The Department of Homeland Security has been pushing a plan that if enacted would require all Americans submit to a facial-recognition scan when departing the country. This step would be a way to expand a 2004 biometric-tracking law meant to target foreigners.

According to the Associated Press, which first reported the plan on Wednesday, facial-scanning pilot programs are already underway at six American airports—Boston, Chicago, Houston, Atlanta, New York City, and Washington DC. More are set to expand next year.

In a recent privacy assessment, DHS noted that the "only way for an individual to ensure he or she is not subject to collection of biometric information when traveling internationally is to refrain from traveling."

In recent years, facial recognition has become more common amongst federal and local law enforcement: a 2016 Georgetown study found that half of adult Americans are already in such biometric databases.

"Americans expect when they fly overseas that their luggage is going to be looked into," Harrison Rudolph, a Georgetown legal fellow, told Ars. "What they don't expect is their face is going to be scanned. This is an expansion of a program that was never authorized for US citizens."

John Wagner, the Customs and Border Protection official in charge of the program, said that the agency will delete such scans within 14 days. But he also said that the agency may keep scans longer after it goes "through the appropriate privacy reviews and approvals."

In May 2017, Wagner and Michael Dougherty, a top CBP official, told a House subcommittee that Atlanta Hartsfield International Airport, the busiest airport in the world, has been testing biometric scans since last summer.

As they wrote:

US Citizens are not exempted from this process for two reasons: first, it is not feasible to require airlines to have two separate boarding processes for US citizens and non-US citizens, and second, to ensure US citizen travelers are the true bearer of the passport they are presenting for travel. If the photograph captured at boarding is matched to a US citizen passport, the photograph is discarded after a short period of time. In essence, for US citizens the document check has been transformed from a manual process by airline personnel or CBP officers into an automated process using a machine. It is important to note that CBP is committed to privacy and has engaged our privacy office at every step in the process to add biometrics to the departure process from the United States.

Alvaro Bedoya, a Georgetown law professor who led the 2016 study, said that DHS' argument does not hold water with him.

"When you [land at] the airport, you are directed into two lines; there's a line for US citizens and green-card holders, and there's everyone else," he told Ars, noting that foreigners are already scanned when they land in the US. "We've determined that it's OK to treat Americans differently on entry, why is it not OK to treat them differently on exit?"

A Senate committee is set to hold a hearing on this issue on Wednesday.

CBP did not immediately respond to Ars’ request for comment.