Long-term exposure to common air pollutants may significantly raise the risk of developing substance use disorders, while living surrounded by greenery appears to offer a measure of protection, according to a large new population-based study drawing on nearly a quarter of a million participants from the UK Biobank.

The research, published in BMC Psychiatry, analyzed data from 247,138 UK Biobank participants who were free of substance use disorders at baseline, tracking them over a median follow-up period of 10.9 years. During that time, 4,982 participants went on to develop a substance use disorder, providing the researchers with a robust dataset to examine the relationship between environmental exposures and mental health outcomes.

The study, led by Huan Liu and Bolun Cheng of Xi’an Jiaotong University in China, along with colleagues including Feng Zhang, represents one of the most comprehensive investigations to date into how the air people breathe and the greenness of their neighborhoods may influence vulnerability to addiction. The findings arrive at a moment when urban planners and public health officials are increasingly recognizing that the built and natural environments shape health in ways that extend well beyond respiratory and cardiovascular disease.

To assess air pollution exposure, the team employed Land Use Regression modeling to estimate concentrations of five pollutants at each participant’s residential address: fine particulate matter known as PM2.5, coarse particulate matter classified as PM2.5−10, the larger particulate fraction PM10, nitrogen dioxide (NO2), and nitrogen oxides (NOX). These pollutants, produced primarily by traffic, industry, and combustion sources, have long been associated with a range of physical health problems, but their role in psychiatric and behavioral conditions has remained far less certain.

For residential greenness, the researchers turned to the 2005–2007 Land Cover Map, measuring three distinct aspects of the environment within a 300-meter buffer around each home: the proportion of green space, the presence of a domestic garden, and the extent of natural environment. This tripartite approach allowed the investigators to distinguish between different types of vegetation and their potentially different effects on health.

Using Cox proportional hazards models, the team calculated hazard ratios with 95 percent confidence intervals to quantify the association between each exposure and the risk of developing a substance use disorder. The results were striking in their consistency. All five air pollutants were positively associated with incident SUD in fully adjusted models that controlled for a wide range of potential confounders.

Fine particulate matter, or PM2.5, showed the strongest effect, with a hazard ratio of 1.111 (95 percent CI: 1.081–1.142), followed by PM10 with a hazard ratio of 1.070 (95 percent CI: 1.054–1.086). These figures indicate that higher exposure to particulate pollution was associated with a measurable increase in the likelihood of developing a substance use disorder over the follow-up period, even after accounting for demographic, socioeconomic, and behavioral factors.

On the protective side of the ledger, two of the three greenness measures emerged as significant. Green space was inversely associated with SUD risk, showing a hazard ratio of 0.997 (95 percent CI: 0.996–0.998), and the natural environment measure produced an identical finding, with a hazard ratio of 0.997 (95 percent CI: 0.995–0.998). While these effect sizes appear modest on a per-unit basis, they represent population-level signals across a very large cohort, suggesting that even incremental increases in neighborhood greenness may contribute to reduced vulnerability.

Perhaps the most intriguing finding of the study concerns the interaction between pollution and greenery. The researchers tested whether residential greenness modified the association between air pollutants and SUD risk by introducing cross-product terms into their models and evaluating them with likelihood-ratio tests. The results pointed to a buffering role for domestic gardens specifically. Among participants in the high-exposure subgroup, the presence of a domestic garden attenuated the positive associations between pollutants and SUD, with cross-product terms of 0.979 (95 percent CI: 0.963–0.996) for PM2.5 and 0.987 (95 percent CI: 0.980–0.995) for PM10.

In practical terms, this suggests that having access to a private garden may partially offset some of the harm associated with living in areas of elevated particulate pollution, offering a form of environmental resilience that public health advocates have long hypothesized but rarely demonstrated with prospective cohort data.

The associations were not uniform across the population. The positive relationships between air pollution and substance use disorder risk were most pronounced among males, participants aged 65 and older, and individuals with a history of smoking. These findings highlight the importance of considering demographic and behavioral modifiers when assessing the health consequences of environmental exposures, and they suggest that certain subgroups may benefit disproportionately from interventions aimed at reducing pollution or increasing green cover.

