By Julie Steenhuysen
CHICAGO (Reuters) - Dozens of workers battling to control radiation at Japan's stricken reactors face a far greater risk of developing cancer than normal, but Tokyo residents are within the safe range for exposure, U.S. nuclear experts said.
Radiation levels in Tokyo, one of the world's most populous cities, rose 10 times above average Tuesday evening, spreading fear among many of the 33 million residents in the metropolitan area.
The best advice experts could give them was to stay indoors, close the windows and avoid breathing bad air -- steps very similar to those for handling a smog alert or avoiding influenza.
While these steps may sound inconsequential, experts said the danger in Tokyo, while worrisome, is slight - at least for now.
"Everything I've seen so far suggests there have been nominal amounts of material released. Therefore, the risks are generally low to the population," Jerrold Bushberg, who directs programs in health physics at the University of California at Davis, said in a telephone interview.
"There may be more significant risks for emergency workers on site. They are dealing with the occupational exposure, but not for the population at large."
Fresh explosions Tuesday at the Fukushima plant, 180 miles north of Tokyo, released low levels of radiation, escalating a crisis triggered by last week's massive earthquake and tsunami. With cooling systems knocked out, the fear is more blasts within the reactors at the complex could eventually cause a major radiation leak.
The levels measured around Tokyo at one point were 40 times above normal but have receded to 10 times. That amounts to roughly the same dose as a chest or abdominal CT scan.
"My instinct tells me it is probably not a health concern for the long term," said Dr. John Chute, a cancer biology expert at Duke University School of Medical Center. But because radiation is so poorly understood, Chute said taking steps to prevent exposure just makes sense.
"If it were me, I'd stay indoors."
WELL BELOW LETHAL
For cancer risks to be elevated, exposure would have to exceed 100 millisieverts in a year, experts say. To be lethal, the blast of radiation would have to top 5,000 millisieverts, delivered in just minutes or hours.
Measurements at the damaged plants are well below lethal at 400 millisieverts. That means unprotected workers may have been exposed to about four times the level deemed to increase the risk for cancer, or 20 times the annual exposure for some nuclear-industry employees and uranium miners.
"There are 40 people or so that are in the process of risking their lives trying to pump sea water into these plants. They are real heroes. If they get the plants full of sea water, then things will cool down and we'll be OK," Kemper said.
Japan's nuclear safety agency said two workers were missing after Tuesday's explosion at a reactor of the Fukushima plant.
Most sensitive to radiation exposure are the thyroid gland and bone marrow. Leukemia, a cancer that arises from bone marrow, is the most common radiation-induced cancer, according to the American Cancer Society.
One method of protection is to take potassium iodide pills just before or after exposure. The substance floods the thyroid gland with so much iodine that the body is unable to absorb any of the harmful radioactive forms of iodine released in a nuclear accident.
Medical studies are inconclusive about the effects of low-level exposure. Most studies have looked at the cancer risk from high levels. It is much more difficult to tease out the increased risk of cancer from low-level radiation exposure from smoking and other lifestyle factors that are known to increase a person's cancer risk, scientists say.
Exposure to heavier doses of radiation over a short period causes burns or radiation sickness, triggering nausea, weakness, hair loss, skin burns and reduced organ function. A large exposure can cause premature aging or death.
The U.S. military took new steps to safeguard its personnel from radiation Tuesday, moving arriving warships to safer waters and cautioning some forces to limit outdoor activity.
For residents of Tokyo, experts said people could take similar precautions, staying inside as much as possible.
"The reason you stay inside is you don't want to get it on your body. The radiation is only serious if you ingest it -- assuming it is low level," Kirby Kemper, a nuclear physics professor at Florida State University, said in a telephone interview.
"The ultimate concern is radiation has the ability to cause cancer. In very high doses, it can have immediate effects," said Peter Caracappa, a nuclear engineering professor at Rensselaer Polytechnic Institute in New York.
(Additional reporting by Bill Berkrot in New York; Editing by Stella Dawson and Frank McGurty)