It is found to be useful and becomes widely used. People start noticing an increase in a symptom which they suspect might be due to the chemical or device being used. Safety assurances are given by manufacturers and government agencies. A search of the diverse sources of scientific studies reveals evidence of associations, cellular changes or animal effects, but there is no direct human evidence of effects. Early human studies often don't find evidence of effects, or some find a small but statistically insignificant rise in symptoms.
Cancers have latencies of decades for many adult cancers.
That is, it takes typically eight to thirty years for damaged cells to develop into full-blown cancers. Subsequently, occupational groups who have a distinctly higher chance of exposure are studied and found to have a higher incidence of the disease symptoms. The study is repeated and confirmed. The association between cell phones and brain tumour is following this pathway. Local people notice that friends or family, with no history of brain cancers, get brain tumours at relatively young ages.