WARNING: Holding a cellphone against your ear may be harmful to your health. The legal departments of cellphone manufacturers give warnings about it . 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 profit for the wireless carriers. The cellphone instructions-cum-wa rn ings were brought to my attention by Devra Davis, an epidemiologist who 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. Henry Lai, a research professor in a bioengineering department, began laboratory radiation studies in 1980 and found that rats exposed to radiofrequency radiation had damaged brain DNA. The unit of measurement for radiofrequency exposure is called the specific absorption rate, or SAR. The Federal Communications Commission mandates that the SAR produced by phones be no more than 1.6 watts per kilogram. One study listed by Mr.Lai found effects like loss of memory in rats exposed to SAR values in the range of 0.0006 to 0.06 watts per kilogram." I did not expect to see effects at low levels," he said. Ms.Davis recommends keeping a phone out of close proximity to the head or body, by using wired headsets or the phone's speaker. Children should text rather than call, she said, and pregnant women should keep phones away from the abdomen. It's advice that I find hard to put into practice myself. The comforting sight of everyone around me with phones pressed against their ears,just like me,makes the risk seem abstract.