将手机贴在耳边或者塞在口袋里可能损害健康。在消费者购置新机后，常对手机厂商印在不起眼位置的小字警告不以为然。例如，苹果公司提醒称iPhone手机距离人体至少要有1.5厘米，黑莓手机生产商RIM公司建议最小距离为2.5厘米。如果手机使用可导致健康问题，后果将非常严重。

WARNING: Holding a cellphone against your ear or storing it in your pocket may be hazardous to your health.

This paraphrases a warning that cellphone manufacturers include in the small print that is often tossed aside when a new phone is purchased. Apple, for example, doesn't want iPhones to come closer to you than 1.5 centimeters; Research In Motion, BlackBerry's manufacturer, recommends 2.5 centimeters.

If health issues arise from cellphone use, the implications are huge. Voice calls - Americans chat on cellphones 2.26 trillion minutes annually - generate $109 billion for the wireless carriers.

Devra Davis, an epidemiologist who has worked for the University of Pittsburgh, has published a book about cellphone radiation, "Disconnect." The book surveys scientific research and concludes the question is not settled.

Brain cancer is a concern that Ms. Davis examines. Over all, there has not been an 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 low-energy radiation could damage cells 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 five centimeters 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 studies have yet been completed on cellphone radiation and children, she says.

Henry Lai, a research professor in the bioengineering department at the University of Washington, began laboratory radiation studies in 1980 and found that rats exposed to radiofrequency radiation had damaged DNA in their brains.

Ms. Davis recommends 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.