President Donald Trump’s promises to renegotiate trade deals and restrict immigration in order to put American workers first has become a point of concern for some along the northern border: Canadian nurses and Detroit hospitals.

Henry Ford Health System, with hospitals in the Detroit area, on March 16 said Canadian registered nurses with specialized training had been denied entry into the United States and might lose their ability to work in the country due to changes under Trump’s administration, the Detroit Free Press reported.

While the issue with Henry Ford nurses was ultimately resolved in favor of the nurses, it raises important issues about the North American Free Trade Agreement (NAFTA), which allows Mexicans and Canadians to come work in the United States.

Trump promised to rework NAFTA, calling it the "worst trade deal ever." The case of the Canadian nurses shows it could end up having real-world repercussions.

NAFTA professional visa

Under NAFTA, citizens (not permanent residents) of Canada and Mexico may be admitted into the United States to work for U.S. or foreign employers.

NAFTA professionals must meet specific qualifications, such as working a prearranged full-time or part-time job and having their profession specifically listed in NAFTA. Among occupations included: registered nurse, lawyer and economist.

While Mexicans must have a NAFTA Professional (TN, which stands for Treaty NAFTA) visa in order to come to the United States, Canadians generally do not need one, but are admitted under a TN nonimmigrant status.

About 30,000 to 40,000 Canadians are in the U.S working with a TN nonimmigrant classification, according to CBC News.

U.S. Customs and Border Protection said the agency had not changed its policies that would affect NAFTA professionals, but it has seen issues with travelers presenting improper or insufficient paperwork in order to be granted a TN nonimmigrant classification.

"Each application for TN status is evaluated by the inspecting officer, and the decision is made on the totality of the evidence provided," said CBP spokesman Kristoffer Grogan. "Every application for TN status is a separate inspection, and the decision to approve or deny is based on the merits of that individual case."

Canadian news outlet Windsor Star quoted Grogan in a March 17 article saying Customs and Border Protection had "done further review" and deemed that specialized nurses fell in the registered nurse category listed in NAFTA.

If NAFTA is renegotiated, there is broad concern over the future of NAFTA professional visas, said Laurie Tannous, an immigration and trade lawyer and special advisor to the Cross-Border Institute at the University of Windsor, in Ontario.

Though the matter with Canadian specialized nurses has been resolved, "we are expecting that other grey areas could be affected going forward," Tannous said.

Canada and Mexico would have to be part in any sort of NAFTA reconfiguration. If Trump decides to pull the United States out of the deal, he would have to give both nations a six-month notice in writing.

Any major change to NAFTA would take time, said Daniel Costa, director of immigration law and policy research at Economic Policy Institute.

In the meantime, Trump’s administration through new regulations and policy guidelines "could do some tinkering around the edges, which could have an impact," Costa said. That includes modifying the requirements for TN applications, restricting the interpretation of who qualifies for eligible occupations, and giving border protection officers new instructions regarding application requirements, he said.

Trump said he would deal with NAFTA’s renegotiation in his first 100 days in office.