Your boss may not care if you smoke weed when you’re not at work. But then again, you might get fired.

While marijuana is now legal after the passage of Proposition 64, Northern California businesses are taking widely different approaches to testing to see if you use the drug.

Related: What you need to know about passing a drug test

If you design computer chips at Intel or web pages at Google, you won’t be tested. But if you drive a Samtrans bus or operate a crane for a construction company, you will be.

At UC Berkeley, professors aren’t tested, but campus police officers are. The network company Cisco Systems doesn’t test; the construction company Cisco does.

In many ways it’s a digital divide, with zero-tolerance for many blue-collar workers and a “don’t ask, don’t tell” policy among tech-savvy cubicle dwellers. And as marijuana is about to become more bountiful in the Golden State, the gap between increased toughness and greater acceptance will only increase.

“I can’t think of a hotter issue in employment law right now,” said Christina Semmer, an employment lawyer with Wilson Turner Kosmo in San Diego. “A lot of employees are thinking ‘Hey, it’s legal, so I can smoke now and still show up for work.’ That’s not correct.”

Proposition 64 explicitly permits “public and private employers to enact and enforce workplace policies pertaining to marijuana,” she said. So while use of marijuana is legal, your employer is not required to allow you to use it.

Some employees are tested only once, before they are hired; others are tested routinely and randomly throughout the year.

But as marijuana becomes more publicly acceptable, Semmer said, policies may become more flexible, except for federal contractors or employers whose workers could endanger public safety.

Many Silicon Valley companies don’t do any drug testing because they worry that they’ll lose young talent in a tight labor market.

“It affects recruitment and retention. In high tech, usage is so common that it is really hard to drug test, so most companies don’t,” said John Sullivan, head of the Human Resources Management Program at San Francisco State.  “The companies who used to do it don’t, because it drove so many people away and doesn’t predict anything.”

But testing is still critical for manufacturers, logistics firms, construction contractors and companies that employ a lot of drivers, pilots and other “safety sensitive” workers, or who have contracts with the federal government.

The issue is complicated by the fact that there’s no standard for determining levels of impairment — unlike alcohol. In California, for instance, if you have a blood alcohol concentration of 0.08 percent, you’re considered too drunk to drive.

Testing positive for marijuana doesn’t mean you’re currently impaired — only that you’ve used it within the past 30 to 45 days. It’s not like heroin, cocaine or alcohol, which are quickly cleansed from your system. Marijuana’s psychoactive component, THC, remains stored in the fat.

Drug testing became routine during the “War on Drugs” era of the Reagan administration — a time when marijuana was completely illegal. Congress mandated a “drug-free workplace” program for federal departments that oversee trucks, trains, aircraft, ships, as well as pipelines and nuclear facilities. Many private companies followed the lead.

But now nearly two-thirds of Americans live in a state that has legalized medical marijuana. And with recreational use approved this month by California, Massachusetts, Nevada and Maine, the percentage of Americans living in states where all pot is legal for adults jumped from 5 percent to 20 percent.

The popularity of drug testing, meanwhile, has waned. Companies that require drug tests fell from 81 percent in 1996 to 62 percent in 2004, according to the American Management Association.

Intel, which instituted a drug-testing policy in 1992, has dropped the requirement, as has Hewlett Packard. Google, Facebook, Twitter and Adobe don’t test either. (Oracle and Agilent refused to discuss their testing policies.)

The white-collar vs. blue-collar divide is driven by several factors. White-collar workers are rarely in a position to physically hurt others. And their employers often see testing as an infringement into employees’ personal lives, believing that what workers do on their own time is their own business. Testing also adds regulatory paperwork and offers little return on the time and money, most tech firms say.

Furthermore, drug testing can drive away the workforce of the future: millennials. Even if they don’t use marijuana, millennials often favor the right to use the drug and are strongly against workplace testing, companies say.

“We evaluate people by the job they do, in the hours between when they walk in and walk out,” said Derek Peterson, CEO of the Irvine-based cannabis agricultural company Terra Tech, whose revenues have grown from $500,000 to almost $30 million in six years.

Peterson doesn’t smoke at work, but uses it to relax at home.  “I can’t function at the level I need to, if I smoke during the day. But the evening is no problem,” said Peterson, who broke his neck eight years ago and still suffers pain. “I’m a better dad. I have more fun. I re-prioritize my social life. And there’s pain relief.”

Companies can still search for other illicit substances by selecting the type of analysis they want, Sullivan said. In places where drug testing is not required by law and cannabis is legal, for example, employers still test for amphetamines, cocaine, opiates and PCP.

But drug testing will remain mandatory or even expand in businesses with more blue-collar workers.

Under federal law, all truck drivers are subject to testing. So are airplane mechanics and luggage handlers. Drug testing also is standard for government agencies, large corporations where safety is paramount, such as gas and oil exploration. Bus drivers and train conductors are also routinely tested.

In the wake of Colorado’s legalization of recreational marijuana in 2014, one in five Colorado employers reported that they implemented more stringent drug testing policies. But that has sometimes created new headaches for hiring managers.

“We have been challenged with finding drug-free applicants to fulfill the staffing needs within our company,” Brandon Kochen, of K.R. Swerdfeger Construction in Colorado Springs, told the journal Underground Construction.

Unfortunately, attorney Semmer said, what random drug testing for marijuana often “means is that the employer sees you may have smoked two nights ago at home. Even if you are not under the influence now, the employer has no way of verifying that.

“How do we decide who is truly impaired at work?” she added. “That is the next frontier in cannabis law.”