Since a thesis statement is wholly dependent upon the claim or assertion you want to prove through evidence and argument, a good thesis statement depends upon a clear understanding of what you think, after detailed analysis and evaluation, your answer is to the question being asked.
In this case, the question is being asked for you (as opposed to being developed by you): Are cell phones, or are they not, dangerous? Your answer to this question will grow out of your research and out of your analysis and evaluation of that research.
To analyze your research, you will want to examine the scientific fact behind the two perspectives: (1) cell phones are dangerous and (2) cell phones are not dangerous. In your analysis, mere opinions on the question will not be relevant. What will be relevant is factually supported data and expert opinion, such as, for example, that of scientists and research physicians who have studied the dynamics and effects of cell phones.
To evaluate your research, you will want to critically assess the factual evidence from experts that you have amassed and find flaws, if there are any, in the scientific studies conducted, the logic presented and the intrusion of unsupported opinion into what ought to be scientifically or otherwise factually supported data.
Two other elements to evaluate are the dates of the studies or expert opinions and how various governments have responded to the cell phone question, for example, in 2013 Australia issued advisories to parents to restrict exposure and limit use of cell phones (mobile phones) for their children (similar advisories went out in the UK as early as 2005).
As we know, cell phone companies develop and introduce new cell phone models quite frequently amid much sociocultural excitement. Since some of these companies incorporate the latest research into their designs to make their cell phones safer--though some companies opt for cheaper costs and more dangerous materials, such as recycled black plastics from China--it is important to evaluate when the data or expert opinions used in your research were publicized: Is the data current or is it out-dated? If it is outdated, there may be a flaw in the data, reasoning or opinion if used as support for either side of the question.
Having analyzed and evaluated your research, you are now in a position to provide your answer in a thesis statement. A thesis statement is a one- to two-sentence statement of the claim you present (your answer to the question), and it expresses in succinct language that you have thoroughly explored the question and that your answer will be defended by specific factual data. To quote the Writing Center of the University of Washington:
[A] thesis conveys to the reader that the claim being offered has been thoroughly explored and is defendable by evidence. It answers the "what" question (what is [your] argument?) and it gives the reader a clue as to the "why" question (why is this argument the most persuasive [answer]?). (Writing Center, University of Washington)
Your thesis statement will be a good one when you have gathered data, analyzed it, evaluated it and come to your own factually grounded answer to the question of whether cell phones are or are not dangerous. Other Universities also offer helpful information on thesis statement writing:
The University of North Carolina, Chapel Hill
Start by making a non-controversial statement, such as "It is a known fact that all new technology meets with opposition, sometimes from conservatives who want "the old ways" preserved for their own sake; sometimes by "Luddites" who lament the complications of "refinements, improvements, streamlinings, etc." of technological change; sometimes by "fatalists" who see every change as a sign of some impending catastrophe predicted by Nostrodamus or someone. Then introduce your topic: "Cell phones have met with controversy on two fronts. First, some fear a medical complication, perhaps caused by some sort of electronic interference of brain waves (possibly even causing cancer); others find a sociological negative consequence (loss of one-to-one human contact, or distancing from reality, or loss of communication abilities on the paralinguistic level -- facial expressions, body "language," voice inflections, etc.)
With this start, you can build your argument, in either direction or in both, guiding the reader through your evidence and rhetoric using the established methods of essay writing.