Key takeaways:
Experts are learning more about the effects of secondhand cannabis (marijuana) smoke.
Cannabis smoke contains toxic chemicals that may be especially harmful for children and other at-risk groups.
In certain situations, it’s possible to get high or fail a drug test from secondhand cannabis smoke.
Even if you don’t smoke cannabis (marijuana), you might still inhale secondhand smoke from time to time. This is the smoke that comes from burning cannabis, which you may inhale when you’re near someone who’s smoking.
Experts are still learning about the effects of secondhand cannabis smoke. But they know it contains many of the same toxic chemicals found in cigarette smoke. Beyond the health risks, secondhand smoke from cannabis or vape pens can also make you intoxicated (high) or cause you to fail a drug test.
Can you get a secondhand (contact) high from marijuana smoke?
Yes, it’s possible to get high from secondhand cannabis smoke. This is also called a “contact high.” Whether or not you get a contact high from cannabis smoke depends on how much THC (tetrahydrocannabinol) you’re exposed to. THC is the chemical in cannabis that causes the high.
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A secondhand high also depends on how much ventilation there is in the space where smoke is present. For example, spending an hour in an unventilated room with cannabis smoke could be enough to get a contact high. But you’re much less likely to get a contact high outdoors or in a large, well-ventilated area.
Will secondhand marijuana smoke make me fail a drug test?
It’s possible for secondhand cannabis smoke to raise drug levels enough to cause a positive cannabis drug test. But formal studies on this are limited.
One study found that nearly half of children exposed to cannabis smoke at home had THC detected in their urine.
A very small study in adults also found detectable levels of THC in blood and urine. Certain factors can make a positive test from secondhand cannabis smoke more likely:
A very poorly ventilated room: The study exposed participants to cannabis smoke in a sealed chamber.
Exposure to a lot of smoke over an extended period of time: Participants in this study stayed in the sealed room for 1 hour while someone freely smoked cannabis.
Testing shortly after exposure: All test samples were collected within 1 ½ days after exposure.
Type of test: How long cannabis can be detected in your system depends on the type of test. For example, cannabis will show up on a urine test for longer than a blood test.
While it’s possible for you to have a positive drug test from secondhand smoke, it’s unlikely. The best way to reduce the risk of failing a drug test from secondhand smoke is to avoid exposure to cannabis smoke whenever possible.
How long does secondhand smoke stay in your system?
It’s hard to predict how long secondhand cannabis smoke will stay in your system after exposure. This will mainly depend on how much smoke you were exposed to and how well the area was ventilated.
In the study with adults mentioned above, some participants still had detectable levels of THC in their urine 34 hours after exposure. But it’s important to note that the researchers used lab tests that were more sensitive than standard drug tests. Because of this, it’s unlikely that a normal drug test would be positive beyond 12 hours after secondhand smoke exposure.
What are the adverse effects of secondhand marijuana smoke?
Research on the harmful effects of secondhand cannabis smoke is limited. But experts warn that secondhand cannabis smoke contains many of the same toxic and cancer-causing chemicals found in tobacco smoke. And secondhand smoke increases the risk of:
Lung damage
Heart disease
Stroke
Lung and other types of cancer
Pregnancy complications
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These risks are especially harmful for at-risk populations, such as children and people with chronic medical conditions.
Below is a look at the growing research on the negative effects of secondhand cannabis smoke.
Lung damage
Any kind of smoke can damage your lungs, including cannabis smoke. The toxic chemicals in cannabis smoke can increase your risk of long-term lung conditions, such as:
Increased mucous production and cough
Breathing problems, like wheezing
Bronchitis
Pneumonia
COPD (chronic obstructive pulmonary disease)
Cancer
Heavy cannabis smoking can increase your risk of cancer. Like tobacco smoke, smoked cannabis contains cancer-causing chemicals. Your risk will depend on how much and how often you smoke.
One study looked at cancer risk over 40 years. It showed that heavy cannabis use was associated with more than twice the risk of developing lung cancer. The long-term effects of light or occasional cannabis use are still unclear.
Heart disease
Smoking cannabis — specifically THC — can affect your heart in dangerous ways. Research shows that exposure to cannabis smoke may put you at greater risk of:
A higher heart rate
Elevated blood pressure
Arrhythmia (irregular heartbeat)
Increased risk of blood clot, heart attack, or stroke
And research suggests that any type of cannabis use can increase your risk of heart attack and stroke. But it’s still unclear how this applies to people exposed to only secondhand cannabis smoke.
Pregnancy risks
Cannabis smoke can also affect a growing baby during pregnancy. Risks can include:
Stillbirth
Preterm birth
Developmental delays
Fetal growth problems
Low birth weight
Brain effects
Early research on the long-term effects of cannabis on the brain has shown mixed results. Some studies have linked cannabis use to long-term problems with:
Memory
Thinking, learning, and IQ
Impulse control
Mental health conditions and addiction
These concerns and risks are especially worrying for teens who start using cannabis when their brain is still developing. Previous studies have found a link between cannabis use and lower scores on cognitive and memory tests.
But newer research suggests that cannabis may not be to blame. Instead, the differences in test scores could be caused by underlying factors that contribute to both cannabis use and thinking skills. For example, education level, employment status, and income may be to blame.
Cannabis can affect how a teen’s brain functions in the short term, according to the researchers. But cannabis doesn’t seem to impact long-term mental health and cognitive ability as previously thought. This is an ongoing topic of research.
Frequently asked questions
Ventilation, filtration, and air filters — especially portable, HEPA-equipped air cleaners — can reduce some of the harmful substances in secondhand smoke. But it’s unlikely that even a top-quality air filter can completely eliminate all of the harmful substances in secondhand smoke.
Smoking cannabis or being exposed to secondhand cannabis smoke can result in a positive drug test. Using other forms of cannabis, such as edibles or cannabis-infused drinks, can also lead to a positive test result. This is true even if you haven’t smoked. Cannabis is stored in your fat cells and is slowly released from your body over time. Because of this, you may still fail a cannabis drug test even if you haven’t smoked it recently.
Ventilation, filtration, and air filters — especially portable, HEPA-equipped air cleaners — can reduce some of the harmful substances in secondhand smoke. But it’s unlikely that even a top-quality air filter can completely eliminate all of the harmful substances in secondhand smoke.
Smoking cannabis or being exposed to secondhand cannabis smoke can result in a positive drug test. Using other forms of cannabis, such as edibles or cannabis-infused drinks, can also lead to a positive test result. This is true even if you haven’t smoked. Cannabis is stored in your fat cells and is slowly released from your body over time. Because of this, you may still fail a cannabis drug test even if you haven’t smoked it recently.
The bottom line
Experts are still learning about the effects of secondhand cannabis smoke. If you’re in a poorly ventilated room with a lot of cannabis smoke, you may absorb the smoke as if you were smoking it yourself. This level of exposure can make you feel high or fail a cannabis drug test. Cannabis smoke also contains known toxins that may increase your risk of cancer, heart disease, and lung disease, especially for young children or other at-risk groups.
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References
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Callaghan, R. C., et al. (2013). Marijuana use and risk of lung cancer: A 40-year cohort study. Cancer Causes and Controls.
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Schaefer, J. D., et al. (2021). Associations between adolescent cannabis use and young-adult functioning in three longitudinal twin studies. Proceedings of the National Academy of Sciences.
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