We've heard a lot about the potentially negative health effects of cell phones — that using them may or may not raise the risk of brain tumors, cause head pain and increase our chances of getting run over — and now it turns out that their ubiquity is causing problems for government scientists whose mission is to track the nation's wellbeing.

You'd never guess, but the National Center for Health Statistics (NCHS), the statistical clearinghouse for the Centers for Disease Control and Prevention (CDC), is the most widely cited source of wireless usage in the U.S. Why? CDC traditionally conducts its national health surveys by randomly dialing landline telephone numbers. But six years ago it realized that was becoming a problem, because people were dropping their landline service in droves. How big a problem? In an effort to find out, the agency began measuring how many U.S. homes use only cell phones.

In a report released today, the NCHS says that the number of wireless-only households rose from 3.5 percent in 2003 to 17.5 percent in the first half of last year. Actual numbers of those households aren't available, but in 2003, 6 million adults and 2 million kids were wireless only, compared to 36 million adults and 12 million children in the first half of last year. By now, more than 20 percent of households are wireless only, NCHS senior scientist Stephen Blumberg tells ScientificAmerican.com. In two states, Oklahoma and Utah, more than a quarter of households keep only mobile phones.

Why is this an issue? Wireless-only folks differ from those who still keep a landline, Blumberg explains. Not only do they skew younger, but they behave differently, too. Introduce those differences into health surveys, he says, and the results are biased.

Half of wireless-only users are under 30 years of age, but the greatest predictor of whether they rely solely on a cell phone is if they own or rent: renters are four times as likely to be wireless-only as are people who own their homes, Blumberg says. On top of that, they're more likely to live in metropolitan versus rural areas, and in or near poverty.

That said, "even if we control for age, renting, income, education, for family structure, where they live, race — minority individuals are more likely to be wireless only — we still see health differences," Blumberg says. "Wireless-only [users] are more likely to binge drink, they are more likely to smoke, they're more likely to be uninsured so therefore, if these wireless-only adults are excluded from these surveys, they may underestimate binge drinking, smoking and the prevalence of uninsured adults. That's why the CDC is concerned."

Three CDC surveys are affected: the Behavioral Risk Factor Surveillance System (which tracks rates of smoking, drinking, unprotected sex and nutrition habits), the National Survey of Children with Special Health Care Needs (which measures the extent to which sick kids need particular services, whether they have unmet needs and how their health affects their family's financial and employment status) and the National Immunization Survey (which tracks immunization rates in children and teens).

So what's CDC doing about this? It’s trying to do some surveying on cell phones, but, perhaps unsurprisingly, people who receive its calls aren’t always thrilled. "People don’t expect to get a survey call on their cell," Blumberg says. "Some still pay by the minute and are concerned they're using up the minutes. Many think of it as a personal device that only friends and family call on. All of that contributes to making interviewing more complicated."

CDC is also trying to survey by address: if it can identify a phone number for a given address, surveyors call, and if not, the agency sends a letter and asks the resident to participate by calling an 800 number, sending a postcard back with their phone number or by doing things the old-fashioned way: answering a questionnaire through snail mail.

Image © iStockphoto/Tomasz Pietryszek