Official UN figures have predicted 4,000 extra cancer deaths attributable to Chernobyl's radioactive fallout.
But Greenpeace says in a report released on Tuesday that recent studies estimate there will be 100,000 extra cancer deaths.
Doctor Oxana Lozova, who works at a children's hospital in Rivne district, 300km (190 miles) west of Chernobyl, said many generations appeared to be affected.
"I think the fallout from Chernobyl has affected the immunity of those who were young children at the time of the disaster," she said.
It spread a cloud of radioactive particles across a huge swathe of western Europe. Several million people still live in contaminated areas.
In its new report, Greenpeace says recent studies suggest the radiation from Chernobyl will cause 100,000 extra cancer deaths, and that official figures compiled by the International Atomic Energy Agency of just a few thousand casualties are a gross simplification of the breadth of human suffering.
The charity says that radiation affects the immune, circulatory and respiratory systems, and causes an increase in foetal abnormalities and birth defects.
They are controversial claims. But Greenpeace acknowledges that it is impossible to know the final impact on human health without more research.
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