Secondhand Marijuana Smoke?

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Smoke rising against a black background

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People often worry how breathing someone else’s marijuana smoke could affect them. Let’s look at some common questions about this, and the answers that science has found.

Can you get a “contact high” from secondhand marijuana smoke? It depends.
You’d have to be in an unventilated room, and very close to someone who is smoking for a long period of time. (“Unventilated” means all windows and doors are closed.)

In one study, 12 participants sat together for an hour in an unventilated room. Six participants smoked marijuana casually during that hour and the other six didn’t smoke anything. 

At the end of the hour, each nonsmoker’s heart rate was a little faster than it was before the experiment. The nonsmokers also reported feeling slightly buzzed or sleepy, and they didn’t perform as well on logic tests as they had before the experiment. 

But when the same experiment was conducted in a ventilated room, there was no change in the nonsmokers’ heart rate or test performance, and they did not report any signs of a “contact high.”

Can you test positive on a drug test after inhaling secondhand marijuana smoke? Probably not in most cases.
But it is possible to detect marijuana in drug tests after exposure in an unventilated room to larger amounts of THC, the ingredient in marijuana that causes the high.   

Drug screens can look for THC in your blood, urine, saliva, or hair. In one study, nonsmokers spent 3 hours in a well-ventilated space with people who casually smoked marijuana. Then, researchers measured the amount of THC in the nonsmokers’ urine. 

Sensitive laboratory tests were able to find THC in the nonsmokers’ urine, but not enough to trigger a positive result on most commonly used drug tests.  

Next, the researchers conducted the same experiment for 1 hour in an unventilated room with marijuana that had higher levels of THC (11.3 percent, compared with 5.3 percent in the previous experiment). This time, some nonsmoking participants did test positive for THC in their urine—but that was rare, and it only happened in the hours right after the experiment.

What are the health effects of inhaling secondhand marijuana smoke? Researchers are working to figure this out.
We don’t know yet how a person is affected if they live with somebody who smokes marijuana regularly, or how higher concentrations of THC in today’s marijuana could affect secondhand smoke.

A study using rodents found that secondhand marijuana smoke can do as much damage to the heart and blood vessels as secondhand tobacco smoke. But researchers haven't fully explored the effect of secondhand marijuana smoke on humans. 

We also know marijuana smoke contains levels of chemicals and tar that are similar to tobacco smoke, raising concerns about risk for cancer and lung disease. We don’t know yet how it affects a person’s health in the long run. 

Research is also being done on possible risks of secondhand vaping; this will be important to know, because a growing number of teens report vaping THC.

As researchers learn more, we’ll keep you updated.

Learn more: What are the risks of inhaling secondhand tobacco smoke?
 

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