The decline of the landline has a greater effect than just a few abandoned phone booths. (Sean Gallup/Getty Images)

New results released Tuesday by the Centers for Disease Control reveal that two of every five American households have ditched their landlines for cellphones -- and there are interesting differences between the health of households that still have landlines and those that are wireless-only.

The data are part of a preliminary study from the National Health Interview Survey, a part of the CDC's statistics department that regularly conducts tens of thousands of phone and in-person interviews to get a snapshot of Americans' health. In 2003, the agency decided to also look at how many households have landlines to better tailor its survey. Since then, it's become the most prominent government agency collecting the data.

Overall, 39.1 percent of adults are in wireless-only households; that number goes up to 47.1 percent when the figure also includes children. You may expect that young adults are primarily driving the trend toward households that only use cellphones -- something that has certainly been true in the past. But this year marked the first time that, by a slim majority, adults 35 and older make up the largest portion of households that rely solely on cellphones. In line with other studies looking at the demographics of technology use, Hispanic adults are also far more likely to be living in households that only had wireless phones.

Adults living in poverty are  also much more likely to only have cellphones; 56.2 of poor households have no landline service, as compared to 36.6 percent of higher-income adults.

Those trends pose some interesting problems for statisticians. The CDC, like political pollsters and social scientists, has had some trouble navigating the move from landlines to cellphones for their surveys. Reaching cellphone users is more difficult and more costly, said the study's lead author, Stephen J. Blumberg.

"People don't expect such calls on their cellphones; they consider it more of an intrusion of their privacy. So they're more reluctant to participate," he said.

But undercounting wireless users can skew crucial health survey information, such as how many American adults are diabetics, heavy smokers, or people who have a reliable place to find health care. And, as it turns out, the NHIS has found several statistically significant differences between wireless-only and landline homes.

Adults in wireless-only households are, for example, less likely to have received their flu shots and are more likely to have faced financial barriers to health care. They're also more likely to smoke and drink heavily. And those correlations stick even when researchers control for factors such as age, income level and home ownership status.

"This suggests to us that there's something about these people's personalities that may lead to health risk behaviors," Blumberg said.

That's not to suggest, he said, that going wireless-only is a risky behavior. But it does underscore how important it is to properly represent these populations, which are sometimes overlooked, in health and other surveys. And even among homes that still have plain old telephones , the growing trend away from landlines can still bias the picture that researchers can collect, because a lot of people who have landlines don't use them.

Many health surveys are doing a better job now of reaching these "wireless-mostly" and wireless-only households, Blumberg said, but undercounting them can lead to problems down the line.

"Public health programs are guided by such numbers," he said. "The public health community may be relying on inaccurate data in deciding which programs  to move forward with."