Canadian nurses working at Michigan hospitals were shocked last week when border security officers stopped them from entering the U.S. because of changes to their working visas under new immigration policies.

Marc Topoleski, an immigration lawyer for Henry Ford Hospital in Detroit, said the changes at the border stem from new U.S. immigration policies, but officials from the Cross Border Institute at the University of Windsor disagree.

According to Laurie Tannous, a special adviser to the institute, the changes come from border security enforcing existing NAFTA regulations that have not been enforced before.

"There is no NAFTA or legal challenge to what they are doing," she said. There were far fewer specialized nurses needed in the U.S. when NAFTA rules were written, Tannous said.

Specialized Canadian nurses stress over rules preventing them from working in U.S. 0:28

Last week, a new Canadian hire at Henry Ford Hospital tried to go to work, but was turned away at the Windsor-Detroit border.

She was told advanced practice nurses and nurse anesthetists no longer qualify for the working visas because of policy changes under U.S. President Donald Trump.

Only advanced practice nurses and nurse anesthetists are being rejected. All Canadian nurses working in the U.S. have non-immigrant NAFTA professional (TN) visas.

Henry Ford Hospital alone has hundreds of Canadians on staff, with about 25 advanced practice nurses or nurse anesthetists with TN visas.

The Henry Ford Hospital in Detroit, which has hundreds of Canadians on staff, is concerned about Canadian nurses being unable to renew their working visas. (David Coates/Detroit News via Associated Press)

Here's a look at the visa that no longer lets them in the U.S., and the visa they're now being told to apply for.

What is a TN visa?

An estimated 30,000 to 40,000 Canadians work in the U.S. under the non-immigrant NAFTA professional (TN) visa, which allows Canadian and Mexican citizens to work in the U.S.

The visa allows experts in certain fields — including nurses — a fast track into the U.S., provided they have a job offer there. Those jobs include but are not limited to:

Accountants.

Engineers.

Graphic designers.

Librarians.

Teachers.

TN visas apply to highly skilled Canadian and Mexican workers in the U.S., which cannot be changed unless or until there are changes to NAFTA. They can be renewed indefinitely every three years.

The Windsor nurses have been advised they need to apply for H-1B visa status.

Given Trump's stance on protecting American jobs, many nursing students at the University of Windsor, including first-year nursing student Tiffany Taylor, fear they might not be able to find work in the U.S., where many graduates flock. (Jason Viau/CBC)

What is an H-1B visa?

Temporary worker permits for high-skilled workers are known as H-1Bs, and are not limited to NAFTA countries. However, applicants coming to the U.S. must have a "specialty occupation," the requirements being:

Theoretical and practical application of a body of highly specialized knowledge.

A bachelor's degree or higher.

A full state licence to practise in the occupation, if one is required.

Silicon Valley companies have used the visa to bring foreigners with technical skills to the U.S. for three to six years. Currently the system is lottery based, and capped at 85,000 workers a year.

While the tech industry in the U.S. insists the H1-B program is vital, it has drawn fire for allegedly disadvantaging American programmers and engineers, especially given that the visas are widely used by outsourcing firms. Attorney General Jeff Sessions is a longtime critic of the program.

Canadian Jason Dumelie came to New York City in 2014 to work as a medical researcher through a TN visa. He worries about his ability to work in the U.S. if President Donald Trump follows through on a promise to rip up the NAFTA exemption. (Steven D'Souza/CBC)

"Apple would not exist without immigration, let alone thrive and innovate the way we do," Apple CEO Tim Cook wrote in a company memo shortly after Trump's first travel ban was announced, an apparent reference not only to the company's foreign-born employees, but to Apple co-founder Steve Jobs, the son of a Syrian immigrant.

Topoleski said H-1B applications can cost between $3,000 and $4,000 US depending on the applicant.

However, United State Citizenship and Immigration Services (USCIS) announced on March 3 that it will temporarily suspend premium processing for all H-1B petitions starting April 3.

U.S. President Donald Trump, left, and wife Melania Trump arrive at a Florida rally on Feb. 18. Melania had used the H-1B visa to work in the U.S., but reports show she was compensated for modelling jobs before she received it. (Susan Walsh/Associated Press)

Trump's wife, Melania, had used the H-1B visa to work in the U.S. as a model on Oct. 18, 1996, after arriving in the country from Slovenia on a visitor visa on Aug. 27 of that year. Under H-1B, fashion models are allowed if they are "distinguished by merit and ability."

However, a recent report by The Associated Press said she was compensated for modelling jobs before she received the H-1B — during that time, her visitor visa allowed her to be in the U.S. and look for work, but not perform paid work.