Toronto police Chief Mark Saunders defended his force's new cannabis policy for officers on Thursday, saying it was based on research and developed with public trust in mind.

Earlier this week, the service confirmed that officers will be prohibited from using recreational marijuana within 28 days of reporting for duty — a far longer timeframe than adopted by many other police services and institutions in Canada.

Ottawa police, for example, will be given free rein to use pot, so long as they are "fit for duty" when they arrive at work. And the Canadian forces will allow personnel to use pot but say they must leave at least eight hours between using cannabis and being on duty.

Speaking on CBC Radio's Metro Morning on Thursday, Saunders said his own force "is dealing with a bit more science" than those who have taken different routes, though he did not elaborate on exactly what that means.

He added that research has shown cannabis can stay in a user's system for up to 28 days. Therefore, it's possible that the THC still in an officer's system could, at least in theory, affect their judgment, he pointed out.

How long the acute psychoactive effects of cannabis last differs from person to person, though generally it is at least six hours, according to Health Canada.

Saunders said some of the differences in police cannabis policies comes down to ambiguity around the definition of "fit for service," adding that his main priority was to ensure the safety of communities and of his officers.

"I have to look at a much bigger picture, and that bigger picture includes public trust," he said, noting that he had to "set a high water mark."

He added Toronto police will make use of 320 field sobriety testers, controversial instruments that allow for roadside screening of saliva.

Cannabis legalization comes into effect on Oct. 17.