By Ted Gansler, MD, MPH
Like other contributors to the Expert Voices blogs, I am occasionally asked to reply to questions from journalists about various cancer-related topics. The most recent question I received is whether it is true that women who carry a cell phone in their bras are at increased risk for developing breast cancer.
This kind of question is surprisingly difficult to answer. It's relatively easy to write about things that are known to cause cancer. It's more difficult to be confident that something does not cause cancer, but one can still provide some guidance if there have been at least a few carefully-conducted epidemiologic studies with negative results. The most challenging requests we receive are often about questions that researchers have not addressed by scientific studies of humans populations. This is one such question.
Cause or Coincidence?
There are a few known instances of breast cancer in young women who have kept cell phones in their bras. (Even when cell phones are not being used for conversation or texting, if they are on then they still periodically emit low energy electromagnetic signals to stay in touch with nearby cell towers.) Because breast cancer is an uncommon and tragic occurrence among young women, these cases have received significant attention on television and on the Internet. But it is the nature of these media to emphasize unusual events, so of course we don't hear much about the millions of women and men who carry phones close to various organs and still remain healthy.
When we say that something is a human carcinogen, we mean that a greater percentage of people exposed to that factor develop cancer, in comparison with people who are not exposed. Because there have not been any scientific studies of cell phones in bras and breast cancer, we do not know these percentages.
Although the average age at diagnosis of breast cancer is over 60 years, about 13,000 women younger than 40 years are diagnosed with breast cancer each year in the US. If even a few percent of all women younger than age 40 carried a phone in their bras, and even if there is absolutely no cause and effect link - in other words, if the phones do not increase breast cancer risk -- we would still expect there to be hundreds of such women who develop breast cancer every year just by chance, not because the cell phone caused it.
Some of the television and Internet stories about young women who developed breast cancer and who carried cells phone in their bras emphasized that these women didn't have inherited BRCA mutations. These DNA changes account for some families with multiple members who develop breast cancer (and some other forms of cancer, too).
Overall, BRCA mutations are responsible for about 5%-10% of breast cancers. Even though this percentage is higher in younger patients, the majority of young women with breast cancer do not have any inherited changes in these genes. So, based on estimates of how many young women without inherited BRCA mutations develop breast cancer and a guess about how many young women carry phones in their bras, it would not be surprising to find hundreds of women in the US with both these characteristics based entirely on coincidence. These estimates don't tell us that such a link is impossible, they just tell us there is not any convincing evidence that it is present so far.
Answers Aren't Always Easy to Find
Among scientists who study associations of risk factors and diseases, individual case reports are considered one of the least convincing types of evidence, because of the possibility that they represent a coincidence rather than cause and effect. Individual reports of uncommon medical events often prompt researchers to conduct systematic studies that can provide more useful information, but that takes time.
Even when research is done, the results may not be clear cut. For instance, studies of cell phones and brain cancer risk have had conflicting results; most of these studies did not suggest any increased risk of brain tumors from cell phone use, but some did suggest that there might be. The International Agency for Research on Cancer (which is part of the World Health Organization) classifies cell phones as a "possible" (but not "known" or even "probable") cause of brain tumors. Several US agencies conclude that evidence is inconsistent and inconclusive, and that more research is needed.
Meanwhile, it is up to individuals to decide what to do about it. Some people will avoid anything that has not been proven to be safe (and, remember that it's much harder to prove something is safe than to prove that it's dangerous). Other people will do things until they are conclusively proven to be dangerous (or, in the case of smokers, drivers who text, etc., even after they have been proven as dangerous), and the vast majority of us are somewhere in between.
I don't blame the patients described in these stories (or their families, friends, and even their doctors) for proposing a cause and effect relation - that carrying cell phones in their bras caused their breast cancer. It is an essential part of human nature to attempt to make sense out of such tragic events. Conclusions based on single cases or a few cases are sometimes true and sometimes are not. That is why epidemiologic research uses sophisticated statistical methods to analyze data from large groups of people. These studies provide important information that can tell us how to avoid things (like smoking) that increase risk and what we can do (like physical activity) to decrease risk of various diseases.
Act on Proven Risks
Although I am not convinced by these individual reports of any link (and I think most cancer researchers agree with this conclusion), these news stories do raise that possibility. If even the possibility of this link concerns you, then find somewhere else to store your phone. And, if you are concerned about breast cancer and other forms of cancer, please learn more about proven ways to reduce your risk of developing and dying from these diseases:
- Avoid weight gain and obesity as an adult
- Be physically active
- Limit alcohol intake
You can learn more about these and other risk-reduction measures on our page Can Breast Cancer be Prevented?.
Although these steps can help reduce your odds of developing breast cancer, they cannot eliminate that risk, so it's important to also follow recommendations for mammography and clinical breast exams, in order to find cancer earlier, when it can be most effectively treated.
Now that you have read my rather long answer (including several subtle points that I think are important to understand) to a short question, I don't mind adding a summary.
- Reports of a few young women who developed breast cancer and who had carried a cell phone in their bra do not prove that the cancers were caused by the phones. These cases may all be a coincidence. But, it is possible that some may be a coincidence and some may be linked. Nobody knows, because no systematic studies have been done.
- In the context of this uncertainty, what should you do? That depends on how worried you are about this unproven-but-not-disproven risk, and how much you would be inconvenienced by finding another place to keep your phone.
Finally, in case you're curious... Where do I keep my cell phone? In my left pants pocket; this consistency helps me remember where it is. Where does my wife keep her phone when she is wearing an outfit without pockets and not carrying a purse? In my right pants pocket.
Dr. Gansler is director of medical content for the American Cancer Society.