Secondhand Marijuana Smoke Exposure

Secondhand Pot Smoke Risks and Drug Testing Implications

secondhand smoke from a lit joint
What are the risks and drug testing implications of secondhand pot exposure?.

We have heard about secondhand tobacco smoke exposure for many years, but with the legalization of marijuana in some states, concerns have been raised about secondhand marijuana smoke exposure as well. These concerns come from two angles. One concerns health. Could secondhand marijuana smoke exposure have a negative effect on the health of exposed non-users? And, for those who do not smoke marijuana but hang out with marijuana smokers, could this exposure affect drug testing? Is secondhand marijuana smoke dangerous or could secondhand pot smoke mess up your drug testing at work? These are important questions to be asking.

How Common is Exposure?

It's difficult to know how common secondhand marijuana smoke exposure is, most notably because it is illegal in many places.

One recent study set out to examine this question by questioning people at two southeastern universities. Researchers found that:

  • 14.5 percent of participants allowed cigarette smoking in the home
  • 17 percent allowed marijuana smoking in the home
  • 35.9 percent allowed cigarette smoking in cars
  • 27.3 percent allowed marijuana smoking in cars

Of course, this study evaluated only a subset of people, but the take away message is that many people are likely exposed to secondhand marijuana smoke.

Possible Health Risks

We know that personal exposure to marijuana can carry some health risks but what about non-users who are exposed to secondhand marijuana smoke?

There are difficulties in evaluating potential hazards of secondhand marijuana smoke; not the least of which is that it is illegal in many areas, making studies difficult. Another is that the potency of marijuana has changed over time; the joints smoked by hippies in the 60's aren't the same as those smoked today. But we have learned a few things and can consider a few things.

  • Research shared at the American Heart Associations (AHA) Scientific Meetings in 2014 suggests that breathing secondhand marijuana smoke may not only be dangerous but may cause as much damage to blood vessels as secondhand tobacco smoke.

This research looked at the effect of secondhand marijuana smoke on blood vessels, albeit in rodents. Rats that were exposed to secondhand marijuana smoke had a 70 percent reduction in blood vessel function. (These results were the same for rats exposed to marijuana smoke containing THC as those not, so it was considered likely that THC alone wasn't the culprit.)

Of even more concern was that whereas blood vessel function returned to normal after 40 minutes for rats exposed to secondhand tobacco smoke, this wasn't the case for the marijuana smoke group; in the rats exposed to secondhand marijuana smoke, blood vessel function remained affected after this interval.

While often we look at studies like this thinking that a lot of smoke over an extended period of time is to be most feared, but a 2016 study made this approach questionable. It was found that even one minute of secondhand marijuana smoke could impair vascular endothelial function in rats. Even though we don't know whether these results on rats reflect what happens in humans, knowing that vascular endothelial dysfunction underlies a leading killer in the U.S. (endothelial dysfunction leading to heart attacks), this information is worth investigating further.

Of course, the next step is determining the significance of reduced blood vessel function, something which has been linked to atherosclerosis.and heart attacks.

  • Another concern surrounds the harmful chemicals in cigarette smoke; Tobacco smoke and marijuana are chemically alike, and therefore many of the harmful chemicals in cigarette smoke are likely to be found in marijuana smoke. We could make assumptions based on this evidence—that the cancer-causing chemicals in secondhand tobacco smoke which result in 34,000 deaths per year in the United States are also released in marijuana smoke—but until we have further studies, no one can say for sure. In one study, levels of ammonia were 20 times higher in secondhand marijuana smoke than secondhand tobacco smoke. Levels of hydrogen cyanide and aromatic amines were 3 to 5 times higher in secondhand marijuana smoke than secondhand tobacco smoke.
  • A final concern is not a risk related to marijuana smoke per se but is a secondhand risk to those who are around those who smoke marijuana. Children and even dogs have suffered from the accidental ingestion of marijuana. From broken bongs that can cut, to the financial complications imposed on nearby nonusers (for example if a child has a parent who faces legal problems due to use,) are all things that need to be considered by those who choose to smoke marijuana.

    Effects of Secondhand Marijuana Smoke on Urine Drug Screens of Non-Smokers

    Can exposure to secondhand marijuana in non-smokers result in positive drug screens?

    Though older studies seemed to say no, a recent study suggests that the answer is yes. In rare cases, anyway. That said, the yes deserves an explanation. It's wasn't easy for a non-user to have a positive test. In the study that said "yes," non-users were subjected to what was called "extreme exposure”—heavy exposure in poorly ventilated rooms—something that an individual would clearly be aware of. Even in this type of situation, the chance of a "false positive" result rapidly decreased with time; drug screens would be normal in a matter of minutes or hours.

    The conclusion of one older study is that it would be improbable that people would unknowingly tolerate the nasty smoke conditions that would result in a positive test. What does this mean? If you are at risk of having a positive test, you're probably hanging with the wrong crowd. 

    Next Steps

    As more states legalize marijuana, issues regarding secondhand exposure are likely to be examined in more depth.

    For Non-Users - Avoid secondhand marijuana smoke. If your loved ones use, ask them to use away from you, and certainly not in a poorly ventilated space.

    For Users Legal doesn't mean harmless. Check out these articles:

    On the other hand, we can't dismiss the possible benefit (while protecting others from secondhand smoke, of course) for the use of medical marijuana for people suffering from medical conditions such as cancer. Fortunately, now that marijuana is legal in many places, studies can be done to determine what role marijuana can play in reducing the pain and loss of appetite these people experience.

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