To ensure the robustness of their findings, the team conducted a series of sensitivity analyses. These included restricting the sample to long-term residents to minimize exposure misclassification from relocation, using alternative outcome definitions based on ICD-10 codes F10-F19 and F11-F19 to capture different ranges of substance-related diagnoses, and recomputing greenness measures at a wider 1000-meter buffer around each residence. The core conclusions held across these variations, strengthening confidence in the results.

The study was performed in accordance with the ethical principles of the Declaration of Helsinki and was registered with the UK Biobank under application number 46478. Ethical approval for the UK Biobank was granted by the North West Multi-centre Research Ethics Committee under approval reference 11/NW/0382. All participants provided written informed consent before any study-related procedures were initiated.

The work was supported by the China Postdoctoral Science Foundation, the Natural Science Basic Research Plan in Shaanxi Province of China, and the Fundamental Research Funds for the Central Universities. Corresponding authors are Bolun Cheng and Feng Zhang, both affiliated with Xi’an Jiaotong University, with Zhang also holding a position in the Department of Psychiatry at the First Affiliated Hospital of Xi’an Jiaotong University.

The findings come amid growing scientific interest in the neurological and psychiatric consequences of air pollution. Particulate matter, particularly PM2.5, is small enough to cross the blood-brain barrier and has been implicated in neuroinflammation, oxidative stress, and alterations in brain structure and function. These biological mechanisms could plausibly influence reward processing, impulse control, and stress reactivity, all of which are central to the development and perpetuation of substance use disorders.

Conversely, greenness has been associated with stress reduction, improved mental well-being, greater opportunities for physical activity and social interaction, and lower levels of psychological distress, each of which may buffer against the onset of addictive behaviors. The current study adds a significant piece of evidence to this emerging picture by demonstrating the relationship prospectively in one of the world’s largest and most deeply phenotyped biobanks.

The authors conclude that long-term exposure to ambient air pollutants was associated with a higher risk of incident substance use disorders and that residential greenness showed protective associations, with the most pronounced patterns observed among older males and individuals with a smoking history. They argue that the findings support integrating greenness exposure into environmental frameworks for the prevention of substance use disorders, a recommendation that carries implications for urban design, housing policy, and environmental regulation.

As cities around the world continue to grapple with the twin challenges of air quality and mental health, studies of this scale provide an evidence base for interventions that address both simultaneously. The prospect that something as tangible as a garden could offer measurable protection against the psychiatric consequences of polluted air adds a compelling dimension to ongoing debates about how the environments we build shape the minds that inhabit them.

The article is published open access under a Creative Commons Attribution 4.0 International License, permitting broad sharing and adaptation with appropriate credit to the original authors and source. It was received on 08 December 2025, accepted on 28 August 2026, and published on 09 September 2026.

Psilocybin therapy involves more than the compound itself, and a new research initiative at the University of Southern California is investigating the effects of combining psilocybin with structured mental training.

A team from the Keck School of Medicine of USC, working with the USC Dornsife Brain and Creativity Institute and the USC Norris Comprehensive Cancer Center, has begun the university’s first clinical trial of psychedelic therapy. The study will examine whether structured mindfulness meditation can boost the effects of psilocybin, the active ingredient in psychedelic mushrooms, on mental well-being and cognitive performance.

Funding for the trial comes from ARPA-H’s EVIDENT initiative, a federal program designed to accelerate the development of rapid-acting treatments for mental and behavioral health conditions. The study will investigate whether adding mindfulness training to psilocybin-assisted therapy yields outcomes distinct from those of psilocybin-assisted therapy alone.

A Schedule 1 Drug With a Breakthrough Label

Although the Controlled Substances Act still classifies psilocybin as a Schedule I substance, the FDA has granted it a breakthrough therapy designation for major depressive disorder and treatment-resistant depression. In clinical settings, psilocybin has shown promise in treating these conditions, and other studies suggest it may also help with addiction.

Psilocybin alters a person’s perception, mood, and thought processes, and can influence how individuals experience their senses and their sense of time and space. With therapeutic support, people often report emotionally meaningful or spiritual experiences during these altered states.

Testing Whether Mental Training Changes the Outcome

About 72 middle-aged adults from the Los Angeles area will take part in the study. All participants will have no psychiatric or medical conditions and no previous experience with either psychedelics or meditation. Researchers will assign participants to one of two groups: one will receive supervised psilocybin alone, while the other will receive psilocybin alongside an eight-week mindfulness training program designed to build attention and focus through structured meditation.

