It threatens the U.S. government's national health survey, the Behavioral Risk Factor Surveillance System, which uses the telephone to interview a nationwide sample of nearly a half-million adults to measure health habits, the officials say.

Interviewers used to call only conventional telephones because nearly 98 percent of households had them.

But that's not true anymore, The Washington Post reports.

The National Center for Health Statistics says in the first half of last year, 16 percent of U.S. adults lived in households that had only cell phones -- up from 7 percent in 2005 -- and the number is rising rapidly.

The BRFSS will include cell phone numbers in every state this year, with a goal of having 10 percent of interviews done that way.

But some cell phone users are reluctant to talk at length to a surveyor because they have to pay for incoming calls, the Post says.

Consequently, it takes about nine calls to working cell phones to get one completed survey, compared with five calls to working land-line numbers, the Pew Research Center for the People and the Press says.