(WXYZ) — Research shows there has been a 33 percent increase in positive drug tests on employees, according to a study by EHS Daily Advisor.

Rob Wilson, president of Employco USA and a human resources expert, says there is an increase in employee usage of marijuana in industries across the board.

“Most importantly, test results showed that there was a double-digit jump of marijuana use in transportation and warehouse fields, which could be very problematic as operating heavy machinery while under the influence of marijuana could be very dangerous indeed," he said.

With 10 states, including Michigan, making it legal to use marijuana recreationally, and 33 states making it legal to use medically, employers might wonder what rights they have when taking a hard line on drug use in the workplace.

“Your ability to monitor drug use among your employees is going to depend on whether or not you are a unionized or private workplace,” says Wilson. “While you have the right to expect and require sobriety from workers on the job, it can become a bit tricky when you suspect drug use and want to act on your fears.”

Wilson says although marijuana use rules are changing, employers can still have a zero-tolerance policy about drug use on the job, even if the employee has the legal right to use marijuana recreationally or medically.

“While it is against the law to discriminate against someone simply because they have a medical marijuana card, as this could be seen as discrimination against someone with a disability, you can still require sobriety among your employees and treat marijuana the same way you would alcohol or prescription drugs like Vicodin,” says Wilson. “Whether an employee is driving heavy machinery or approving loans, you need your workers to be clear-headed and capable of performing at a high level.”

