Radiation is everywhere. Some naturally occurs in gases, such as radon, released from the ground. Some comes from the sun. And some is man-made in the form of X-rays, radio waves and nuclear power.
The average annual radiation dose of a person living in the United States is 6.2 millisieverts. This includes 2.4 millisieverts per year of natural or background radiation, according to the National Council on Radiation Protection and Measurements, an advisory group. A millisievert is a measure of radiation absorbed into living tissue.
The Chernobyl emergency workers who died within a month of the accident received doses of around 6,000 millisieverts. Over many years the average dose evacuated residents of the area received was 490 millisieverts, according to the World Nuclear Association, a nuclear information and analysis group. The typical dose of a CT scan is about 10 millisieverts, according to Radiology Info.
Over a short time, a 1,000-millisievert dose causes acute radiation sickness; a 10,000-millisievert dose would be fatal within a few weeks.
The effects of long-term exposure to relatively low doses of radiation is controversial.
The World Health Organization, the International Atomic Energy Agency and the United Nations Scientific Committee on the Effects of Atomic Radiations all say there is no good evidence to support claims that radiation can lead to cancer at doses less than 100 millisieverts.
One low-dose skeptic is scientist Gerry Thomas, a professor at Imperial College London.
"Our acceptance of exposure to radiation is somewhat schizophrenic," she wrote in a recent research paper. "We accept that the use of high doses of radiation is still one of the most valuable weapons in our fight against cancer, and believe that bathing in radioactive spas is beneficial. On the other hand, we are fearful of exposure to man-made radiation as a result of accidents related to power generation, even though we understand that the doses are orders of magnitude lower than those we use every day in medicine."
This view is contested by independent scientists and nuclear specialists.
"Many (low dose) deniers find it difficult to accept that most cancers occur only after long latency periods, and that they can arise many decades after exposure, but cancers are still being registered from the atomic bombs at Hiroshima and Nagasaki in 1945. That is over 70 years ago," said Ian Fairlie, a British-based radiation biologist who is evaluating the health effects of the Chernobyl nuclear disaster.
"Human evidence and radiation biology theory both demonstrate that there is no safe dose of radiation," Fairlie said.
