Washington (CNN) -- To the list of instructions you hear at airport checkpoints, add this: "Put your palms forward, please."

The Transportation Security Administration soon will begin randomly swabbing passengers' hands at checkpoints and airport gates to test them for traces of explosives.

Previously, screeners swabbed some carry-on luggage and other objects as they searched for the needle in the security haystack -- components of terrorist bombs in an endless stream of luggage.

But after the Christmas Day attempted bombing of Northwest Flight 253 over Detroit, Michigan, the TSA began a program of swabbing passengers' hands, which could be contaminated by explosive materials, experts say. The TSA will greatly expand the swabbing in the coming weeks, the agency said.

"The point is to make sure that the air environment is a safe environment," Homeland Security Secretary Janet Napolitano told CNN. "We know that al Qaeda [and other] terrorists continue to think of aviation as a way to attack the United States. One way we keep it safe is by new technology [and] random use of different types of technology."

Security experts consulted by CNN said swabbing hands is a good move, and privacy advocates said they support the new swabbing protocols, provided the agency tests only for security-related objects and does not discriminate when it selects people to be tested.

It's a "very good idea," said security expert Tony Fainberg. TSA screeners currently swab luggage handles and parts of bags that are likely be contaminated by human hands, he said, and swabbing a person's hands increases the chances of finding explosive materials. "Looking at the hands means you will probably get a better dose," he said.

Under the new protocols, tests will be conducted at various locations -- including in checkpoint lines, during the screening process and at gates. Newer, more portable machines make it easier to conduct tests away from fixed locations such as the checkpoint.

The TSA has more than 7,000 explosive trace detection (ETD) machines and has purchased 400 additional units with $16 million in federal stimulus money. The president's fiscal 2011 budget calls for $60 million to purchase approximately 800 portable ETD machines.

Napolitano said the tests will not significantly increase wait times at airport checkpoints.

The American Civil Liberties Union has "always supported explosive detection as a good form of security that doesn't really invade privacy," said Jay Stanley, an attorney and privacy expert with the organization.

Stanley said the ACLU is chiefly concerned that the TSA does not discriminate when selecting people for enhanced screening -- something the agency said it does not do -- and that it treat people with dignity.

"We would not want to see it implemented in a discriminatory fashion, for example, in a disproportionate way against Muslims and Arabs or, for example, people with red hair or anything else. Security experts from across the spectrum will tell you that that's not just unfair and unjust and not the American way, it's also a terrible way to do security," Stanley said.

Swabbing also should not be used to test for nonsecurity-related contraband, such as drugs, he said. "Under the Constitution, searches in airports are only for the purpose of protecting the security of airline transportation; they are not general law enforcement stops. And so it wouldn't be permissible for the government to use these trace portal detectors to look for drugs," Stanley said.

The TSA said the machines test only for explosives. It declined to specify which explosives, citing security reasons.

Because some legal substances -- such as fertilizers and heart medicines -- can result in "false positives," Stanley said the ACLU also wants to ensure that people who test positive be treated respectfully.

"It's important that the government treat people who do show up as a positive -- fairly and with dignity -- and not parade them off in handcuffs and treat them as terrorists, but do rational things to investigate what the problem might be," he said.

But swabbing hands does not, by itself, raise civil liberty problems, Stanley said. "There's really not a big privacy interest at stake here," he said. "They are basically looking for particles of explosives, which is not something that people normally have."