With recreational marijuana use about to become legal in Illinois Jan. 1, the National Safety Council is urging employers to forbid workers in safety-sensitive jobs from using cannabis, even when they're off the clock.

The Itasca, Ill.-based nonprofit said it "believes there is no level of cannabis use that is safe or acceptable for employees who work in safety-sensitive positions." The group said research has shown that cannabis can affect psychomotor skills and cognitive ability.

Safety-sensitive positions might include workers in health care, transportation, law enforcement or those who work with certain chemicals, said Jane Terry, vice president of government affairs for the council.

Currently, there is no physical test for impairment due to marijuana, making any use by workers in those jobs inadvisable, Terry said.

"Do you want those types of individuals to be in a position where maybe you can't tell if they're impaired or not?" Terry asked. "We hope this position will give some guidance to employers and really give some idea of how to think about this to policymakers as well."

About 81% of employers that responded to a 2019 survey for the group said they were concerned about cannabis having a negative effect on their workforce.

U.S. Department of Transportation regulations bar many transportation workers from using marijuana, said Stephanie Dodge Gournis, a partner at law firm Drinker Biddle & Reath in Chicago. Many companies that contract with the federal government and receive federal grants are supposed to maintain drug-free workplace policies. Federal law prohibits marijuana use.

But the legalization of recreational marijuana is raising questions for many other types of employers about how to handle pot use among employees.

On one hand, the new Illinois law says that employers can't discriminate against employees or job applicants based on their use of marijuana outside of work, Gournis said.

But the law also says that employers may continue to enforce drug-free workplaces and "reasonable" zero tolerance policies "concerning drug testing, smoking, consumption, storage or use of cannabis in the workplace or while on call."

Workers are considered to be on-call when they're scheduled, with at least 24 hours' notice, to be on standby or working.

Under the Illinois law, employers may consider workers to be impaired if the employer "has a good faith belief" that employees are showing symptoms that affect their job performance, such as those related to speech, physical dexterity, agility, coordination and demeanor, among others.

Courts that have ruled on the issue have often sided with employers that require their employees to abstain, Gournis said.