WARNING: Holding a cellphone against your ear may be hazardous to your health. So may stuffing it in a pocket against your body.
I'm paraphrasing here. But the legal departments of cellphone manufacturers slip a warning about holding the phone against your head or body into the fine print of the little slip that you toss aside when unpacking your phone. Apple, for example, doesn't want iPhones to come closer than 5/8 of an inch; Research In Motion, BlackBerry's manufacturer, is still more cautious: keep a distance of about an inch.
The warnings may be missed by an awful lot of customers. The United States has 292 million wireless numbers in use, approaching one for every adult and child, according to C.T.I.A.-The Wireless Association, the cellphone industry's primary trade group. It says that as of June, about a quarter of domestic households were wireless-only.
If health issues arise from ordinary use of this hardware, it would affect not just many customers but also a huge industry. Our voice calls -- we chat on our cellphones 2.26 trillion minutes annually, according to the C.T.I.A. -- generate $109 billion for the wireless carriers.
The cellphone instructions-cum-warnings were brought to my attention by Devra Davis, an epidemiologist who has worked for the University of Pittsburgh and has published a book about cellphone radiation, "Disconnect." I had assumed that radiation specialists had long ago established that worries about low-energy radiation were unfounded. Her book, however, surveys the scientific investigations and concludes that the question is not yet settled.
Brain cancer is a concern that Ms. Davis takes up. Over all, there has not been a general increase in its incidence since cellphones arrived. But the average masks an increase in brain cancer in the 20-to-29 age group and a drop for the older population.
"Most cancers have multiple causes," she says, but she points to laboratory research that suggests mechanisms by which low-energy radiation could damage cells in ways that could possibly lead to cancer.
Children are more vulnerable to radiation than adults, Ms. Davis and other scientists point out. Radiation that penetrates only two inches into the brain of an adult will reach much deeper into the brains of children because their skulls are thinner and their brains contain more absorptive fluid. No field studies have been completed to date on cellphone radiation and children, she says.