Why Smokers Have a Lower Risk of Parkinson's: Scientists Discover a Surprising Clue in Carbon Monoxide
For decades, scientists have wrestled with one of the strangest findings in neurological research.
People who smoke cigarettes—a habit responsible for enormous amounts of preventable disease and premature death—have consistently shown a lower incidence of Parkinson's disease in many observational studies.
The finding seems almost impossible to reconcile with everything we know about tobacco.
Smoking damages blood vessels, increases cancer risk, contributes to heart disease, and exposes the body to thousands of harmful chemicals.
Yet when researchers examine populations over time, smokers often appear less likely to receive a Parkinson's diagnosis than people who have never smoked.
Why?
For years, nicotine was one possible explanation.
But a major study published in JAMA Neurology on September 8, 2026, has drawn attention to another candidate.
Carbon monoxide—the invisible, potentially deadly gas found in cigarette smoke.
Researchers from the University of Oxford and collaborating Chinese institutions followed more than half a million adults and discovered that even among people who had never smoked, higher levels of carbon monoxide in their breath were associated with a lower risk of developing Parkinson's disease.
The result is striking because carbon monoxide is not exclusively a product of smoke or pollution.
The human body also produces extremely small amounts of it naturally.
At carefully regulated physiological concentrations, carbon monoxide participates in cellular signaling pathways.
And previous laboratory experiments suggest that manipulating some of those pathways might help protect nerve cells from damage.
Could a gas best known for causing accidental poisoning also hold clues to future Parkinson's treatments?
The new evidence makes that possibility worth investigating.
But there is an essential warning:
The study has not proved that carbon monoxide prevents Parkinson's disease. It certainly does not establish that smoking, inhaling carbon monoxide, or deliberately increasing exposure to exhaust fumes is beneficial.
What researchers have identified is an intriguing association that could help guide safer, carefully controlled medical research.
The Study That Sparked the Discovery
These figures describe an observational cohort and adjusted relative-risk estimates, not a controlled experiment establishing prevention. JAMA Network
The study, titled Smoking, Exhaled Carbon Monoxide, and Risk of Parkinson Disease, was led by Clara Bueno Lopez and colleagues.
Researchers used data from the China Kadoorie Biobank, a large long-running health study involving adults from ten geographically diverse areas of China.
Participants were recruited between June 2004 and July 2008.
At baseline, researchers recorded information about smoking habits and measured the carbon monoxide concentration in exhaled breath.
They subsequently used health and mortality records to identify new diagnoses of Parkinson's disease and other conditions.
During approximately 12 years of follow-up, the researchers recorded 1,131 Parkinson's cases and 2,949 cases of other specified neurodegenerative conditions.
The long follow-up period and large population made it possible to investigate relationships that would be difficult to detect in a small study.
The central findings were unusual enough to challenge existing assumptions about the relationship between tobacco use and neurological disease.
Why Have Smokers Consistently Shown Lower Parkinson's Risk?
The inverse association between smoking and Parkinson's disease is not new.
Epidemiologists have documented it across multiple populations for decades.
In the 2026 analysis, regular smokers had an adjusted hazard ratio of 0.70 compared with people who had never smoked regularly.
In ordinary language, that corresponds to an approximately 30% lower relative rate of Parkinson's diagnosis during the study period.
But this is not the same as showing that cigarettes directly prevented the disease.
The Difference Between Association and Protection
An observational study can identify patterns among groups of people.
It cannot automatically establish what causes those patterns.
For example, scientists have considered whether individuals in the early stages of Parkinson's disease might be less inclined toward smoking or nicotine use because of changes in brain systems involved in reward and motivation.
Other factors may influence both smoking behavior and Parkinson's diagnosis.
Researchers attempt to account for such explanations through statistical adjustments and additional analyses.
But uncertainty remains.
That is why a finding such as a lower observed risk among smokers should not be converted into a recommendation to smoke.
Why Scientists Looked Beyond Nicotine
Nicotine interacts with receptors in the nervous system and has long attracted interest in Parkinson's research.
However, cigarettes contain many biologically active compounds.
The Oxford researchers investigated whether carbon monoxide might help explain part of the observed relationship.
Their reasoning was based on an unusual fact about human biology: carbon monoxide is not solely an environmental poison.
