Japan's nuclear regulator says radiation levels around the crippled Fukushima Daiichi nuclear power plant have decreased to about a quarter of 2011 levels.
The Nuclear Regulation Authority says radiation levels recorded last September fell by about 78 percent compared to levels recorded in October 2011. Data was compiled in almost identical circumstances on both occasions.
Officials have been periodically monitoring radiation levels within an 80-kilometer radius of the plant by helicopter since the 2011 accident. They calculate radiation levels about one meter above the ground over a one-hour period and disclose the results in a nine-color map.
Officials say the drop is largely because radiation levels of radioactive substances decrease over time. They also attribute the reduction to decontamination work and rainfall, which has washed away harmful substances.