The study is based on the hypothesis that mindfulness training, which has demonstrated mental and physical health benefits independently, may enhance the effects of psilocybin-assisted therapy when used together.

“Mindfulness meditation practice provides people with the tools to deconstruct unhelpful narratives, a process that may be amplified by psilocybin-assisted therapy,” said Rael Cahn, MD, PhD, director of the USC Center for Mindfulness Science and the study’s co-lead. The trial, he said, is meant to clarify how combining the two approaches “may enhance both the potential immediate and longer-term effects of psychedelic medicine.”

An Unusually Deep Data Set

The team will collect extensive biological, neurological, and psychological data from participants throughout the study. Participants will undergo EEG scans, brain MRI and fMRI imaging, and provide saliva, blood, and stool samples. Researchers will administer psychological and cognitive assessments before and after treatment and will survey participants again at three, six, and twelve months.

The team will use the data to track changes across psychological, cognitive, neurological, and biological measures, including well-being, spirituality, brain activity, and markers of inflammation and brain health. The open-label design means both participants and researchers are aware of group assignments, which allows for more comprehensive data collection at the expense of some blinding rigor.

Through the EVIDENT initiative, researchers will contribute anonymized trial results to a national repository. The resource will help researchers identify broader patterns in rapid-acting mental health treatments.

“Psilocybin-assisted therapy has the potential to revolutionize how we approach mental health research,” said Caryn Lerman, PhD, director of the USC Norris Comprehensive Cancer Center and the study’s other co-lead. Beyond addiction, she said, there’s growing evidence the treatment could improve “quality of life and emotional well-being for people facing serious illness and end-of-life challenges.”

A Test Case for the Next Wave of Psychedelic Medicine

If mindfulness training is shown to enhance the effects of psilocybin therapy, future protocols for psychedelic treatment may incorporate mental training as an additional component of treatment. Regardless of the outcome, the study will generate one of the most detailed biological and psychological datasets from a psilocybin trial to date.

Whether mindfulness ultimately amplifies psilocybin’s effects remains an open question. By tracking participants’ brains, biology, and psychological well-being over the course of a year, the USC team hopes to generate some of the clearest data yet on how mindset and psychedelic medicine interact.

Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.

In recent years, the global landscape of cannabis policy has undergone significant transformations, stirring complex debates about the implications of legalization, decriminalization, and commercial sales on public health and societal well-being. A comprehensive study published in The Lancet Psychiatry on June 17, 2026, provides the most extensive analysis to date of international cannabis policies and their associations with cannabis use, cannabis use disorder, and other psychiatric outcomes. Led by an interdisciplinary team spearheaded by experts from the University of Bath in the UK, this systematic review draws on a wealth of data spanning across continents including the Americas, Europe, Africa, Australia, New Zealand, and Asia, elucidating the nuanced consequences of various regulatory approaches to cannabis.

The study underscores a critical distinction between policy frameworks that decriminalize or tightly regulate cannabis and those that permit large-scale commercial sales. Decriminalization typically involves removing criminal penalties for personal possession while maintaining restrictions on production and distribution. The data shows that such approaches do not generally lead to an uptick in cannabis consumption. Conversely, jurisdictions that have embraced fully commercialized cannabis markets—with broad retail availability and significant profit-driven incentives, such as many US states and Canada—experience higher consumption rates. These markets tend to propel an increase not just in use, but also in cannabis potency and the prevalence of addiction, revealing troubling public health ramifications hitherto underestimated.

Uruguay’s pioneering legalization model exemplifies a tightly regulated framework. Since its inception, Uruguay has implemented strict controls, limiting cannabis products’ availability and potency, distributing through pharmacy networks, cannabis social clubs, and permitting personal cultivation under strict rules. This structure, focused on harm minimization rather than commercial proliferation, correlates with relatively stable cannabis use rates. Such regulatory restraint contrasts sharply with the dynamics in North America, where deregulation has coincided with escalating rates of daily cannabis consumption and a worrying rise in addiction rates. Notably, in the US, the number of daily cannabis consumers now surpasses daily alcohol consumers, a landmark shift underlining the profound public health challenges precipitated by commercial legalization.