It also exists as a naturally produced signaling molecule inside the body.
And research in laboratory animals had already raised the possibility that carefully controlled carbon monoxide-related pathways might influence processes associated with neurodegeneration.
The Surprising Discovery Among People Who Never Smoked
This was the most important part of the new study.
Among people who had never smoked, the researchers compared Parkinson's risk across different levels of carbon monoxide measured in exhaled breath.
The association remained even after statistical adjustments for factors including season, time of day, solid-fuel use, and passive smoking exposure.
When the never-smoking participants were divided using a threshold of three parts per million, those with higher measured exhaled CO had an adjusted hazard ratio of approximately 0.71 for Parkinson's disease compared with those below that level.
That corresponds to about a 29% lower relative hazard.
And some comparisons across higher carbon monoxide categories suggested reductions approaching one-third or more.
What the Results Looked Like
Parkinson's risk association among never-smokers
Adjusted hazard ratios by baseline exhaled CO concentration. Reference group = 1.00; lower values indicate lower observed relative hazard.
The association was broadly graded, although it was not perfectly linear.
Importantly, researchers did not observe the same clear association for the other neurodegenerative conditions they examined.
That possible specificity strengthened their interest in whether carbon monoxide-related biological mechanisms deserve further investigation.
But it did not eliminate alternative explanations.
Carbon Monoxide: How Can a Poison Also Be a Signaling Molecule?
Carbon monoxide has an understandably frightening reputation.
It is a colorless, odorless gas capable of causing serious poisoning and death.
When inhaled, it binds to hemoglobin, interfering with oxygen delivery and contributing to cellular injury.
That is why carbon monoxide from faulty heaters, generators, fires, engines, and other combustion sources is dangerous.
However, the body also generates tiny quantities of carbon monoxide during normal biological processes.
The Body Produces Carbon Monoxide Naturally
One source involves the breakdown of heme, a component of hemoglobin and other proteins.
Enzymes known as heme oxygenases help break down heme, generating several products, including small amounts of carbon monoxide.
Scientists have investigated whether endogenous carbon monoxide can participate in cell signaling and influence inflammation, oxidative stress, and blood-vessel function.
This is a reminder that biological effects often depend on context and concentration.
A compound can be part of normal physiological signaling while becoming highly toxic at greater exposures.
That does not make environmental carbon monoxide safe.
The distinction is between tightly regulated biological chemistry and uncontrolled external exposure.
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Could Carbon Monoxide Help Protect Brain Cells?
Parkinson's disease involves progressive changes in brain networks responsible for movement and other functions.
One important feature is the loss of dopamine-producing neurons in a region called the substantia nigra.
Scientists also investigate processes involving alpha-synuclein accumulation, oxidative stress, mitochondrial dysfunction, and inflammation.
These processes do not explain every aspect of Parkinson's, but they provide important targets for therapeutic research.
What Earlier Animal Studies Found
In August 2024, scientists affiliated with Massachusetts General Hospital and Harvard Medical School published research in npj Parkinson's Disease exploring low-dose carbon monoxide in rodent models.
The researchers found that controlled administration reduced measures of neurodegeneration and alpha-synuclein pathology under the experimental conditions.
They also observed activation of pathways involving heme oxygenase-1, a protein linked with cellular responses to stress.
These results provided a possible biological explanation for the association seen in epidemiological studies.
But there is a substantial difference between observing protective effects in animal models and proving a safe, effective treatment in humans.
The Role of Oxidative Stress
Oxidative stress occurs when the production of reactive molecules exceeds the body's capacity to manage their effects.
These molecules can damage proteins, lipids, and other cellular structures.
Oxidative stress is one of several processes implicated in Parkinson's disease.
Researchers are interested in whether specific carbon monoxide-related signaling pathways might improve cellular stress responses.
However, the same gas can cause severe injury when exposure is uncontrolled.
Any future treatment would require a precisely engineered delivery method, rigorous dose assessment, and careful safety monitoring.
A Carbon Monoxide Drug Is Already Being Studied in People With Parkinson's
The research has progressed beyond laboratory models.
A clinical trial registered as NCT07005180, called LoCaMoTE-PD, is investigating an experimental oral low-dose carbon monoxide product known as HBI-002.
