Radiation levels in an 80-kilometer radius around the disaster-stricken Fukushima No. 1 Nuclear Power Plant are down 40 percent, possibly due to the effects of rainfall and decontamination work, according to a government ministry.

The Ministry of Education, Culture, Sports, Science and Technology (MEXT) on March 1 released a radiation map taken of the area showing airborne radiation one meter from ground level, measured from a helicopter on Nov. 16 last year. Compared to measurements taken around a year earlier on Nov. 5, 2011, the measurements were on average around 40 percent down.

During the time between the measurements, radioactive cesium would have naturally lessened by around 21 percent. It was the sixth radiation measurement of the area from a helicopter since the nuclear disaster. Measurements were also taken from air for areas in eight prefectures outside the 80-kilometer zone, where earlier surveys found higher radiation levels, on Dec. 28. Most of those measurements were below one microsievert per hour.

The radiation map can be seen on MEXT's homepage.