Watch what you put in that suitcase

It's one of the biggest changes in the way Americans will travel the skies in the post-9/11 era: Starting Wednesday, each piece of baggage dropped off at airport ticket counters or curbside stands will be screened for bombs.

Roughly 10 percent of these bags - or 274,000 pieces of luggage each day - will be pried open by federal agents and searched by hand.

The change may already be altering the culture of flying - for instance, by prompting Americans to avoid packing chocolate, fruitcake, or other foods that look suspiciously like explosives to high-tech scanners. Americans' tendency to overpack has suddenly gotten riskier, too: Screeners may struggle to reclose a bag, meaning it could be delayed and miss its flight.

Yet, in a sign of the security of the times, federal agents pawing through underwear or toiletry kits is the latest piece of privacy many Americans seem willing to give up for safety's sake. And money appears to be little hindrance: The price tag for 5,700 baggage-screening machines needed nationwide is $1.3 billion. That doesn't include huge setup costs for the devices, which may become obsolete in as little as two years.

In all, the change marks the beginning of a massive effort to guard against suitcase bombs like the one that brought down Pan Am flight 103 over Scotland in 1988. And it's just one piece of the effort to boost airline security in the wake of 9/11.

"I will pack differently," says Freeman Lin, a university researcher traveling through Boston's Logan Airport.

He cites cheese and fruitcake as new packing taboos and says it's "a hindrance" that some screening machines can, for instance, ruin rolls of camera film. But it's worth the price, he says. "We've got to try" to improve security - even if current efforts are relatively crude.

Indeed, one reason so many bags are to be hand-screened is that the $1 million van-sized scanners - similar to hospital CAT-scan machines - have false-positive rates of roughly 35 percent.

So because peanut butter has about the same density as some explosives, a jar of Skippy can trigger an alarm. Once it does, a federal agent looks at a video image of the bag. About half the time, the agent decides the item poses no risk.

Otherwise, the bag is diverted to a second station. There screeners use a $45,000 device that can find traces of explosive material. To get the most accurate reading, however, the inside of the bag must be swabbed. So about 60 percent of the bags at this station will be opened.

In all, about 10 percent of the 1 billion bags Americans check each year will be opened by some of the 22,000 new federal baggage screeners. To speed this process, the Transportation Security Administration (TSA) tells travelers not to lock their bags.

It will provide red cable-tie devices, which can be easily snipped off if needed, to protect luggage. If a bag is opened, a sheet of paper is put inside explaining the procedure - and a blue snip-off lock is installed.

Most airports will eventually put baggage-screening out of sight, so passengers won't be present if their bags are opened. But many travelers seem accepting of the new scrutiny.

"The loss of privacy is a major thing, but it's necessary," says Mary Harrild from Sharon, Mass., who was at Boston's airport recently. "There's a difference if someone's checking your bag without you knowing," she says. But now every traveler has been fully warned, so it's fair game, she adds.

Even privacy defenders don't have constitutional objections. "We believe these are justified administrative searches under the Fourth Amendment," says Barry Steinhardt of the American Civil Liberties Union in New York.

Complications come, however, if screeners find illegal items such as drugs. They're trained to report them to police - so they don't become virtual antidrug law-enforcement agents. But the reality may be more complicated: Screeners will have to decide whether to disrupt the flow of bags by taking time to call authorities.

With so many bags being opened, there's also the potential for theft. This month in Miami six airline baggage handlers were arrested for stealing from luggage. But the TSA hopes background checks and higher pay will limit the theft by screeners.

Still, while the airlines are liable for bag damage or stolen items up to $2,500 per passenger, the TSA is very vague about its responsibility for missing material.

"If we've gone through your bag, and you get to the hotel and something's not there, call us immediately," says spokesman Ed Martell. Issues will be handled case by case.

Finally there's the question of whether the process will actually prevent bombs from getting on planes.

Experts worry it's a one-dimensional system that, if breached, has little backup. Also, it's so labor-intensive that quality may fall off over time.

"There's a feel-good factor," says George Mason University aviation expert Ken Button. "But that becomes cumbersome after awhile, and the inspectors cease to be efficient." Yet clearly first steps are needed. "Right now we're crawling," says Mr. Lin, the Boston traveler. "Then we will walk, then run."