It's a big ocean between northeastern Japan and the United States and thousands of miles from the crippled nuclear power plant to much of Asia. That means there's little chance - at least for now - that radiation from the shattered reactors could pose a serious threat to the wider world.
Experts say the amount of radioactivity emitted by the facility has been relatively minor and should dissipate quickly over the Pacific Ocean.
"Every mile of ocean it crosses, the more it disperses," said Peter Caracappa, a radiation safety officer and clinical assistant professor of nuclear engineering at Rensselaer Polytechnic Institute in Troy, N.Y.
The only people at immediate risk are workers inside the plant and the people living closest to it. The danger of radiation exposure elsewhere is minuscule - unless the plant sustains a complete meltdown, which would sharply escalate the dangers.
Japanese officials told the International Atomic Energy Agency on Tuesday that a fire had broken out in a fuel storage pond where used nuclear fuel is kept cool and that radiation had been "released directly into the atmosphere."