The Kyshtym disaster, which occurred at Mayak in Russia on 29 September 1957, was rated as a level 6 on the International Nuclear Event Scale, the third most severe incident after Chernobyl and Fukushima. Because of the intense secrecy surrounding Mayak, it is difficult to estimate the death toll of Kyshtym. One book claims that "in 1992, a study conducted by the Institute of Biophysics at the former Soviet Health Ministry in Chelyabinsk found that 8,015 people had died within the preceding 32 years as a result of the accident." By contrast, only 6,000 death certificates have been found for residents of the Tech riverside between 1950 and 1982 from all causes of death, though perhaps the Soviet study considered a larger geographic area affected by the airborne plume. The most commonly quoted estimate is 200 deaths due to cancer, but the origin of this number is not clear. More recent epidemiological studies suggest that around 49 to 55 cancer deaths among riverside residents can be associated to radiation exposure. This would include the effects of all radioactive releases into the river, 98% of which happened long before the 1957 accident, but it would not include the effects of the airborne plume that was carried north-east. The area closest to the accident produced 66 diagnosed cases of chronic radiation syndrome, providing the bulk of the data about this condition.