ST. LOUIS — When the freezing water reached her waist, numbing her legs and causing her feet to shake uncontrollably, Brenna Garnett found what she needed. She had braked hard to avoid an animal, hit a tree, and skidded down a steep embankment into a creek. She could hear traffic above her, but the wrecked car - her parents' car - wasn't visible from the road and no one heard her cries. Looking around for something to help, the high school senior suddenly saw her mother's cellular telephone floating on the surface of the water. Although wet and deep in a ravine on the back roads near her southern Illinois home, the phone worked. Within minutes of dialing 911 operator, emergency personnel rescued her. She experienced only scratches and bruises. Brenna believes in the cell phone. "If you're on the highway or on the road, it's a lifesaver," she says. In thousands of ways, wireless technology is transforming our lives. The changes are subtle rather than dramatic. But as more people hook up to the technology, the changes are accelerating. They're also paradoxical. The same technology that saves lives also distracts drivers and causes accidents. The wireless phone call that closes an important deal may interrupt a family's meal at a restaurant and annoy nearby patrons. Undoubtedly, the technology makes us more efficient. It also cuts into our private time. Wireless technology encompasses many things: pagers, newfangled radios, and computer modems. But its most frequent incarnation - and the virtual poster boy for all its benefits and frustrations - is the wireless telephone. The use of these phones is growing so quickly that it's not uncommon to hear them ring in airplane cabins and supermarket aisles. By one estimate, half a billion people in the world will have a wireless phone by 2002 - more than double today's figure. Wireless replacing wired phones In the United States, 1 in 4 Americans totes one. And the figures are even higher for some other countries, such as Finland and Israel, where costs are so similar to the price of owning a traditional wired phone that consumers are rushing to go wireless. In some third-world countries, where traditional phone service is poor or nonexistent, it's often cheaper to build a cellular system than to bury cable for a traditional system. "The [wireless] technology has progressed to the point that its actually cheap to provide basic telephone service," says Alex Hills, vice provost and chief information officer for Carnegie Mellon University in Pittsburgh. "The mobility is frosting on the cake." Their popularity is so high, it's hard to imagine wireless technology has a downside. But critics say it does - physically and emotionally. "I think it's the wrong direction for us to be going in as a society," says Peter Crabb, psychology professor at Pennsylvania State University in Abington. "Cell phones are encouraging people to lose their self-control." Safety is the technology's biggest bugaboo. Wireless phones undoubtedly save lives. Every day, cell-phone users place an average of 98,000 calls to 911 or other emergency numbers, according to the Cellular Telecommunications Industry Association (CTIA), based in Washington, D.C. That's up 18 percent from the total a year and a half ago. Law-enforcement officials generally support mobile phones because users immediately report accidents, crimes, and instances of drunk driving. Driving and car talk is dangerous Increasingly, however, police say mobile phones also cause accidents. "Although there is a serious under-reporting bias in the data, there are trends which show that cellular telephone use is a growing factor in crashes," the National Highway Traffic Safety Administration concluded in a report last year. Surprisingly, the worst distraction was not dialing but talking, the report found. "Driver inattention is the most frequently cited pre-crash condition for drivers who use cellular telephones." "It can be a distraction," acknowledges Tim Ayers, a CTIA spokesman. "Once you get behind the wheel, your first job is to drive, not call." Although it resists laws that would restrict calling in moving vehicles, the industry is making an effort to encourage safe driving. Another problem: What to do with all those ugly phone towers that dot the countryside? Enterprising wireless companies have hidden them in silos and church steeples. Near his home in eastern Pennsylvania, Dr. Crabb has come across a cell-phone tower disguised as a tree. "To me that is an insult," he says. "It looks like one of those aluminum Christmas trees you get.... The more people want to use cell phones and mobile phones, the more they are contributing to the damage of the environment." Some observers also point to the technology's damage to the user's emotional landscape. "It's what it does to your time," says Paul Saffo, a cell-phone user and director of the Institute for the Future in Menlo Park, Calif. "We're becoming steadily more interrupt driven: 'Oh, I'd better clear out those voice-mail messages.' We're ending up dividing our lives into six-minute chunks." But quick bursts of communication often yield big benefits. "Timing is everything," says Lisa-Joy Zgorski, communications manager for The Century Foundation, a Washington think tank. Recently, while dining with her boss, she was able to schedule him for an important radio interview. The economic benefits of mobile communication could be huge. Business people can use their commuting time to extend their workday. Executives available anytime can close important deals. "On balance, it will be constructive because of the economic impact and the fostering of ties," especially in the developing world, says Herschel Shosteck, president of his own telecommunications consulting firm in Wheaton, Md. "That will be an unrecognized contributor to the growth of developing countries." But some people misuse the technology. They whip out a phone at an inappropriate moment and talk too loudly. Miss Manners has weighed in on the subject (it's a no-no at the dinner table, during a concert, or at a religious service). Then there are the show-offs. About a year ago, dining alone at a hip Los Angeles restaurant, Ms. Zgorski watched a man talking on his phone at a nearby table. "He looked so natural," she recalls. Then his phone rang - and Zgorski realized he'd been trying to impress her with an imaginary conversation. "He looked up and I gave him a little wave," she remembers. "He turned all sorts of colors." Keeping the private private "We've gotten into some very strange territory," says Crabb of Pennsylvania State University. Phone conversations are supposed to be private. But "now we have private behavior intruding on public places.... It's simply not healthy. We've got to keep private and public separate." The situation is sometimes worse overseas. "Go to Israel or China and nobody has any compunction about picking up the cell phone in the middle of a [face-to-face] conversation," says Mr. Saffo of the Institute for the Future. "Each society is making its own collective decision" about the social limits of the technology. Technology may solve some of these problems. Saffo never gives his phone number out, relying instead on a computerized telephone assistant to screen calls and forward the important ones. And wireless messaging is also taking off, which could cut down on cellular phone calls. Carnegie Mellon University, for example, is building a wireless system to give everyone on campus mobile access to the Internet. Using a computer that fits in the palm of their hand, students will be able to send e-mail on the go. Professors could deliver worksheets to students in class using electronics rather than paper. Already, some faculty are carrying around wirelessly connected laptop computers. "It's really handy to be able to look something up during a meeting," Mr. Hills says. But "I've been in meetings where people are tapping away, seemingly taking notes about the meeting but really writing e-mail or sending messages back and forth with comments about the speaker." "At some point people have to control behavior," he says.