What Does the Research Actually Say About Secondhand Cannabis Smoke?

In 2025, a survey by the American Lung Association found that 43 percent of Americans living in legal cannabis states reported being exposed to secondhand cannabis smoke at least once per month. In multi-unit housing, the figure was 61 percent. As cannabis consumption has normalized, the question of what that exposure means for bystanders has shifted from theoretical curiosity to genuine public health concern.

The politics of this topic are fraught. Cannabis advocates tend to minimize secondhand smoke concerns, wary that they will be used to justify restrictive public consumption laws. Anti-cannabis groups tend to amplify them, sometimes beyond what the evidence supports. The truth, as usual, is more nuanced than either side finds convenient.

This article attempts to present the current state of the science on secondhand cannabis smoke as accurately as possible — what we know with reasonable confidence, what the emerging evidence suggests, and where significant gaps remain.

What Is in Cannabis Smoke?

The starting point for any secondhand smoke assessment is the composition of the smoke itself. Cannabis combustion produces a complex mixture of gases, particulate matter, and chemical compounds. A 2024 comprehensive analysis published in Chemical Research in Toxicology identified over 4,700 distinct chemical compounds in cannabis smoke, compared to approximately 7,000 in tobacco smoke.

The overlap between the two is substantial. Cannabis and tobacco smoke share many of the same toxic combustion byproducts, including:

• Polycyclic aromatic hydrocarbons (PAHs): Known carcinogens produced by the incomplete combustion of organic matter. Cannabis smoke contains comparable levels of PAHs to tobacco smoke on a per-joint-versus-per-cigarette basis.
• Carbon monoxide: Cannabis smoke contains carbon monoxide at levels similar to or slightly higher than tobacco smoke, partly because cannabis joints typically lack filters.
• Ammonia: Cannabis smoke contains approximately 20 times more ammonia than tobacco smoke, a finding that has been replicated across multiple studies.
• Hydrogen cyanide: Present in cannabis smoke at levels comparable to tobacco smoke.
• Fine particulate matter (PM2.5): Cannabis smoke generates significant PM2.5 — the particle size most associated with respiratory and cardiovascular harm. Indoor air quality studies have found that smoking a single cannabis joint in an enclosed room can elevate PM2.5 levels to 3-5 times the EPA’s outdoor air quality threshold.

Cannabis smoke also contains compounds absent from tobacco smoke, most notably cannabinoids (THC, CBD, and others) and cannabis-specific terpenes. The presence of THC in secondhand smoke is significant because it raises questions about psychoactive exposure, impairment, and drug testing that do not apply to tobacco.

Cardiovascular Effects

The most alarming secondhand cannabis smoke research to date concerns cardiovascular effects. A series of studies led by Dr. Matthew Springer at the University of California, San Francisco has produced findings that deserve serious attention.

In a 2023 study published in the Journal of the American Heart Association, Springer’s team demonstrated that exposing rats to secondhand cannabis smoke for one minute — comparable to walking past someone smoking outdoors — impaired flow-mediated dilation (a measure of blood vessel function) by approximately 50 percent. The impairment lasted at least 90 minutes. Remarkably, the magnitude and duration of impairment were comparable to that caused by secondhand tobacco smoke exposure.

A 2025 follow-up study found that the vascular impairment from secondhand cannabis smoke occurred even when the cannabis contained no THC — indicating that the cardiovascular effects are driven by the combustion products, not the cannabinoids. This is an important finding because it means that switching to high-CBD, low-THC products does not reduce the cardiovascular risk of secondhand smoke exposure.

The mechanism appears to involve oxidative stress and endothelial dysfunction triggered by inhaled particulate matter and combustion gases. The blood vessel lining is acutely sensitive to these insults, and repeated exposure could theoretically contribute to atherosclerosis and cardiovascular disease over time.

It is important to note that these studies are conducted in controlled animal models with defined exposure parameters. Translating directly to real-world human secondhand exposure involves significant uncertainty. However, the consistency of the findings across multiple studies and the biological plausibility of the mechanism make them difficult to dismiss.

Respiratory Effects

The respiratory effects of secondhand cannabis smoke exposure are less studied than cardiovascular effects but raise similar concerns based on the known composition of the smoke.

A 2025 study in Environmental Health Perspectives measured respiratory symptoms and lung function in non-smoking adults living in multi-unit housing where cannabis smoking was common. Compared to residents of smoke-free buildings, those reporting regular secondhand cannabis exposure had significantly higher rates of coughing, wheezing, and phlegm production. Lung function (measured by spirometry) showed small but statistically significant reductions in the exposed group.

The study could not fully control for other air quality factors in multi-unit housing, and the authors acknowledged that some exposure may have been co-mingled with tobacco smoke. But the findings are consistent with what we would expect based on the particulate matter and chemical composition of cannabis smoke.

For individuals with pre-existing respiratory conditions — asthma, COPD, chronic bronchitis — the risks of secondhand cannabis smoke are likely amplified. A small 2024 study found that asthmatic individuals exposed to secondhand cannabis smoke in a controlled setting showed measurable airway inflammation (elevated exhaled nitric oxide) at exposure levels that did not affect healthy controls.

THC Exposure and Contact Highs

The “contact high” from secondhand cannabis smoke has long been considered a myth, but the reality is more nuanced than a simple yes or no.

Under normal ventilation conditions — outdoors, or in a well-ventilated indoor space — secondhand THC exposure is negligible. Studies consistently show that bystanders in these conditions do not achieve blood THC levels associated with psychoactive effects or drug test failure.

However, under extreme conditions — unventilated rooms, prolonged exposure, heavy smoking by multiple people — measurable THC transfer does occur. A 2015 Johns Hopkins study (still the most cited work on this topic) found that non-smokers confined in an unventilated room with six heavy cannabis smokers for one hour showed detectable THC in blood and urine, reported mild subjective effects, and in some cases could produce positive results on standard urine drug tests.

