G-men might come knocking on your door in coming weeks, but for a change, it's a good thing.

NHANES, or the National Health and Nutrition Examination Survey, is a little-known federal health survey that has long helped Americans change to healthier lifestyles.

And, for the first time since the 1960s, when NHANES began collecting health information, Lancaster County will play a role in shaping the statistical data distributed through the Centers for Disease Control and Prevention and other outlets.

Starting today, 13 field workers will visit randomly selected county households to gather preliminary data. Those households previously received letters requesting their participation.

Based on the data gathered, some households will be asked to participate in extensive medical testing, the survey's next phase, starting in mid-May. Those results will be combined with other data from elsewhere in the country to create a statistical picture of the nation's health.

NHANES began with a pool of 2,200 county addresses, but field workers probably will only need to visit about half that number, officials said.

Although most Americans aren't familiar with NHANES, it's the survey that resulted in the familiar pediatric growth charts displayed in doctors' offices across the country.

It's also the survey that showed Americans had too much lead in their blood. By the 1990s, after lead-free gasoline was phased in, elevated lead levels appeared in only 4 percent of Americans.

NHANES data also put cholesterol in the spotlight when it showed one-third of adults had elevated levels in the 1960s. Partly because of those statistics and collaborative efforts to reduce cholesterol, fewer than one in five Americans is estimated to have high cholesterol today.

The data also provide annual estimates on a range of health conditions, such as the number of Americans who are overweight, obese or have heart disease or diabetes.

"I like to call it 'the nation's report card on health,' " said NHANES senior study manager Nora L. Martinello said.

NHANES statistics are used by public health officials, legislators and physicians to develop health policies, design programs to prevent and treat ailments, and expand Americans' understanding of health issues.

"NHANES is very much like a health exam center on wheels that goes out into actual communities to get data on real Americans," CDC Director Julie L. Gerberding said in a media release. "It truly is a unique resource for health information in this country, and without it we wouldn't have data on a number of important health conditions."

Each person providing data for the survey represents 25,000 to 60,000 Americans with similar profiles. The household addresses are chosen randomly from census data to obtain a statistical sampling.

Testing, for those individuals who are selected and agree to participate, takes place in a high-tech mobile exam center. It includes a physician examination, dietary interview, body measurement and a blood panel of 120 tests.

Depending on participants' ages, some may also receive hearing and vision tests, screening for degenerative eye disease and a skeletal health screening.

No medical treatment is included, but respondents receive a medical report with an explanation from the survey medical staff.

"For adults, they get about $4,500 of medical tests for free that we encourage them to share with their doctor," Martinello said. "Children who participate often get community service credits or extra credit in school."

Test participants also receive $100 compensation and reimbursement for travel expenses.

"So there is a time commitment to all this, but there are other benefits to participating too," Martinello said.

Lancaster County is among 15 participating counties in Pennsylvania, North Carolina and Virginia. In all, about 6,000 randomly selected residents will have the chance to participate in this year's data collection.

Martinello said typically, 80 percent of those asked to participate agree.

Field workers will have official CDC photo identification when they arrive at homes.

"We're the new kids in town, and not everybody is willing to open the door with all the problems there are in the world," Martinello said. "We're excited to be here. Every community has its challenges, but we meet some of the most incredible people in the world."

For more information about NHANES, visit cdc.gov/nchs/nhanes.htm.

E-mail: slindt@lnpnews.com