Engineers still struggled today with Japan's crippled Fukushima Daiichi nuclear plant, and engineers warned that time may be running out before melting fuel rods spread dangerous radiation along the Japanese coastline.
The radiation could be deadly in the areas surrounding the plant. The Japanese government has ordered people to stay at least 12 miles away; the U.S. Nuclear Regulatory Commission cautiously urged Americans in Japan to stay at least 50 miles away.
"This is very, very radioactive material," said Kenneth Bergeron, a physicist who has worked at Sandia National Laboratories. "If there is core on the floor and containment penetration, there will be significant public health consequences."
But could the danger spread to American shores? Nuclear engineers and meteorologists said the U.S., including Alaska and Hawaii, is safe.
"These releases from the plant, because they're not elevated, because they're not getting up high in the atmosphere, they won't travel very far," said Kathryn Higley, director of the department of nuclear engineering at Oregon State University. "There are so many factors in our favor. Rain will knock it down. There are 5,000 miles of ocean between us and Japan. It will be diluted, it will mix with sea spray, long before it gets remotely close to us."
President Obama, in his first statement on the nuclear crisis, also said Americans should not worry.
"I want to be very clear," he said at the White House Thursday. "We do not expect harmful levels of radiation to reach the West Coast, Hawaii, Alaska or U.S. territories in the Pacific."
Nevertheless, the Environmental Protection Agency said that it was adding several monitors to its RadNet system of radiation detectors. The small monitors are scattered around the country and send their readings to EPA computers. The EPA said it was setting up two new ones in Hawaii, two in Guam, and three in Alaska.
The agency said it acted "in an abundance of caution," to gather new data. "As the Nuclear Regulatory Commission has said, we do not expect to see radiation at harmful levels reaching the U.S. from damaged Japanese nuclear power plants."
The high-altitude winds over Japan are primarily out of the west -- good news for Japan in a worst-case scenario, if there were a large release of radiation into the air.
And if there is a worst case, with radioactive particles carried long-distance by upper-level winds?
In that case, "we will get some fallout on the West Coast 2-3 days after its release in Japan," said Edward Morse, a nuclear engineer at the University of California, Berkeley, in an e-mail to ABC News. "The levels will not be threatening to life and health but they will be observable."
At Texas A&M University, Prof. Ken Bowman and graduate student Cameron Homeyer put together a computer projection showing that if any radiation from the March 14 explosion at the plant reached altitudes of 4-5 miles (6 km), it might pass over northern Alaska on Friday. But Bowman said his model was not designed to show the intensity of any plume getting that far.
"If any radiation were to make it here, it would be merely background levels," said Jere Jenkins, the director of Radiation Laboratories at Purdue University in West Lafayette, Ind. "Nothing for people on the West Coast or people in the United States to be concerned about."
Higley said she has been spending a lot of time over the last few days urging calm.
"We have monitoring capability here in the U.S. that is extraordinarily sensitive. We can detect radiation that is like a hundred-thousandth of what you get from a regular X-ray, and we don't expect to see even that.
"For the stuff to travel, it has to be picked up by the wind," she said, "higher-level winds that have global distribution. And that's just not happening. This is a little like a campfire -- the smoke is all near the ground."
The worry remained acute, of course, along the Japanese coast, where even outside the 12-mile evacuation zone, people were urged to stay indoors if they lived within 18 miles of the Daiichi plant.
"We are all-out urging the Japanese to get more people back in there to do emergency operation there, that the next 24 to 48 hours are critical," an American official told ABC News. If cesium and other radioactive elements with long half-lives get into the air, "that could be deadly for decades," the official said.
But Jeff Masters, a former meteorologist at the National Weather Service who now works at Wunderground.com, ran a computer model and concluded that radiation would not get very far.
"Ground-level releases of radioactivity are typically not able to be transported long distances in significant quantities, since most of the material settles to the ground a few kilometers from the source," he wrote.
"Given that the radioactivity has to travel 3,000 miles to reach Anchorage, Alaska, and 5,000 miles to reach California, a very large amount of dilution will occur, along with potential loss due to rain-out.
"Any radiation at current levels of emission that might reach these places may not even be detectable," he said, "much less be a threat to human health."