BALTIMORE - Body-scanning machines that show images of people underneath their clothing are being installed in 10 of the nation's busiest airports in one of the biggest public uses of security devices that reveal intimate body parts.

The Transportation Security Administration recently started using body scans on randomly chosen airline passengers in Los Angeles, Baltimore, Denver, Albuquerque and New York's Kennedy airport.

Airports in Dallas, Detroit, Las Vegas and Miami will be added this month. Reagan National Airport near Washington starts using a body scanner today. A total of 38 machines will be in use within weeks.

"It's the wave of the future," said James Schear, the TSA security director at Baltimore-Washington International Airport, where two body scanners are in use at one checkpoint.

Schear said the scanners could eventually replace metal detectors at the nation's 2,000 airport checkpoints and the pat-downs done on passengers who need extra screening. "We're just scratching the surface of what we can do with whole-body imaging," Schear said.

In Phoenix Sky Harbor International Airport, where scanners have been tested since last year as an alternative to pat-downs, 90 percent of passengers choose to be scanned, the TSA says.

"Most passengers don't think it's any big deal," Schear said. "They think it's a piece of security they're willing to do."

The TSA effort could encourage scanners' use in rail stations, arenas and office buildings, the American Civil Liberties Union said. "This may well set a precedent that others will follow," said Barry Steinhardt, head of the ACLU technology project.

Scanners are used in a few courthouses, jails and U.S. embassies, as well as overseas border crossings, military checkpoints and some foreign airports such as Amsterdam's Schiphol.

'The ultimate answer'

The scanners bounce harmless "millimeter waves" off passengers who are selected to stand inside a portal with arms raised after clearing the metal detector. A TSA screener in a nearby room views the black-and-white image and looks for objects on a screen that are shaded differently from the body. Finding a suspicious object, a screener radios a colleague at the checkpoint to search the passenger.

The TSA says it protects privacy by blurring passengers' faces and deleting images right after viewing. Yet the images are detailed, clearly showing a person's gender. "You can actually see the sweat on someone's back," Schear said.

The scanners aim to strengthen airport security by spotting plastic and ceramic weapons and explosives that evade metal detectors and are the biggest threat to aviation. Government audits have found that screeners miss a large number of weapons, bombs and bomb parts such as wires and timers that agents sneak through checkpoints.

"I'm delighted by this development," said Clark Kent Ervin, the former Homeland Security inspector general whose reports urged the use of body scanners. "This really is the ultimate answer to increasing screeners' ability to spot concealed weapons."

The scanners do a good job seeing under clothing but cannot see through plastic or rubber materials that resemble skin, said Peter Siegel, a senior scientist at the California Institute of Technology. "You probably could find very common materials that you could wrap around you that would effectively obscure things," Siegel said.

'You have to go along with it'

Passengers who went through a scanner at the Baltimore airport last week were intrigued, reassured and occasionally wary. The process took about 30 seconds on average.

Stepping into the 9-foot-tall glass booth, Eileen Reardon of Baltimore looked startled when an electronic glass door slid around the outside of the machine to create the image of her body. "Some of this stuff seems a little crazy," Reardon said, "but in this day and age, you have to go along with it."

Scott Shafer of Phoenix didn't mind a screener looking at him underneath his shorts and polo shirt from a nearby room. The door is kept shut and blocked with floor screens. "I don't know that person back there. I'll never seem them," Shafer said. "Everything personal is taken out of the equation."

Steinhardt of the ACLU said passengers would be alarmed if they saw the image of their body. "It all seems very clinical and non-threatening - you go through this portal and don't have any idea what's at the other end," he said.

Passengers scanned in Baltimore said they did not know what the scanner did and were not told why they were directed into the booth.

Magazine-sized signs are posted around the checkpoint explaining the scanners, but passengers said they did not notice them.

Darin Scott of Miami was annoyed by the process.

"If you don't ask questions, they don't tell you anything," Scott said. When he asked a screener technical questions about the scanner, "he could not answer," Scott said.

TSA spokeswoman Sterling Payne said the agency is studying passenger reaction and could "get more creative" about informing passengers. "If passengers have questions," she said, "they need to ask the questions."

Passengers can decline to go through a scanner, but they will face a pat-down.

Schear, the Baltimore security director, said only 4 percent of passengers decline.