There's no more ubiquitous feature of modern life than smartphones. Convenience, necessity and plain fun make the devices a near requirement, but it all comes with a ... doubt: Is there a health risk to a device that's become almost a human appendage? The California Department of Public Health [has published] guidelines that come close to suggesting that there's uncertain harm. As people use the devices, more often and at an earlier age, health risks could eventually show up. A Kaiser Permanente study ... found a higher rate of miscarriage for those exposed to radiation emitted by cell phones, power lines and wireless networks. Health dangers indicated in other studies include brain tumors, lower sperm count and impacts on memory, learning and sleep. State health officials are flashing a yellow light. If people want to take steps to minimize risk, the agency has suggestions. On the list is parking a smartphone away from the bedside table at night, storing the device in a purse or backpack instead of a pocket during the day, and generally keeping the phone at a distance from the body. Ease back on phone use when the signal is weak. Such advice sounds easy to accept. But coming from a major state agency, the idea of preventive steps to decrease exposure is sending a message. There are hundreds of millions of phones in this country, and California's top health experts aren't convinced there's no human risk.