Drug testing can detect second-hand marijuana smoke

Smoking marijuana has become legal in more US states and smoking it in public more common. Increased public use has led to people wondering about the impact of second-hand marijuana smoke.

Popular culture has joked for years about people getting a ‘contact high’ from second-hand marijuana smoke. Several scientific studies on this assumption show that it is not true.

Another concern was whether exposure to second-hand marijuana smoke could cause you to fail a drug test. Studies have found that drug testing can detect chemicals from second-hand marijuana smoke in your system.

During a study at Johns Hopkins University, researchers placed people who had never smoked marijuana in a sealed room with people smoking various strengths of marijuana. After exposure to marijuana smoke, the non-smokers’ urine was tested for the presence of THC. All the participants tested positive at a THC level of 20 ng/mL, which remained steady for up to 22 hours.

In some US jurisdictions, the workplace drug-testing threshold for existing employees is 20 ng/mL, but the threshold for new hires is 50 ng/mL, as in the UK.

So, should you worry about failing a drug test if exposed to second-hand marijuana smoke? Subjects in the Johns Hopkins study were in a sealed room, a condition not common in everyday situations. Unless you spend a lot of time with serious marijuana smokers in an airtight space, second-hand marijuana smoke should not cause you to fail a drug test.