According to its ClinicalTrials.gov registration, the study is a phase 2a, randomized, placebo-controlled trial sponsored by Hillhurst Biopharmaceuticals.
Its planned enrollment is 36 people with Parkinson's disease.
Participants receive the investigational product or placebo under controlled research conditions, with researchers monitoring safety, tolerability, drug exposure, and biological markers.
The trial is designed to evaluate an experimental treatment, not to establish that people should use carbon monoxide to prevent Parkinson's.
As of the latest official record examined, no trial results had been posted.
Is This a Harvard Clinical Trial?
There is a connection to researchers at Harvard Medical School and Massachusetts General Hospital through the earlier preclinical research.
But the registered HBI-002 trial is sponsored by Hillhurst Biopharmaceuticals, with the Parkinson Study Group listed as a collaborator.
It would therefore be misleading to describe the clinical trial simply as a Harvard-run study.
The scientific pathway is better understood as a progression from laboratory and animal research to independently organized human drug development.
What Would a Successful Trial Actually Mean?
A positive early-stage trial could establish that a particular formulation can be administered within a defined protocol with an acceptable safety profile.
It might also reveal useful changes in biological markers.
But that would not automatically prove that the drug prevents Parkinson's disease or slows progression.
Those questions would require larger and longer clinical trials, appropriate comparison groups, and meaningful clinical outcomes.
A therapy designed to alter carbon monoxide signaling would also need to demonstrate that its benefits outweigh its potential risks.
Why This Study Does Not Make Smoking Healthy
The temptation to misunderstand the research is obvious.
If smokers develop Parkinson's less frequently, and carbon monoxide may partly explain the association, does smoking offer some hidden protection?
That conclusion would be dangerous.
The same 2026 study that reported a lower Parkinson's incidence among smokers also confirmed significantly increased risks of lung cancer, ischemic heart disease, stroke, and death from all causes.
Those findings are consistent with the overwhelming evidence that smoking causes severe long-term harm.
One Potential Benefit Cannot Cancel Numerous Proven Harms
A behavior can be associated with a reduced risk of one particular disease while substantially increasing the risk of several others.
Medical decisions cannot be based on one outcome alone.
For example, imagine a treatment that appeared to lower the risk of a rare condition but dramatically increased the risk of cancer and cardiovascular disease.
That trade-off would require careful consideration.
With cigarette smoking, the harms are already well established.
The observational association with Parkinson's does not reverse them.
The appropriate response to this research is to investigate potentially useful biological pathways—not to encourage tobacco use.
Secondhand Smoke Is Not a Parkinson's Prevention Strategy Either
Some readers may wonder whether exposure to secondhand smoke could provide a smaller, supposedly protective effect.
There is no established evidence supporting that approach.
In the 2026 analysis, passive smoking among never-smokers did not show a clear association with reduced Parkinson's risk.
And secondhand smoke carries well-documented health risks.
There is no medical justification for intentionally exposing yourself or others to cigarette smoke in an attempt to prevent Parkinson's disease.
Why Deliberately Inhaling Carbon Monoxide Would Be Extremely Dangerous
Carbon monoxide poisoning can cause headache, dizziness, nausea, confusion, loss of consciousness, neurological injury, and death.
The gas is particularly dangerous because it has no smell or color.
People may be exposed without realizing it.
Even if future medicines successfully use carbon monoxide-related pathways, those medicines would not be equivalent to inhaling smoke or exhaust.
A pharmaceutical formulation is designed around controlled dosing, carefully selected participants, monitoring, and defined safety limits.
Environmental exposure is unpredictable.
Breath Carbon Monoxide Is Not a Dosage Recommendation
The researchers measured carbon monoxide in participants' exhaled breath.
Those measurements were used as an indicator for epidemiological analysis.
They were not intended to identify an ideal concentration that people should attempt to reproduce.
Exhaled carbon monoxide can reflect multiple sources and processes, including environmental exposure and internal production.
Higher measurements do not necessarily mean someone has a beneficial amount of carbon monoxide circulating in the brain.
The study's statistical associations cannot be transformed into a safe exposure target.
What the Researchers Still Cannot Explain
The Oxford team's findings are compelling, but several uncertainties remain.
