Clark University have published a very comprehensive document Health Risks of Ionizing Radiation: An Overview of Epidemiological Studies, (Russ et al. 2006). In their introductory chapter they state that the harmful effects of ionising radiation, in this case, x-rays, were observed very quickly after their discovery, specifically,
X-rays were first discovered in late 1895
and dangers associated with exposure became
apparent very quickly. In 1896 the first injuries
due to x-ray exposure were recorded and in 1904
Thomas Edison’s assistant Clarence Dally was the
first person recorded to have died as a result of x-ray
More detail of this is provided in the article A Brief History of Radiation Protection Standards (Inkret et al. 1995),
In July 1896, only one month after the discovery
of x rays, a severe case of x-ray-induced dermatitis
was published, and in 1902, the first dose limit of
about 10 rad per day (or 3000 rad per year), was recommended.
This limit was not based on any biological limit, but on the, then, limit of detection. But, it was a start. The link between radiation and cancer came about in 1903:
By 1903, animal studies had
shown that x rays could produce cancer and kill living
tissue and that the organs most vulnerable to radiation
damage were the skin, the blood-forming organs,
and the reproductive organs.
and specifically for the element radium (from Russ et al. 2006):
Radium was discovered in 1898 and its use in
medicine also spread very quickly. In the 1920s,
sickness and death in watch dial painters, who
ingested small amounts of radium in their work,
taught scientists and doctors that internal exposure
to radium could be harmful.
Around the same time, (from Inkert et al. 1995), standards were being formalised and developed particularly in reference to 'tolerance doses', with:
tolerance dose was "assumed to be a radiation dose to which the body can be subjected
without production of harmful effects.” Mutscheller presented his recommendation
in a paper entitled, “Physical Standards of Protection Against Roentgen
Ray Dangers,” which was published in 1925. Quite fortuitously, F. M. Sievert arrived
at about the same limits using a similar approach.
A Sievert is now a unit of ionising radiation dosage.
Leading to the first internationally recognised radiation safety guidelines being published in 1928, and the founding of the International Commission on Radiological Protection.