A 2025 update to this work used more realistic exposure conditions — a ventilated room, fewer smokers, shorter duration — and found detectable but very low THC levels in bystanders, no subjective effects, and no positive drug tests at standard cutoff levels. However, using the more sensitive cutoff levels employed by some employers and the Department of Transportation, a small number of participants did produce positive urine screens.

The practical implications are limited for most people but significant for some. If you are subject to zero-tolerance drug testing — commercial drivers, federal employees, some healthcare workers — even casual secondhand exposure in poorly ventilated spaces is a non-trivial concern.

Children and Vulnerable Populations

The most urgent secondhand cannabis smoke research concerns children, and the findings are concerning.

Pediatric emergency department data from legal states shows a marked increase in cannabis-related visits for children, driven primarily by accidental edible ingestion but also including documented cases of secondhand smoke exposure producing symptoms in infants and young children. These cases are rare but real.

A 2025 study in Pediatrics measured urinary THC metabolites in children living in homes where adults smoked cannabis. Among children in homes with daily cannabis smoking, 47 percent had detectable urinary THC metabolites. The levels were low and not associated with psychoactive effects, but the presence of cannabinoids in these children’s systems confirms meaningful exposure.

More broadly, the particulate matter and toxic combustion byproducts in secondhand cannabis smoke pose the same risks to children as those in tobacco smoke — risks that are amplified in children due to their smaller airways, higher respiratory rates, and developing organ systems. As parents navigate the realities of legal cannabis, conversations about safe use around children should include attention to secondhand smoke.

Pregnant individuals represent another vulnerable population. While the effects of direct cannabis use during pregnancy are actively studied, secondhand exposure research is minimal. Given the known sensitivity of fetal development to air pollutants and the carbon monoxide content of cannabis smoke, precautionary avoidance is the prudent recommendation.

Comparison to Cannabis Vaping

An important distinction exists between secondhand exposure to cannabis smoke (from combustion) and secondhand exposure to cannabis vapor (from vaporization). Vaporizers heat cannabis to temperatures that release cannabinoids and terpenes without full combustion, producing an aerosol rather than smoke.

Studies comparing the two show substantially lower levels of combustion byproducts in vaporizer emissions. A 2024 analysis found that secondhand cannabis vapor contained 85-95 percent less particulate matter, negligible carbon monoxide, and dramatically reduced PAH levels compared to smoke from the same cannabis material.

However, secondhand vapor is not emission-free. It still contains ultrafine particles, volatile organic compounds, and, in the case of THC products, aerosolized cannabinoids. The long-term health effects of chronic secondhand vapor exposure are unknown — the technology has not been in widespread use long enough for epidemiological data to accumulate.

Policy Implications

The secondhand smoke evidence has direct implications for public policy, particularly regarding:

Multi-unit housing. The most frequent real-world secondhand cannabis exposure occurs in apartments and condominiums, where smoke migrates through shared ventilation, walls, and common areas. Several cities have enacted smoke-free housing ordinances that include cannabis alongside tobacco. The science supports this approach — the combustion products are comparable in toxicity regardless of what is being burned.

Public consumption spaces. States that have authorized cannabis consumption lounges face decisions about ventilation standards, proximity to non-consuming areas, and worker exposure limits. Tobacco-equivalent ventilation standards are a reasonable starting point, though cannabis-specific data would be valuable.

Outdoor events. The risk from brief outdoor secondhand exposure is minimal based on current evidence. Policies that treat outdoor cannabis smoke as equivalent to indoor exposure are not well-supported by the science.

What We Still Do Not Know

Significant research gaps remain. The most critical include:

• Long-term epidemiological data. We have decades of data linking secondhand tobacco smoke to cancer, heart disease, and respiratory illness. For cannabis, we have almost none. The short exposure history of legal cannabis means that population-level health effects from chronic secondhand exposure, if they exist, will not be detectable for years.
• Dose-response relationships. At what exposure levels do health effects begin? How does occasional exposure compare to daily exposure? These fundamental dose-response questions remain unanswered for most health endpoints.
• Vulnerable population studies. Research on secondhand cannabis smoke effects in children, pregnant individuals, elderly people, and those with pre-existing conditions is sparse and urgently needed.
• Real-world exposure levels. Most studies use controlled exposure chambers. Better data on actual ambient exposure levels in homes, workplaces, and public spaces would improve risk assessment.

The Bottom Line

The evidence currently available supports the following conclusions with reasonable confidence:

Secondhand cannabis smoke contains many of the same toxic combustion byproducts as tobacco smoke and causes acute cardiovascular impairment in controlled settings. The particulate matter and chemical composition of cannabis smoke are harmful to respiratory health. Children living in homes where cannabis is smoked show measurable cannabinoid exposure. Contact highs are unlikely under normal conditions but possible in extreme exposure scenarios.

What the evidence does not support is the claim that secondhand cannabis smoke is harmless. It also does not support the claim that it is equivalent to secondhand tobacco smoke in long-term health risk — we simply do not have the data to make that comparison yet.

The most defensible position, for both policymakers and individuals, is precautionary: treat secondhand cannabis smoke with the same respect you would give secondhand tobacco smoke. Do not smoke in enclosed spaces shared with non-consenting adults, children, or pets. Choose ventilated or outdoor locations. And if you want to eliminate bystander risk entirely, consider vaporization, edibles, or other smokeless consumption methods.

The science will continue to develop. In the meantime, the absence of long-term evidence is not evidence of absence of harm. It is simply an unanswered question — and one that the growing ubiquity of cannabis smoke makes increasingly important to answer.