Smartphone and souvenir bag in hand, Rob Dockerty stood on Wall Street in New York City's lower Manhattan Wednesday, facing south under light rain. “We arrived Sunday,” the 23-year-old English tourist said as he snapped a photo of the New York Stock Exchange and the nearby towering Christmas tree, with his mom and dad in tow. “We’ve been everywhere: The Empire State Building, the Statue of Liberty, the World Trade Center, the 9/11 memorial.”

Every month, millions of foreign nationals like the Dockertys come to the U.S. from a set of mostly European countries under an initiative that does not require visas. That visa waiver waiver program is now under scrutiny and was recently flagged by both Democrats and Republicans as the latest domestic security concern, following last month's Paris terror attacks. But tourism officials and domestic security experts warn lawmakers shouldn’t get too carried away with potential reforms to the program if they want to avoid a repeat of the decade after the Sept. 11 terror attacks, when the now $1.5 trillion American tourism industry took a dip that cost the U.S economy $600 billion and nearly 500,000 jobs.

Dockerty and his parents visited sites that cost at least $30 each, and though he didn't mention the rate he and his family were paying, the hotel in Times Square where they were staying can cost as much as several hundred dollars a night.

"Between 2000 and 2009, we had a significant level of growth in international global travel but that pace of growth did not keep up in the United States,” said Patricia Rojas-Ungar, the vice president of government relations for the U.S. Travel Association, which lobbies on behalf of the American tourism industry. “So, we consider that time the lost decade."

The Electronic System for Travel Authorization (ESTA) regulates which citizens from 38 preapproved countries can legally visit the U.S. for up to 90 days without applying for a tourist visa. Those nations and the U.S. have agreed to a certain level of mutual data sharing about passengers moving between the countries to ensure this quick transfer is safe. The visa waiver program allows quick travel and impromptu trips and helps bring $2.5 billion into the U.S. economy every day to support roughly one in nine American jobs. International tourism to the United States has since rebounded.

Following the Nov. 13 terror attacks in Paris, American lawmakers have been concerned a similar attack launched by Islamic militants with Western passports could take place on U.S. soil, prompting them to re-examine if there is adequate scrutiny of people entering the country for both short and long stays. House Republicans and Senate Democrats have called for changes to ESTA to ensure foreign travelers entering the United States are not terrorists. The measures lawmakers have discussed range from ending the program entirely and requiring all tourists to apply for a visa to implementing more biometric screening on travelers, including checking physical characteristics such as fingerprints.

“The Paris terror attacks were committed by French and Belgian nationals, which means it would have been possible for them to board a plane to this country using the visa waiver program,” Sen. Diane Feinstein, D-Calif., said in a statement released to announce a bill she had introduced to reform the system. “As growing numbers of foreign nationals travel to the Middle East to train and fight, and with 45 million lost and stolen passports on the black market, we must do all we can to secure the program.”

Visitors coming through ESTA are required to provide passport information online at least 72 hours before they board their flight to the U.S., and those passports must have electronic biometric information embedded into the travel document. U.S. Customs and Border Patrol then has that time before the flight embarks to check the data against criminal databases and terrorist watch groups and lists.

“There is the approach that some have suggested and that is shutting down the visa waiver program and having everybody who comes to the United States be interviewed and more thoroughly vetted,” said Rey Koslowski, an associate professor of political science at the State University of New York at Albany and an expert on homeland security issues. “That was something that was considered after 9/11 but then when members of Congress looked at how much it might cost to end the program they decided not to do it.”

That 2002 Government Accountability Office report determined the impact of eliminating the program would not be cheap. Initial costs to the State Department to boost personnel to handle the increased number of visa applications and the collection of further biometric data were estimated to run $739 million to $1.28 billion and then run $522 million to $810 million annually thereafter.

The United States refocused ESTA in the aftermath of the 9/11 terrorist attacks to respond to the growing threat of global terror. All 19 of the hijackers involved were in the U.S. on visas that required an in-person screening of applicants. Instead of the original focus of the visa waiver program, which looked to ensure that people didn’t stay beyond their allotted time period to work in the country, the U.S. became increasingly concerned with making sure homeland security threats were avoided following 9/11. The program has since required all new visas contain microchips with biometric data for travelers to avoid forgeries. Lawmakers are concerned those measures won't be enough to address the growing threat posed by the Islamic State group and individuals inspired by the terror organization.

There are always more security measures that can be taken to make things safer, experts in national security said. For example, tracking the behavior of individuals to ensure countries are able to communicate who is entering and leaving dangerous zones like Syria, where they could receive extremist training and then return with Western passports, could help improve security. However, every decision should be weighed against its effect on the economy or international relations, security experts and tourism officials said.

“The waiver process is put in for a reason. It’s basically: 'We’re a global economy.' If you’re a company that’s in Germany, for example, and you need to have somebody come over here and look at a site or negotiate a conference, you want to be able to get in and do that relatively easily,” said Rick Mathews, the director of the National Center for Security and Preparedness at State University New York Albany. “Business needs to be able to move quickly. Just like a tourist, you may not know four or five months in advance on exactly what dates you’re coming. You might, but some people may want to take a quick trip over to see something.”

Dockerty and his family weren’t on an impromptu trip, but they said the level of security at the airport felt adequate. The process and the security checks he experienced were quick and orderly, but also thorough, he said. Requiring tourists to apply for something like a visa would be a nuisance, he said.

“It'd probably have been such a pain,” he said. “I thought [security] was good enough. It made me feel safe.”