TORONTO -- As Canada and the U.S. move to extend the border closure by another month, there are still concerns about how many people may be arriving in the country by air.

Although Canadians are not permitted to drive to the U.S. for leisure travel, they can still fly to the country.

Ambarish Chandra, an associate professor of economics at the University of Toronto, told that this has "always been the case" since the start of the pandemic.

"The government never closed down travel by air, but there's still restrictions. You can't just hop on a plane and fly to Canada. You're going to be asked when you enter what the purpose of a trip and in principle, you could be turned back and denied," Chandra explained in a phone interview on Wednesday.

Chandra said the border measures focus more on restricting the type of travel that is allowed, rather than the mode of transportation. Tourists and cross-border visits remain prohibited, but essential travel for trade and commerce is still permitted, by land and air.

However, the U.S. Customs and Border Protection says Canadian air passengers can still enter the country as long as they haven't visited Brazil, China, Iran, Ireland, the U.K. or countries in the Schengen Area for 14 days prior.

Depending on which state they are entering, they may not even have to self-isolate upon arrival. The U.S. Centers for Disease Control and Prevention (CDC) only recommends that international travellers quarantine if they might have been exposed to the virus. It is not a requirement unless specified by a state.

However, the permission to fly is not mutual.

Canada prohibits U.S. visitors, except those with immediate family in Canada, from entering the country by all modes of transportation, including by plane.

The border restrictions apply specifically to land crossings because these entry points are less challenging to control and monitor, according to Chandra.

"The policy from the start was to stop all non-essential travel. That's much easier to do by land. and it's also much more practical because most travel over the land border is actually non-essential," Chandra said.

"People cross the border both ways to shop or just to go on short vacations, which is harder to do and requires a lot more planning to do when you're doing it by air."

Despite the restrictions, the Canada Border Services Agency (CBSA) saying many people are still trying to enter Canada from the United States for sightseeing and shopping.

Between March 22 and Sept. 2, the CBSA said it has turned away more than 18,000 travellers for non-essential reasons. This includes foreign nationals by boat, land, and air from the U.S.

When Canadians return home from abroad by land or air, they are still required to self-isolate for 14 days as per federal guidelines. Chandra said the border restrictions help enforce this.

"The rules that are in place [are] to stop foreign travellers from making non-essential trips to Canada, but also to intercept Canadians returning and get them to quarantine and self-isolate, which is still the policy in place," Chandra said.

A spokesperson for the Minister of Public Safety and Emergency Preparedness told on Wednesday that the federal government has "brought forward significant restrictions at" the Canada-U.S. border and will continue "doing what is necessary to keep Canadians safe."

"Going forward, we will continue to evaluate the best public health information available to us to make a decision on when and how to reopen our border. This decision will be made in Canada, with the best interest of Canadians as our top priority," the spokesperson said in an email.

Senior government sources tell CTV News that Canada and the U.S. are expected to extend existing border restrictions until November. The current agreement was set to expire on Sept. 21 but sources say the restrictions will remain in place until it is felt that the COVID-19 pandemic is under control.

The travel ban was first imposed in March and has been renewed every month since.

The federal government says it will continue to adjust border restrictions as the COVID-19 pandemic evolves, but Chandra said it won't stop Canadians from travelling.

"Canada can never stop citizens from leaving the country, and of course citizens are allowed to come back by definition… and while they may feel more comfortable travelling now, there's still policies in place they have to follow," he said.