Do you have any research on that topic?
We know that when we are exposed to radio frequency waves, some of the transmitted energy is absorbed into our heads and bodies. As there is still no clear answer as to whether there are any possible health effects from non-ionising radiation, studies that are investigating this area are hugely important.
Results of the studies available to us right now indicate that we should be concerned and follow advices to minimize our exposure to radiation and at the same time there are studies that proved no correlation between cancer and exposure to RF.
We recommend to get familiar with important factors to be taken into consideration when drawing conclusions from studies in that field prepared by American Cancer Society:
“First, studies have not yet been able to follow people for very long periods of time. When tumors form after a known cancer-causing exposure, it often takes decades for them to develop. Because cell phones have been in widespread use for only about 20 years in most countries, it is not possible to rule out future health effects that have not yet appeared.
Second, cell phone usage is constantly changing. People are using their cell phones much more than they were even 10 years ago, and the phones themselves are very different from what was used in the past. This makes it hard to know if the results of studies looking at cell phone use in years past would still apply today.
Third, most of the studies published so far have focused on adults, rather than children. (One case-control study looking at children and teens did not find a significant link to brain tumors, but the small size of the study limited its power to detect modest risks.) Cell phone use is now widespread even among younger children. It is possible that if there are health effects, they might be more pronounced in children because their bodies might be more sensitive to RF energy. Another concern is that children’s lifetime exposure to the energy from cell phones will be greater than adults’, who started using them at a later age.
Finally, the measurement of cell phone use in most studies has been crude. Most have been case-control studies, which have relied on people’s memories about their past cell phone use. In these types of studies, it can be hard to interpret any possible link between cancer and an exposure. People with cancer are often looking for a possible reason for it, so they may sometimes (even subconsciously) recall their phone usage differently than people without cancer.
With these limitations in mind, it is important that the possible risk of cell phone exposure continue to be researched using strong study methods, especially with regard to use by children and longer-term use.” 
Example that was very evident for us however is regarding breast cancer.
There are number of cases when Multifocal Breast Cancer in Young Women was linked with Prolonged Contact between Their Breasts and Their Cellular Phones. You can read more here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3789302/
So if you know anyone that likes to carry their mobile phone in the bra, please share that information with her.
1. Cosmos- cohort study of mobile phone use and health [Online] http://www.ukcosmos.org/Science#PublicationsPanelScience(Accessed: 03.11.2018)
2. American Cancer society- cellular phones [Online] https://www.cancer.org/cancer/cancer-causes/radiation-exposure/cellular-phones.html(Accessed: 03.11.2018)