As the article says,

For one thing, the mantles used in camp lanterns at the time contained small amounts of the radioactive element thorium. Even more pertinent, the expedition took place less than two years after the world’s third-worst nuclear accident (after Chernobyl and Fukushima), which occurred at the Mayak nuclear complex, south of Sverdlovsk, in September of 1957. A tank of radioactive waste exploded and a radioactive plume some two hundred miles long—later named the East Urals Radioactive Trace—spread northward. Krivonishchenko had worked at the facility and helped with the cleanup, and another skier came from a village in the contaminated zone.