I think the majority of the casualties they list in their near million figure are from the hundreds of thousands of people that participated in clean up efforts that have all died now. I have not though read as to the cause of their deaths and if it could be linked to their actions in clean up.
From the NatGeo article...
"
During the days after the explosion thousands of other workers, called liquidators, were rushed to Chernobyl to tame the radioactive inferno. Coal miners dug underneath the seething core to allow liquid nitrogen to be pumped in and cool the nuclear fuel. Helicopter pilots dumped 5,000 tons of lead, sand, clay, and other material in an effort to douse the flames. Soldiers made timed dashes onto the roof to shovel smoking graphite blocks blown out of the reactor back into the core. Referred to, sardonically, as "bio-robots," many of the 3,400 surreally brave men who took part in this operation absorbed a lifetime radiation dose in seconds.
On May 6 the fires in the mangled reactor were finally extinguished, and an army of liquidators went to work building the sarcophagus and consolidating radioactive waste at several hundred dumps near Chernobyl. In those early days doctors monitoring the liquidators watched white blood cell counts drop and feared for their health. Most recovered."
