The rate of positive tests for marijuana and other substances among U.S. workers reached a two-decade high in 2021, according to a new study by Quest Diagnostics.

The overall positivity rates reached 4.6% in 2021, compared with 4.4% in 2020. The increase was driven by a rise in positive marijuana tests, The Wall Street Journal reports. The findings come from an analysis of more than 11 million drug test results collected during 2021.

The positivity rate increased more than 30% from an all-time low in 2010-2012, the study found.

Last year fewer companies tested their employees for THC, the psychoactive substance in marijuana, according to Quest.

“It is important for workers to know that certain employers are required to test for marijuana under federal law and if they use marijuana, they can still lose their jobs,” Quest Diagnostic’s Barry Sample, Ph.D., said in a news release. “People who use drugs during working hours or before work can still be impaired and dangerous to co-workers, the general public and themselves.”