As part of an informal volunteer network, researchers from universities and government agencies pooled monitoring data from about 150 monitoring stations across 25 countries in Europe. They focused on radioactive iodine, which is of particular concern for human health, because it can disrupt thyroid function and potentially cause thyroid cancer. Although the radioisotope 131I has a half-life of only eight days, it can travel great distances when it is in the gaseous form.
The scientists measured the ratios of iodine and other isotopes as a way to fingerprint the source of the radioactive material. They found that the ratios matched those reported from Japan during the crisis, confirming that the detected radioactive material was from Fukushima. Also the researchers determined that most of the particulate 131I remained in the atmosphere and didn't fall out through precipitation during the journey from Japan, because the ratio of gaseous to particulate 131I in Europe matched the one measured in Japan.
Total 131I concentrations peaked at 4 millibecquerel per m3 at European monitoring stations. Background 131I levels in Europe are usually negligible.
All measured iodine levels were 1,000 to 10,000 times lower than concentrations detected in Europe after the 1986 nuclear accident at the Chernobyl power plant in Ukraine, says co-author Lars-Erik De Geer, research director of the Swedish Defense Research Agency. Because the amount of radioactive material released during that crisis didn't lead to significant health effects in the rest of Europe, the researchers concluded that the Fukushima radioisotope levels posed no health risk.
In the Proceedings of the National Academy of Sciences, U.S. researchers report this week on levels of radioactive sulfur that traveled from Japan to the California coast.
The European researchers also compared their measurements to computer models of how radioisotopes would travel around the globe from Fukushima. They found that the concentrations predicted by the models were lower than those detected at the monitoring stations.
Gerhard Wotawa of the Central Institute for Meteorology and Geodynamics in Vienna, Austria, says these results show that researchers need to revisit the underlying physics in their models to improve predictions for the next nuclear emergency.
Dan Jaffe, an atmospheric chemist at the University of Washington, Bothell agrees: "I'd like to see this as an opportunity to ask, 'how can we improve things?'"