Secondhand weed smoke contains many of the same toxic chemicals found in tobacco smoke, and in some cases at higher concentrations. Beyond the chemical exposure, it can affect your heart, your lungs, and even produce mild psychoactive effects if you’re in an enclosed space. Here’s what the research actually shows.
What’s in Secondhand Cannabis Smoke
Cannabis smoke isn’t cleaner than tobacco smoke. A study published in Chemical Research in Toxicology found that marijuana smoke contains ammonia at levels up to 20 times greater than tobacco smoke. Hydrogen cyanide, nitric oxide, and certain aromatic amines were present at three to five times the concentrations found in tobacco smoke.
The picture with cancer-linked compounds is nuanced. The smoke that comes directly off the burning end of a joint (called sidestream smoke, which is what bystanders primarily breathe) actually contains higher concentrations of polycyclic aromatic hydrocarbons than tobacco sidestream smoke. These are compounds strongly associated with cancer risk. So while the person smoking may inhale fewer of these chemicals per puff compared to a cigarette, the people around them may be getting a larger dose from the air.
Cardiovascular Effects Start Quickly
One of the most striking findings comes from research presented through the American Heart Association. In a controlled experiment, just one minute of exposure to secondhand cannabis smoke caused blood vessels to lose roughly two-thirds of their ability to dilate properly. This measurement, called flow-mediated dilation, dropped from about 7.5% before exposure to just 2.3% ten minutes afterward. Forty minutes later, it still hadn’t recovered.
What makes this especially important is that the impairment had nothing to do with THC. When researchers repeated the experiment using cannabis with the THC removed, the blood vessel damage was nearly identical. The combustion byproducts alone, the particulate matter and chemicals in the smoke, were enough to cause the effect. Clean air exposure, by contrast, produced no change at all. This means that even THC-free hemp smoke in an enclosed space poses cardiovascular risk to bystanders.
The “Contact High” Is Real, but Conditional
Whether you can actually get high from someone else’s smoke depends almost entirely on ventilation. In a controlled study that placed non-smokers in a small, sealed room with people actively smoking cannabis, participants reported mild to moderate sedative effects. They also showed minor increases in heart rate and measurable impairment on a task involving working memory and processing speed.
Blood and urine tests confirmed that THC had entered their systems. Under these extreme, unventilated conditions, one participant even produced a urine sample that tested positive under standard federal workplace drug testing guidelines (the 50 ng/mL screening threshold). That said, this required a worst-case scenario: a small, sealed room with no air circulation and heavy smoking. In a ventilated room or outdoors, THC absorption drops dramatically, and subjective effects largely disappear.
Could It Make You Fail a Drug Test?
This is one of the most common concerns, and the short answer is: it’s unlikely but not impossible. In the study described above, only one urine specimen out of all non-smoker participants crossed the 50 ng/mL cutoff used in standard workplace screening, and that was after prolonged exposure in an unventilated room. The scenario was deliberately extreme.
If you spent a few minutes around someone smoking at a party, in a park, or in a room with open windows, your odds of failing a standard drug test are very low. But if you’re regularly exposed in a small, poorly ventilated space, especially for extended periods, trace amounts of THC can accumulate in your system. For anyone facing zero-tolerance drug testing, that’s worth knowing.
Risks for Children
Children are more vulnerable to secondhand cannabis smoke for the same reasons they’re more vulnerable to tobacco smoke: smaller lungs, faster breathing rates, and developing brains. The CDC notes that THC can be passed to infants and children through secondhand smoke and that exposed children can experience psychoactive effects, including feeling high.
Studies have found strong associations between having a cannabis user in the home and children having detectable THC levels in their bodies. The long-term implications of chronic low-level THC exposure during brain development aren’t fully mapped out yet, but the presence of a psychoactive substance in a child’s bloodstream is a concern on its own. Smoking outdoors and away from children significantly reduces this exposure.
Risks for Pets
Dogs and cats are more sensitive to THC than humans. While most marijuana toxicity cases in pets involve ingestion (eating an edible or a discarded joint), inhalation can also produce symptoms. According to veterinary researchers at Tufts University, the most commonly reported signs include urinary incontinence, disorientation, a drunken or unsteady gait, lethargy, and low heart rate. Most mild to moderate cases resolve fully, but severely affected animals can develop seizures, respiratory depression, or even fall into a comatose state. Veterinary reports of fatalities from marijuana exposure are increasing.
Small pets in enclosed spaces are at the highest risk. If you smoke indoors around a small dog or cat, the concentration of smoke they’re breathing relative to their body size is much greater than what you experience.
Vaping vs. Smoking: Not Necessarily Safer for Bystanders
Many people assume that vaping cannabis produces less secondhand exposure than smoking a joint. The aerosol from a cannabis vape pen does differ from combusted smoke in some ways, but research measuring fine particulate matter (the tiny particles that penetrate deep into the lungs) found that cannabis vaping produces two to six times more particulates per puff than a tobacco cigarette. The aerosol dissipates faster and may contain fewer combustion-specific toxins, but the particle load bystanders inhale is not trivial.
How Ventilation Changes Everything
Across nearly every study on this topic, ventilation is the single biggest factor determining how much secondhand exposure actually reaches bystanders. In sealed, unventilated rooms, non-smokers absorb measurable THC, experience mild cognitive impairment, and show cardiovascular changes. In ventilated spaces, these effects shrink to near zero. Outdoors, exposure drops further still.
If you’re living with someone who smokes cannabis and you want to minimize your exposure, the most effective steps are straightforward: smoking should happen outdoors, windows should stay open, and shared enclosed spaces like cars and small apartments need active airflow. For anyone with asthma, heart conditions, or developing lungs, even brief exposure in a closed room carries real, measurable health consequences.