Science is a wonderful thing. Not only does it give us fun gadgets and fascinating theories, but it also allows us to draw highly-accurate conclusions about events happening far, far away. Since the earthquake in Japan, a team at the University of Washington at Seattle has been testing air filters from university buildings for radioactive particles. Their results have allowed them to gauge exactly where the real threat comes from Fukushima Diachi. Even though, they are approximately 4800 miles apart.
The first comes from the amount of iodine-131 and tellurium-132 which are both short-lived with half lives of 8 and 3 days respectively. That indicates that they must have come from fuel rods that were recently active rather than from spent fuel...
Finally, there are a huge number of possible breakdown products from nuclear fission in a reactor and yet the Seattle team found evidence of only three fission product elements--iodine, cesium and tellurium. "This points to a specifific process of release into the atmosphere," they say.
Cesium Iodide is highly soluble in water. So these guys speculate that what they're seeing is the result of contaminated steam being released into the atmosphere.