There are many well documented cases of radiation induced cancer in humans. The early scientists who worked with x-rays and radioactive substances did not realise the risk. Many died from skin and bone cancer and leukemia . Leukemia is a disease characterised by a great excess of immature white cells in the blood and can be likened to a blood cancer. Marie Curie , for example, who first isolated radium from uranium ore died of leukemia as did her daughter- assistant, (her husband on the other hand died in a traffic accident).
In the 1920's, watch dials were painted with a radium based luminous paint. The employees, all women, who did this work often licked their paint brushes to give them a sharp point and ingested a small quantity of the paint each time they did this. The radium in the paint collected in the bones of these employees and resulted in bone tumours 8 to 40 years later.
Fig. 
shows the distribution of cancers among the 1346 radium
dial painters that were followed up. It
demonstrates quite clearly that the risk of cancer increases with the
radiation dose.
Figure: Cancer cases in radium dial
painters
In Great Britain more than 6500 patients with arthritis of the spine were
treated with large doses of x-rays . The average
dose was 
. The disease called ankylosing
spondylitis , causes a painful stiffening of the
joints in the backbone.
Of the 6500 patients, 30 developed leukemia compared with an expected incidence of 7 cases.
The largest number of human beings exposed to high levels of whole body radiation are the survivors of the Hiroshima and Nagasaki atom bomb attacks. Nearly 80,000 of these people have been carefully studied in the years since the war.
Of this number of survivors, 126 died of leukemia. This is nearly double the normal figure for this number of people. The incidence of leukemia was related to the distance from the explosion and therefore to the radiation dose received. The highest incidence was in those survivors closest to the explosion. This provided clear evidence of the dose dependent relationship of leukemia to radiation, i.e. the higher the dose, the greater the risk.
The study of these survivors also indicated an increase in frequency of stomach , lung and breast cancers . These have taken much longer to develop and some are still appearing now.
The A-bomb data from Japan shows that that there is a delay (called the latency ) between the radiation exposure and the death from cancer it induces. The mean latency from leukemia is in the region of ten years, and for the other cancers it is more than twenty years. Even now, more than 40 years after the explosions, excess cancer deaths are still occurring.