Jonathan Martin / The Seattle Times

SEATTLE — Anyone with a job who plans to attend a marijuana “Legalization Day” party might as well pack their employment contract along with their rolling papers.

Washington voters this month agreed to make marijuana legal for recreational use for adults 21 and up, but the new law gives no protection in the workplace. If employees show up with marijuana in their systems — even residual amounts from a few weeks back — there’s no guarantee the boss will look the other way.

In this new frontier of drug policy, employers could simply view employees’ after-hours use of marijuana like alcohol, intervening only when necessary. But few appear to be loosening bright-line drug policies, as if Initiative 502 never passed, say the region’s top employment lawyers.

Last week, the city of Seattle joined other employers in reminding its 10,500 workers that, because it gets federal funding, and because federal law still considers marijuana a banned substance, it must maintain a drug-free workplace.

But enforcing such policies for after-hours use is likely to be contentious, especially for unionized workers, because most workplace marijuana tests don’t differentiate past use — even weeks prior — from the impairing buzz of a freshly smoked joint.

“We think 502 changes everything,” said Dan Swedlow, senior staff attorney at the 16,000-member Teamsters Local 117. “We’re clearly headed for a showdown with some employers.”

It may not be an issue for many workers: Pre-employment testing is rare among technology and creative-class jobs, and random testing even rarer.

But there is no wiggle room for many manufacturing and transportation jobs, and even less so in public safety, and national employers appear unlikely to flex policies for Washington state.

“I think people who voted for 502 will be really surprised that if you use it in your home, in accordance with the initiative, you can still get fired,” said Seattle employment-law attorney Michael Subit.

Workplace drug tests became widespread after President Ronald Reagan issued an executive order in 1986 mandating drug-free policies. A federal standard intended for big-rig truckers — 50 nanograms of carboxy-THC per milliliter of blood — became widely adopted by employers in most industries, from bakers to warehouse workers.

Courts across the country have upheld the right of employers to drug-test and fire workers with THC in their urine, even those with valid medical marijuana authorizations. The Washington Supreme Court in 2011 ruled in favor of employers, upholding the firing of a call-center worker who used marijuana to treat migraines.

One of the few groups exempted from drug testing: elected officials, thanks to a 1997 U.S. Supreme Court case, Chandler v. Miller.

Employers have good reasons — from productivity to absenteeism to cheaper insurance coverage — to maintain drug-free workplaces, said James Shore, an employment lawyer with Stoel Rives in Seattle.

In the wake of I-502, he advises employers to update policies to prohibit drugs illegal under state or federal law — “with an exclamation point on federal law,” he said — and ban any detectable amount. “It takes the gray area away.”

Mark Berry, an employment-law attorney with Davis Wright Tremaine, said employers like “a bright-line standard” of zero tolerance because marijuana affects people differently, according to past usage and body size.

“It may or may not relate to impairment, but because there is a widely used standard, that’s what is used,” he said.

At Swedish Medical Center’s urinalysis collection clinic in Ballard, Wash., the toilet water is bright blue, fresh samples must be body temperature, and staff keep a sharp eye out for urine-test cheating devices like the “Whizzinator.”

Marijuana consumed within the past seven to 10 days usually triggers a positive result, said Swedish’s Dr. Ray Jarris. In the future, tests — such as oral swabs — may be able to more precisely pinpoint recent usage, but not yet, he said.

“The issue of marijuana impairment is a really difficult issue,” said Jarris. “But that’s not what the (federal) DOT cares about, and it’s not what most employers care about.”

That frustrates Paul Armentano, a marijuana-science expert at NORML, a leading national drug-reform group. The tests detect carboxy-THC, a non-psychoactive compound in marijuana stored in fat cells, not the active THC, which provides the high.

That means a positive drug test “could be someone who used two grams six hours ago, or someone who used two weeks ago,” he said.

The Nov. 6 passage of legalization measures in Washington and Colorado, as well as medical-marijuana laws in 18 states, reflect a new public attitude about marijuana and civil-liberties issues related to testing, he said.