The number of U.S. workers testing positive for illegal drugs has reached its highest level in a decade, according to a new study.

Quest Diagnostics, one of the nation’s largest medical-screening laboratories, surveyed more than 11 million drug tests over the last three years and found that the percentage of workers testing positive for illicit drugs has risen steadily over the past three years, increasing to 4% in 2015 after decades of decline.

Pulling data from more than 9.5 million urine, 900,000 oral fluid, and 200,000 hair laboratory-based tests that were performed by Quest for employers in 2015, the study found that usage rates of amphetamines, marijuana, and heroin have all increased in the past five years in the general workforce.

Drug usage among “safety-sensitive” professions, (truck drivers, pilots, ship captains, subway engineers, and other transportation workers), increased from 1.7% to 1.8%. While modest, that incline should “be of concern to employers, especially those with safety-sensitive employees,” Barry Sample, director of science and technology for Quest’s employer solutions business, said in the study.

Although positive results are thrown out if a worker has a prescription for a legal drug, the survey still reflects illegal use, mainly driven by drugs like amphetamine, cocaine, and heroin.

 

The study also found that positives for marijuana increased 26% since 2011, and almost half (45%) of those who had a positive drug test for any substance in 2015 showed evidence of marijuana use. Positive test rates were at a 3.5% annual low until 2012, when they began to climb—the same time when initiatives to legalize marijuana use in states like Colorado and Washington were approved.

The most shocking detail from the study is its finding on heroin usage. Heroin positives increased 146% in the general workforce between 2011 and 2015, and 84% in the safety-sensitive workforce.

But as more workers tested positive for heroin, fewer tested positive for opioids such as Oxycodone, which is sold under brands like Oxycontin and Roxicodone. The increase of the former is connected to the decline of the latter, according to the Wall Street Journal. With increased crackdowns, obtaining prescription opioids has gotten harder, pushing drug users toward heroin, which is more easily accessible.