The study identifies the expansion of the for-profit cannabis industry as a pivotal factor in increasing health risks. Mirroring patterns observed historically with tobacco and alcohol industries, large-scale cannabis corporations emphasize product innovation, marketing, and aggressive sales strategies. This has catalyzed the availability of increasingly potent cannabis products, which significantly elevates the risk of dependence and the likelihood of adverse psychiatric outcomes, including psychotic disorders. Furthermore, the surge in hospital admissions due to cannabis-related psychosis in regions with commercial cannabis markets casts a serious shadow over the narrative that legalization inherently reduces harm.

In contrast, decriminalization policies appear to offer a more balanced approach, mitigating the harms associated with criminal prosecution—especially in marginalized communities disproportionately affected by enforcement—while not necessarily increasing use. The researchers highlight this as a crucial insight with implications for evolving policy debates, such as those unfolding in the United Kingdom. Here, cannabis remains a Class B controlled substance with severe penalties, although prominent calls for reform, like the 2025 London Drugs Commission report, advocate for shifting possession offenses away from the criminal justice system toward healthcare frameworks, aiming to reduce social and racial inequities.

Medical cannabis, a rapidly expanding sector, emerged as another focal concern within the study. Regulations remain highly inconsistent globally, and often poorly governed, raising critical questions about safety and therapeutic efficacy. The evidence base, as reviewed in the study, remains inconclusive or limited for many medical indications, especially psychiatric disorders. Despite increasing prescriptions of cannabinoids to treat conditions such as insomnia, autism traits, or substance use disorders, outcomes from randomized clinical trials suggest only modest benefits and lack robust evidence to support claims for efficacy in anxiety, PTSD, depression, or psychosis management. Moreover, unregulated medical cannabis access may inadvertently increase risks of harm due to insufficient clinical oversight and dosage control.

A significant aspect of the analysis relates to cannabis’ psychiatric implications beyond addiction. While daily use has been found to synergistically interact with other risk factors to elevate psychosis risk, the associations with depression, anxiety, and suicidality remain equivocal and warrant further investigation. This nuanced understanding challenges oversimplified narratives around cannabis as either categorically harmful or benign and underscores the critical need for targeted, evidence-based public health interventions.

Professor Tom Freeman, lead author from the University of Bath’s Department of Psychology, emphasizes the decisive role of policy modality over the mere presence of policy changes. He asserts that decriminalization or strict legal regulation can disentangle the harms of criminalization from those stemming from increased cannabis availability and potency. By contrast, commercialized legalization prioritizes market expansion, often at the expense of public health safeguards, exacerbating addiction, and psychiatric morbidity.

The research team also notes a geographical research gap, as much existing evidence centers on North America. They advocate for expanded studies in diverse contexts to fully capture the global ramifications of shifting cannabis policies. Such comprehensive surveillance is vital as an increasing number of countries contemplate or implement reforms, thus influencing future public health trajectories internationally.

The robustness of the conclusions stems from the study’s systematic review methodology, synthesizing findings from numerous epidemiological investigations, clinical trials, and policy analyses. This integrative approach provides a panoramic yet detailed picture of cannabis policy consequences, supporting evidence-based policymaking in a politically charged and rapidly evolving arena.

As cannabis policies continue to evolve worldwide, this landmark study serves as a clarion call for caution, nuanced regulation, and rigorous scientific evaluation. In balancing decriminalization and legal regulation against the risks posed by unbridled commercial markets, policymakers face complex trade-offs with profound implications for population health and social justice.

Subject of Research: Cannabis policies and their effects on cannabis use, addiction, and psychiatric disorders in human populations.

Article Title: International cannabis policies and their association with cannabis use, cannabis use disorder, and other psychiatric disorders published in The Lancet Psychiatry

News Publication Date: 17-Jun-2026

Web References:
The Lancet Psychiatry Article

References:
Systematic review published in The Lancet Psychiatry, 2026

Keywords: Cannabis policy, legalization, decriminalization, cannabis addiction, psychosis, psychiatric disorders, commercial cannabis markets, public health, medical cannabis, cannabis potency, substance use disorder

In a small randomized trial in the United Kingdom, adults who added a serving of 100% fruit juice or a smoothie to a healthier diet reported lower depression scores after four weeks.

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