Marijuana Research Report
What are the effects of secondhand exposure to marijuana smoke?
People often ask about the possible psychoactive effect of exposure to secondhand marijuana smoke and whether a person who has inhaled secondhand marijuana smoke could fail a drug test. Researchers measured the amount of THC in the blood of people who do not smoke marijuana and had spent 3 hours in a well-ventilated space with people casually smoking marijuana; THC was present in the blood of the nonsmoking participants, but the amount was well below the level needed to fail a drug test. Another study that varied the levels of ventilation and the potency of the marijuana found that some nonsmoking participants exposed for an hour to high-THC marijuana (11.3% THC concentration) in an unventilated room showed positive urine assays in the hours directly following exposure 80 ; a follow-up study showed that nonsmoking people in a confined space with people smoking high-THC marijuana reported mild subjective effects of the drug—a “contact high”—and displayed mild impairments on performance in motor tasks. 81
Toxins and tar levels known to be present in marijuana smoke (see What are marijuana's effects on lung health?) raise concerns about exposure among vulnerable populations, such as children and people with asthma.
Secondhand Marijuana Smoke Exposure
Secondhand Pot Smoke Risks and Drug Testing Implications
Secondhand marijuana smoke can negatively affect the health of exposed non-pot smokers The risks of secondhand tobacco smoke exposure have been known 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.
Possible Health Risks
We know that personal use of marijuana carries some health risks but what about non-users who are exposed to secondhand marijuana smoke? Do adults or children who are exposed need to worry?
Limitations in Studying Health Risks
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 60s aren’t the same as those smoked today. That said, several risks and potential risks have been identified.
In a study of 43 children, age 1 month to 2 years, who were admitted to hospitals in Colorado from 2013 to 2015 for bronchiolitis, urine samples tested for marijuana metabolites revealed that 16% of the children had a detectable level of exposure to secondhand marijuana smoke.
Another study that provided a preliminary look at health outcomes of children living in homes where marijuana is used showed a “relatively strong. association. between indoor cannabis smoking and adverse health outcomes in children” indicating a significant need for further study.
Effect on Blood Vessels
Tobacco smoke (either in smokers or inhaled as secondhand smoke) can clearly damage blood vessels, with the risk of heart attacks and peripheral vascular disease in people who smoke or are exposed to secondhand smoke only a few examples.
Research shared at the American Heart Association (AHA) Scientific Meeting in 2014 suggested that secondhand marijuana smoke should likely be considered a public health problem.
A Significant Cause for Concern
Breathing secondhand marijuana smoke 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% 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, 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 three times to five times higher in secondhand marijuana smoke than secondhand tobacco smoke.
And like tobacco smoke, marijuana contains a number of carcinogens (compounds known to cause cancer) such as benzene, cadmium, nickel, and more.
Other Secondhand Risks
Another 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 on Urine Drug Screens
Many people have questioned whether secondhand marijuana smoke in non-smokers can result in positive drug screens. Though older studies seemed to say no, a 2015 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.
Public Health Impact
Certainly, the findings of changes in blood vessels with secondhand marijuana smoke raises concern about the public health impact of exposure, but a thorough understanding of risks, as well as preventive measures that should be taken, is lacking at the current time.
Scope of the Problem
It’s difficult to know how common secondhand marijuana smoke exposure is, most notably because it is illegal in many places. A 2015 study set out to examine this question by questioning people at two southeastern universities. Researchers found that:
- 14.5% of participants allowed cigarette smoking in the home
- 17% allowed marijuana smoking in the home
- 35.9% allowed cigarette smoking in cars
- 27.3% allowed marijuana smoking in cars
Of course, this study evaluated only a subset of people, but the takeaway message is that many people are likely exposed to secondhand marijuana smoke.
Exposure in Open-Air Stadiums
Again, it must be noted that studies looking at the potential impact of secondhand marijuana smoke are limited. A 2019 evaluation looked at the effect of secondhand marijuana smoke on the health of police officers working at open-air stadium events.
Findings included detectable levels of THC in personal and area air samples, the presence of THC in the urine of 34% (but negative blood tests), and symptoms potentially attributable to the exposure including dry, red eyes, dry mouth, headache, and coughing. The officers, however, did not experience a “high” related to the exposure.
Accidental Ingestion in Children
While accidental ingestion of marijuana is a separate issue from secondhand smoke, we would be remiss to not mention it here.
A 2017 systematic review published in the Journal of Pediatrics concluded that accidental ingestion of marijuana by children is a serious public health concern, and that physicians and the public should be aware of this concern in children who develop the sudden onset of lethargy or loss of coordination.
As more states legalize marijuana, issues regarding secondhand exposure are likely to be examined in more depth.
Avoid secondhand marijuana smoke. If your loved ones use, ask them to use away from you, and certainly not in a poorly ventilated space.
Remember that legal doesn’t mean harmless. Consider the risk of secondhand smoke to non-smokers nearby, as well as the risk to children.
Driving while under the influence of marijuana has the potential to result in injuries to both self, and other passengers in the car, as people intoxicated by marijuana are roughly 25% more likely to crash. And, keep in mind that long-term use of marijuana can result in addiction in some people.
A Word From Verywell
Many people use marijuana recreationally, and cannabis may have possible benefits to people suffering from medical conditions such as cancer. Hopefully, now that marijuana is legal in many places, studies can further define its possible benefit in comparison with potential risks.
Still, priority should be given to protect non-smokers from the effects of exposure. Edibles may eliminate the concern over secondhand marijuana smoke exposure, but accidental ingestion remains a concern, and those who choose this route and are around children should take precautions recommended for any substance that could cause poisoning.
How does secondhand marijuana smoke exposure affect the health of nearby non-pot smokers, and what impact does this have on drug testing?