Thousands of residents evacuated Fukushima after the Daiichi Nuclear Power Plant disaster occurred as a result of the Tōhoku earthquake and tsunami that hit Japan in March 2011.

There were, however, no formal evacuations in Fukushima Prefecture's Date City regardless of radiation worries. Now, six years after the incident, a study offers assurance to residents.

Findings of the study to be published in the Journal of Radiological Protection offer assurance to those who live just over 37 miles from the location of the nuclear disaster that they do not have to worry about dangerous levels of radiation over the course of their lifetime. For those who left as a precautionary measure, the results could also mean it is safe to go back home to this region.

60 Percent Drop In Cesium Radiation Levels

Researchers of the study claim that that the cesium radiation levels in Date have dropped by 60 percent over a two-year period from 2011 to 2013. Radiation levels dropped so quickly residents should not suffer from any harm.

The researchers found that in Zone A, the most contaminated part of the city, median lifetime radiation dose was only 18 millisieverts. This means the place may actually be safer than usual as this level is less than what is typically received by a person over a lifetime from the radioactive elements in the Earth's crust and the high-energy particles that bombard the atmosphere.

The International Commission on Radiological Protection considers a dose of between 1 and 20 millisieverts per year as acceptable. In fact, Japan allows workers at damaged plants to accumulate as much as 100 millisieverts over a period of five years.

"We found that the mean additional lifetime dose of residents living in Date City is not expected to exceed 18 mSv," the researchers wrote in their study. "This method of combining individual doses and the ambient doses, as developed in this study, has made it possible to predict with reasonable certainty the lifetime doses of residents who continue to live in this radiologically contaminated area."

Drop In Radiation Levels Due To Natural Causes

Researchers think that the drop in radiation level is largely due to natural causes such as snow, rain and the decay of cesium itself rather than to active and expensive decontamination efforts such as roof washing and topsoil removal.

Personal radiation measurements that were collected from more than 400 people who live in Zone A do not show significant evidence of drop in radiation levels around the time of decontamination work that started in October 2012, which means decontamination efforts may not have a crucial impact on the radiation levels.

"Population-wise, we didn't see a big decrease in the individual dose," said study author Ryugo Hayano, from the University of Tokyo. Nonetheless, the researchers said this does not mean that decontamination should not have been carried out. "There may be individuals or families for whom the decontamination was effective."

While the study suggests people in Date City are safe from radiation levels, not every area in the region is free from radiation worries. There are still towns with strong radiation levels ruling out the possibility of former residents returning to their former homes.