Ten days after an earthquake and tsunami crippled a nuclear plant in Japan, officials are detecting abnormal levels of radiation in what may seem like a scattershot assortment of foods: milk from Fukushima Prefecture, where the reactors sit; spinach from Ibaraki Prefecture to the south; canola from Gunma Prefecture to the west; and chrysanthemum greens from Chiba to the south. Shipments of the milk and spinach have been banned.

Experts hesitate to predict where the radiation will go. Once radioactive elements that can harm health are released into the outdoors, their travel patterns are as mercurial as the weather and as complicated as the food chains and biochemical pathways along which they move.

When and where radioactive contamination becomes a problem depends on a vast array of factors: the specific element released, which way the wind is blowing, whether rain will bring suspended radioactivity to earth, and what types of crops and animals are in an exposed area.

Research related to the 1986 Chernobyl accident makes clear that for decades, scientists will be able to detect the presence of radioactive particles released by the crippled Japanese reactors thousands of miles away. Scientists and doctors in Japan and abroad will be monitoring the results to see if those measurements reach dangerous levels. So far there is no indication that anyone has been harmed by eating contaminated food.