Fallout from Japan's crippled nuclear reactors already is being detected thousands of miles away. But scientists who track pollution blowing across the Pacific Ocean say the amount of radioactivity should pose no danger to the United States.
Like other forms of pollution, radioactivity attaches to dust and fine particles that are carried by wind, rain and snow and can be breathed in or ingested. As the disaster in Japan unfolds, the U.S. Environmental Protection Agency is sending extra staff to the West Coast to monitor radiation levels in air, cow's milk and rain. Other agencies, including the Nuclear Regulatory Commission, the National Oceanic and Atmospheric Administration and the Department of Energy, are tracking weather patterns and using computer models to predict how radioactivity from Japan might spread.
But even as government officials step up their monitoring efforts, scientists say it is important to keep the scope of the current crisis in perspective.
"While tragic, this is a single source of radioactivity that will be highly diluted if it crosses the Pacific," said Daniel Jaffe, an atmospheric scientist at the University of Washington who is tracking the fallout from Japan. "It is highly unlikely this would pose any risk to the West Coast, let alone Chicago."
Jaffe is among a handful of researchers who have documented how noxious pollution from China's growing number of coal-fired power plants is wafting across the ocean to California, Oregon and Washington. "Even that pollution, emitted on a continental scale, is diluted by orders of magnitude by the time it reaches us," Jaffe said.
Experts say it is still too early to estimate how much radioactive material will be released into the atmosphere 6,000 miles away from Chicago. The greatest danger to nearby areas in Japan, experts said, will come if one or more of the reactors completely melts down and containment vessels fail to limit the radiation. There also are concerns about radiation spreading from the large amounts of spent fuel in storage pools that are not shielded as well as the reactors.
Wind and rain will help dictate how far the fallout spreads. Winds could disperse the material over a larger area, where it could be diluted by the Earth's atmosphere. Rain or snow would force more of the radioactive debris to the ground or ocean.
Although reactor cores contain enormous amounts of radioactive material, how far dispersed materials might travel depends in part on the force with which they are released, said Brian Toon, an atmospheric researcher at the University of Colorado at Boulder.
An intense fire or a larger explosion could drive the material higher into the air, Toon said, possibly high enough that it would catch the jet stream and travel a considerable distance.
"You can see already in Japan that that material is … near the surface," Toon said. "Therefore it will probably go right out over the oceans and be washed out quickly and very little radioactive material will go very far."
Many experts predict the health consequences of radiation in Japan will pale compared with deaths caused by the earthquake and tsunami that set off the nuclear crisis.
The closest thing to the situation unfolding now is the 1986 disaster at the Chernobyl nuclear plant in Ukraine, where a reactor blast and fire put 400 times more radioactive material into the atmosphere than the atomic bomb dropped on Hiroshima, according to the International Atomic Energy Agency. Fallout spread around the globe, but years of study have shown the health effects were concentrated in areas near the plant.
Moreover, experts say, an epidemic of thyroid cancer seen after Chernobyl could have been prevented if people living near the damaged reactor had been advised to avoid drinking milk from cows that had eaten contaminated grass near the reactor. In Japan, government officials have evacuated people near the plant, told others to remain indoors and distributed potassium iodide pills that can protect the body from some of radiation's harmful effects.
A measurable amount of radiation from Chernobyl reached the United States, Toon said, "but it's nothing compared to all the radiation tests that were done in Nevada in the 1950s."
More recently in the U.S., the 1979 accident at the Three Mile Island reactor prompted Pennsylvania officials to create a registry that tracked the health of more than 30,000 people who lived within five miles of the plant at the time. When state officials found little evidence of increased cancer rates, they shut down the database in 1997.
The Japan disaster has prompted some worried Americans to buy Geiger counters and stock up on potassium iodide pills, but health officials in Illinois and other states are cautioning Americans to avoid potassium iodide supplements. In a statement, the Illinois Department of Public Health said people could be "putting their health at risk" from the side effects of taking the pills.
"The amount of radiation you get going from LA to New York on an airplane is probably greater than anything anyone will ever get from Japan," said Glenn Braunstein, director of the Thyroid Cancer Center at Cedars-Sinai Medical Center in Los Angeles.
In case a nuclear disaster strikes closer to home, potassium iodide supplements are stockpiled around the country.
Dr. Gady Har-El, a head and neck surgeon at New York's Lenox Hill Hospital, said he has noticed that some patients from Belarus, Ukraine and Russia who lived in their native regions during the Chernobyl disaster express more concern about thyroid abnormalities, even when he tells them there's no cause for immediate worry.
"Sometimes I have a hard time convincing them that there's no need to rush into surgery," Har-El said. "I don't know what the psychological mechanism behind it is. But it's there."
The fact that radiation is invisible can stoke fears about possible exposure. And because nuclear materials are frequently associated with weaponry and national security, worries about radiation can feed paranoia, said Daniel Lieberman, a professor of psychiatry at George Washington University in Washington.
"We take risks every day that are hundreds of times greater than risks of radiation, like driving over the speed limit," Lieberman said. "But if we're driving, we feel in control. With radiation, there's an absence of control. We can't see it."
Tribune Washington correspondent Andrew Zajac contributed.