Past events have helped researchers pinpoint when radiation exposure gets dangerous.
Exposure to radiation can cause a range of health effects, from acute gastrointestinal illness to cancer.
Nuclear events from the past are helping experts predict and prevent medical fall-out from the current crisis in Japan.
Radiation dose levels at the Fukushima Daiichi plant have been moderate so far, but it could get worse.
Radiation continues to seep out of an earthquake-damaged nuclear plant in Japan. The public faces inconsistent reports about how bad the damage is. And many people are anxiously wondering, even panicking, about what the implications might be for their health.
While scientists can't predict the future, they can put the current crisis into perspective. And so far, experts say, the situation is worrisome -- but it pales in comparison to radiation levels seen in some of the past's worst nuclear disasters.
For now, the biggest health concerns in Japan face cleanup workers and residents who live in a 12 to 50-mile radius from the Fukushima Daiichi plant. Those groups may face elevated risks of thyroid problems, leukemia and other cancers in the years and decades to come.
If the situation worsens, though, health threats will become much more serious, far more widespread and potentially fatal. Depending on wind and weather patterns, effects could reverberate as far as the western United States. And once radiation settles into the Earth, it can persist for thousands or millions of years.
"If [the situation] does change to the extent that they had a full-blown meltdown of the entire core and an explosion, then that would be another Chernobyl," said Scott Davis, an epidemiologist at the Fred Hutchinson Cancer Research Center and the University of Washington in Seattle. "Then, it's a different ballgame."
Scientists measure radiation dose levels with units called millisieverts, or mSv. Because radiation streams toward us all the time from outer space and from the Earth's crust -- along with small discharges from medical facilities, nuclear power plants, and other man-made sources -- most people are exposed to background levels ranging between 1 and 10 mSv over the course of a year, depending on the area and the altitude.
One chest X-ray delivers 0.1 mSv just to a single part of the body, said medical physicist James Hevezi, chair of the American College of Radiation Medical Physics Commission. And medical radiation workers are monitored to prevent total exposures of more than 50 mSv per year above and beyond background levels. At low levels like these, medical concerns are minimal.
At higher doses, risks rise. That's because radiation knocks electrons off of atoms as it passes through our bodies. When that happens enough, cells die, DNA breaks, and tissues become damaged.
Radiation affects everyone's body in different ways. And damaging effects can be enhanced by trauma, injuries or other illnesses. But in general, starting at exposure levels of about 1,000 mSv (or 1 sievert) per hour, radiation sickness can cause nausea, vomiting, diarrhea and skin blisters.
Exposures of between 3,500 and 5,000 mSv for a period of minutes to hours lead to death within 30 days for about half of people, according to the United States Nuclear Regulatory Commission. Even higher levels cause immediate death.
At exposures above normal background levels but below the acutely ill or fatal limits, effects are less dramatic but more insidious. People don't necessarily know they've been affected until symptoms start to appear months, years or even decades later.
"When you see a truck coming at you, you know you're going to get hurt real bad," Hevezi said. "The problem with radiation is that we don't sense it. We don't feel it going on."
After an extreme exposure event, thyroid problems are usually the first issues to show up in the first months to years, because the gland is particularly sensitive to radiation. Childhood leukemia also appears early. Down the line, people develop solid tumors, and cancers of the breast, colon, lungs and elsewhere.
"Radiation is often referred to as a universal carcinogen because of its ability to produce cancers in a number of tissues and organs," said Jerrold Bushberg, director of the health physics programs at the University of California, Davis. For every 1,000 mSv of exposure, he added, the risk of dying from cancer goes up by about 5 percent above the 24 percent natural mortality rate from the disease.
History offers plenty of examples of how radiation can affect health. Researchers have detected few to no problems from relatively small exposures -- such as the 10-mSv doses for people closest to atomic bomb testing sites, or the 20-mSv peak that occurred during a 1979 accident at the Three-Mile Island power-plant in Pennsylvania.
During the 1945 atomic bomb explosions in Hiroshima and Nagasaki, on the other hand, peak radiation levels ranged from 10,000 mSv to more than 100,000 mSv, said Bushberg. Many people died instantly, and radiation sickness killed many more within months.
For most of the people who survived the blast, though, average doses ranged from 10 to 100 mSv, Bushberg said. And studies of that population have helped researchers learn much of what they know about the links between radiation and cancer.
Compared to the atomic bomb blasts, workers and firefighters at the 1986 Chernobyl disaster experienced lower, but still high exposures, according to the USNRC, ranging from 800 to 16,000 mSv. Yet average radiation doses for Ukrainian evacuees from the area were 17 mSv.
For Belarussian evacuees, the average was 31 mSv. And so far, the only documented health effects from that event are thousands of cases of thyroid cancer in children and teenagers who drank milk that was contaminated with radioactive iodine.
It's hard to know what's going to happen in Japan. At the malfunctioning Fukushima Daiichi plant, radiation dose levels have reached 400 mSv per hour in one of the reactors, according to reports, though it's not clear if people have been exposed to the highest levels.
In Tokyo, radiation levels rose briefly to just three or four times above the normal background level, then fell back to normal again.
"I don't want to speculate the worst because who knows what is going to happen," Bushberg said. "At the moment, the amount of radiation released has been modest. I don't have any specific concern for people in general."