While the damaged facilities at Fukushima have dumped a lot of radioactivity into the environment, most of it has ended up either in the ocean, or in the groundwater at the site itself. Outside the 20km exclusion zone, most of the radiation came from a single plume released in the first few days of the crisis. The plume drifted to the northwest, leading to the evacuation of some communities outside of the exclusion areas.

Does any of the radiation that traveled in that plume pose a threat to the people who have since returned? To find out, a large consortium of Japanese scientists performed a monitoring project on residents in three areas near Fukushima. They found that, while radiation exposure is elevated compared to natural background, the levels are still well below safety limits, and the long-term health risks are small enough that we're unlikely to detect the impact of the added exposure.

They did find, however, that residents involved in the cleanup had the highest exposures, which suggests continued monitoring of these workers should be a priority.

The research team identified three areas to recruit residents. One was to the southwest, away from where the airborne radiation spread. A second was to the north, just east of the area where the plume extended outside the exclusion zone. And finally, there was a site to the northwest, just outside the area evacuated because of the plume. Residents in these areas were given monitoring badges, their food was sampled, and dust samples were obtained from their environment.

The Japanese government is monitoring and removing food that shows signs of significantly elevated radioactivity. Still, many residents of the areas maintain home gardens that could provide a route for radioactive contamination. The study, however, showed that most of the residents were seeing food-borne doses in the microSievert/year range, well below the limit of 1 Sv/year. Airborne exposure was also minimal.

These factors left environmental exposure as the largest potential risk. And it was larger than either of the others, with annual doses likely to be over a microSievert per year. "The mean of the dose rates in 2012 was greater than the ordinary permissible dose level of 1 mSv/y (31), particularly in the [area at the northern edge of the plume]," the authors note, "but was less than the permissible annual dose of 20 mSv/y during radiation emergencies " All told, it was about equal to the natural background radiation in Japan (which is below that of the natural background in the US—2 mSv/y vs. 6 mSv/y). When used to estimate the health risks of cumulative exposure, it suggests that the risk of developing a solid tumor is only increased by one percent. Since that risk starts out low, any change in cancer incidence is likely going to be below our ability to detect.

In the area near the edge of where the plume extended over land, there were some individuals who had elevated levels of environmental exposure. Follow-up interviews revealed that these people typically worked outdoors in the nearby forests, often as part of the effort to clean up the contamination. Again, this exposure is still below safety levels, but it is significantly higher than the exposure of other people in the area. And the highest dose they saw was from someone who snuck into his former home in the exclusion zone. These high exposure individuals suggest that the cleanup work may need to go on for some time, and the workers involved in it will need to be monitored carefully.

Of course, critically, this analysis excludes people from the area inside the site of the accident, or any of the people working to contain and clean up existing leaks. But it shows that, in part because of the happenstance of weather and the site's location on the Pacific, Japan itself has escaped most of the worst of Fukushima's radiation.

PNAS, 2014. DOI: 10.1073/pnas.1315684111 (About DOIs).