Yes, secondhand weed smoke can make you feel sick, especially in poorly ventilated spaces. The effects range from mild drowsiness and increased heart rate to longer-lasting impacts on blood vessel function. How much it affects you depends largely on airflow, how long you’re exposed, and how much smoke is in the air.
What Secondhand Weed Smoke Does to Your Body
When non-smokers sit in a room with cannabis smoke and no ventilation, their bodies absorb enough THC to produce measurable changes. In a controlled study published in Drug and Alcohol Dependence, non-smokers exposed to secondhand cannabis smoke in an unventilated room experienced a small but significant jump in heart rate, from about 66 beats per minute to nearly 72. They also reported feeling sedated, hungry, and less alert during the hour after exposure. Their performance on a cognitive task that measures processing speed and attention also dropped.
These effects mirror a mild version of what the smoker experiences: drowsiness, brain fog, and that classic “munchies” sensation. In a well-ventilated room, these effects largely disappear, which tells you that airflow is the single biggest factor in whether secondhand exposure actually affects you.
Blood Vessel Damage Happens Fast
The most striking finding from recent research isn’t about getting a contact high. It’s about what happens to your blood vessels. Just one minute of exposure to secondhand marijuana smoke impairs your blood vessels’ ability to dilate normally, a function that’s essential for healthy circulation. This impairment is comparable to what happens with secondhand tobacco smoke, but with one key difference: recovery takes much longer with cannabis smoke.
After tobacco smoke exposure, blood vessel function typically bounces back within about 30 minutes. After marijuana smoke exposure, blood vessels remained impaired for at least 90 minutes in a study from the Journal of the American Heart Association. The researchers found this effect was independent of THC itself, meaning it’s driven by the smoke particles and chemicals, not the drug. Cannabis smoke contains many of the same toxic and cancer-causing compounds found in tobacco smoke, and according to the CDC, some of those chemicals are present in higher concentrations in cannabis smoke.
For someone with existing heart disease, asthma, or circulation problems, this is worth taking seriously. Even brief exposure in an enclosed space puts temporary stress on the cardiovascular system.
The Contact High Is Real (in the Right Conditions)
The “contact high” from secondhand weed smoke isn’t a myth, but it requires specific conditions to happen. In a ventilated room, non-smokers generally don’t absorb enough THC to feel any psychoactive effects. In an unventilated, smoky room, it’s a different story. Non-smokers in that setting reported noticeable drug effects, including feeling pleasant sedation that lasted about an hour.
The key variables are room size, number of people smoking, potency of the cannabis, length of time you’re in the room, and whether windows or fans are moving air around. A quick walk past someone smoking outdoors is unlikely to produce any noticeable effect. Sitting in a closed car or small apartment with active smokers for an extended period is a very different level of exposure.
Can It Make You Fail a Drug Test?
This is one of the most common concerns, and the answer is mostly reassuring. At the standard cutoff used by federal workplace drug testing (50 ng/mL), secondhand exposure almost never triggers a positive result. In a study that put non-smokers in extreme smoke exposure conditions, only a single specimen out of hundreds tested positive at the 50 ng/mL threshold, a 0.4% positivity rate.
The picture changes at lower cutoff levels. At 20 ng/mL, which some employers and testing programs use, multiple positive results appeared across exposure sessions. So if you’re facing a drug test with a lower threshold, heavy secondhand exposure in an enclosed space could theoretically cause a problem, though it would take unusually intense, prolonged exposure.
For the vast majority of people worried about a standard workplace or pre-employment screening, casual secondhand exposure is very unlikely to cause a positive result.
Respiratory Symptoms and Irritation
Beyond the THC-related effects, cannabis smoke is still smoke. It contains particulate matter, carbon monoxide, and irritants that affect the lungs and airways the same way any combustion smoke does. If you’re exposed in an enclosed space, you may experience coughing, throat irritation, eye burning, or headaches. People with asthma or other respiratory conditions are particularly vulnerable and may notice wheezing or tightness in the chest.
These symptoms aren’t unique to cannabis. They happen with any kind of smoke inhalation. But because cannabis sessions often last longer than a single cigarette and tend to happen indoors, cumulative exposure during one sitting can be significant.
Risks for Children and Pets
Children are more vulnerable to secondhand cannabis smoke because of their smaller body size, faster breathing rate, and developing organs. Their systems process THC differently than adults, and even passive exposure can potentially cause sedation or other effects that are disproportionate to what an adult would experience in the same environment.
Dogs are especially sensitive to THC. Symptoms of cannabis exposure in dogs can appear within 30 minutes and last up to 72 hours. Signs include difficulty walking, extreme lethargy, a dazed or disoriented look, dilated or glassy eyes, vomiting, drooling, and urinary incontinence. Some dogs also develop tremors, agitation, or increased sensitivity to sounds and touch. In severe cases, seizures or coma can occur. If your dog shows these signs after being around cannabis smoke, veterinary care is important because the sedation can set in quickly enough to make home remedies like inducing vomiting dangerous.
Cats are also at risk, though dogs account for the majority of reported cases, partly because they’re more likely to eat cannabis products in addition to inhaling smoke.
How to Reduce Your Exposure
Ventilation is the most effective way to minimize the effects of secondhand cannabis smoke. Research consistently shows that the same exposure that produces measurable THC absorption, cognitive effects, and elevated heart rate in an unventilated room produces little to no effect when air is circulating. Opening windows, using fans, or simply moving to a different room makes a substantial difference.
If you live with someone who smokes cannabis, asking them to smoke outdoors or in a well-ventilated area protects both you and any children or pets in the home. Air purifiers with HEPA filters can help with particulate matter but won’t eliminate gaseous compounds. The simplest and most reliable approach is physical distance and fresh air.