More American workers are testing positive for marijuana, a new report finds, as lawmakers in New Jersey and Illinois push to join nearly a dozen more states where recreational use of the drug is now legal.

The number of workers and job applicants who tested positive for marijuana climbed 10% last year to 2.3%, according to an analysis of 10 million urine, saliva and hair samples by Quest Diagnostics Inc., one the nation’s largest drug-testing laboratories. The Quest data, which is set to be released Thursday, found 4.4% of the analyzed samples contained traces of both legal and illegal controlled substances including pot, prescription painkillers and other drugs—the highest such rate since 2004.

Since then, the number of drug tests showing signs of cocaine, heroin, prescription sleep aids and certain opiates like morphine has fallen sharply. But the tests indicate marijuana use has steadily risen for the general U.S. workforce, including among employees in safety-sensitive jobs such as airplane pilots, nuclear power-plant operators and train conductors.

“Marijuana use is on the rise in society, so it’s not surprising that we’re starting to see that filter into the workplace,” said Barry Sample, Quest’s senior director for science and technology. Quest has been analyzing workplace drug-testing data since 1988.

Most urine and saliva tests indicate only whether pot’s psychoactive ingredient—tetrahydrocannabinol, or THC—is present but not how long ago the drug was used or whether someone is currently impaired, a distinction some employers increasingly say is important. Because THC can linger in the body for months, a positive marijuana test is sometimes interpreted as a “lifestyle” indicator, rather than confirmation of current or regular use, Dr. Sample said.