Ever since the Fukushima No. 1 nuclear power plant began spewing radiation on March 11, many have struggled to understand the unfamiliar units used to measure it.
A few days after the quake and tsunami damaged the reactors and cut lifelines in the disaster zone, news spread that radiation levels in the air had climbed sharply across Fukushima Prefecture, and the words “microsievert” and “millisievert” began to enter the popular lexicon. One millisievert equals 1,000 microsieverts.
Soon reports emerged that the government had placed a ban on spinach and other green vegetables as well as milk produced near the nuclear power plant because radiation levels exceeded allowable limits. Suddenly, another term — “becquerels” — was sprung on the public.
But sievert and becquerel are just two of many units — many named after renowned nuclear physicists — used to measure radiation, depending on which aspect of radiation is being discussed.
For example, becquerels are used to delineate the amount of radiation being emitted by a radioactive material.
On March 23, the health ministry reported that 210 becquerels of iodine-131 per liter of water, more than double the government’s limit of 100 becquerels for infants but below 300 for adults, were detected in water at a purification plant in Tokyo. The level stood at 2.9 becquerels as of Sunday.
One becquerel simply indicates that one nucleus is decaying per second. When a nucleus breaks down, it releases energy in the form of radiation.
But it does not take into account the strength of the various types of radiation that are emitted.
For that, “the ‘dose’ received by people is measured in ‘millisieverts,’ ” according to the International Atomic Energy Agency.
The average annual dose one gets from food, radon in the air, water and cosmic radiation and other sources over a year is 2.4 millisieverts worldwide and 1.5 millisieverts in Japan.
One millisievert is one-thousandth of a sievert.
Exposure to 100 millisieverts raises one’s long-term cancer risk by 0.5 percent, according to the National Institute of Radiological Science website.
The site notes this is much lower than the cancer risk posed by smoking or a bad diet.
Another unit — the “gray” — measures the amount of radiation absorbed per unit of weight of a material.
The various units can be roughly divided into those that measure radiation emitted and radiation received, according to the Federation of Electric Power Companies of Japan.
Further confusing the matter, different countries favor different measures.
Sieverts and becquerels are linked to the metric system. This International System of Units (Systeme international d’unites in French, or SI) has been nearly universally adopted, including by Japan. But there are some exceptions, including the United States.
“Most scientists in the international community measure radiation using the SI,” according to the U.S. Department of Health and Human Services website. “In the United States, however, the conventional system of measurement is still widely used.”
Instead of the becquerel, for example, some scientists in the U.S. still use the “curie.” One curie is equal to 37 billion becquerels.
To measure the amount of radiation absorbed by a person, scientists in the U.S. use the “rem” instead of the sievert. One sievert (1,000 millisieverts) is equal to 100 rems.
As for the weight-based gray, another SI unit, the U.S. replaces that with yet another unit called the “rad.” One gray is equal to 100 rads.
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