Sieverts per hour.

What are the short- and long-term health effects of radiation?
In the short term, exposure to high levels of radiation can cause radiation sickness, leading to nausea, weakness, hair loss, and, in some cases, death. Exposure to radiation can also have long-term health effects, increasing the risk of cancer.

What levels of radiation cause short-term (acute) effects?
Exposure to more than 0.5 Sievert of radiation has been linked to acute health effects, beginning with the onset of nausea. After 1 Sievert of exposure, individuals are at risk of more severe health effects, including hemorrhage. Exposure to several Sieverts of radiation can result in death.

What levels of radiation cause long-term effects, such as cancer?
There is no known safe level of exposure to radiation. According to the U.S. National Academy of Sciences, a single dose of 0.1 Sieverts would result in approximately 1 person in 100 developing cancer over their lifetime. Lower doses produce proportionally smaller risks. For example, a single exposure of 0.01 Sieverts would cause 1 person in 1,000 to develop cancer during their lifetime.

What have the levels of radiation exposure been at the Fukushima plant, or elsewhere in Japan?
The highest radiation reading on March 15 at the Fukushima plant was a reading of 0.4 Sieverts/hour between reactors 3 and 4. [CURRENT LEVELS MAY WELL BE HIGHER.] It is unknown how long that level of radiation persisted, but experts have stated that such high levels of radiation are definitely capable of harming human health. Since then, radiation levels have declined, with the International Atomic Energy Agency reporting at 2 a.m. Eastern time on March 15 a reading of 0.6 millisieverts per hour.

A helpful chart illustrating the effects of various levels of radiation exposure can be found here.

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