1. Carbon Monoxide May Be a Marker Rather Than the Cause
The amount of carbon monoxide someone exhales could be associated with an underlying physiological process.
That process might influence Parkinson's risk independently.
For example, differences in heme metabolism, inflammation, or other biological characteristics could affect both breath measurements and neurological health.
In that scenario, carbon monoxide might be a useful clue without being the protective agent itself.
2. Environmental Exposure Was Not Fully Measured
The researchers accounted for several potential sources of exposure, including solid-fuel use and passive smoking.
However, they acknowledged that they could not completely distinguish carbon monoxide produced naturally inside the body from that originating in the environment.
This is a significant limitation.
Two people with similar breath readings could have reached those measurements through different combinations of physiological and environmental factors.
Those differences might matter for interpreting the association.
3. Breath Measurements Were Taken at Baseline
Participants had carbon monoxide measured when they entered the study.
Their exposure and physiology could have changed over the following years.
A single measurement cannot perfectly represent long-term biological conditions.
Repeated measurements in future research could help establish whether sustained differences are associated with Parkinson's risk.
4. The Findings Need Replication
The study involved a very large population in China.
That is a substantial strength.
But results from one population may not apply identically to other populations with different environmental exposures, smoking patterns, genetics, and healthcare systems.
Independent replication would increase confidence in the findings.
5. The Nicotine Hypothesis Has Not Been Ruled Out
The researchers specifically noted that they could not exclude possible contributions from nicotine or other tobacco constituents.
Carbon monoxide is a new and promising candidate explanation.
It is not a confirmed replacement for every competing hypothesis.
These limitations are acknowledged in the original JAMA Neurology paper.
What Would It Take to Prove That Carbon Monoxide Protects Against Parkinson's?
The strongest evidence would come from carefully controlled clinical studies demonstrating that a specific intervention improves meaningful health outcomes.
Researchers would need to establish that the treatment is safe, produces a reproducible biological effect, and leads to measurable clinical benefits.
For prevention, such studies would be particularly demanding because Parkinson's disease develops over many years and does not affect everyone exposed to a suspected risk factor.
For treatment, investigators might examine changes in symptoms, disease progression, imaging findings, or validated biological markers.
A well-designed trial would also need to distinguish genuine treatment effects from changes caused by other factors.
Until those studies are completed, carbon monoxide remains an investigational avenue rather than a proven Parkinson's therapy.
Why the Discovery Matters Even If Carbon Monoxide Isn't the Final Answer
Scientific discoveries do not always lead directly to treatments.
Sometimes their greatest value is revealing a biological pathway that researchers previously overlooked.
The association between smoking and Parkinson's disease has been recognized for decades.
But explaining it has remained difficult.
The new study offers a testable possibility.
Perhaps carbon monoxide itself influences relevant cellular processes.
Perhaps it is a marker of another protective mechanism.
Or perhaps multiple biological and behavioral factors contribute to the overall association.
Each possibility can guide new experiments.
Even if carbon monoxide ultimately proves unsuitable as a medication, researchers could potentially identify safer ways to influence related signaling pathways.
That is an important distinction between investigating a mechanism and recommending exposure to a hazardous substance.
Hasyin Iqbal
Frequently Asked Questions
Do smokers really have a 30% lower risk of Parkinson's disease?
Several observational studies have reported an inverse association. In the 2026 JAMA Neurology study, regular smoking was associated with an approximately 30% lower relative hazard of Parkinson's disease. This does not prove smoking prevents Parkinson's, and smoking substantially increases other serious health risks.
Can carbon monoxide protect the brain?
Laboratory and animal studies suggest that carefully controlled, low-dose carbon monoxide-related signaling may affect inflammation, oxidative stress, and neurodegenerative processes. However, protective effects have not been established in humans with Parkinson's disease.
Does the human body naturally produce carbon monoxide?
Yes. Small quantities are produced during normal processes such as heme breakdown. At physiological levels, carbon monoxide participates in cellular signaling. This does not mean inhaled environmental carbon monoxide is safe.
Is there a carbon monoxide drug for Parkinson's?
An experimental oral product called HBI-002 is undergoing clinical investigation. It is not an established or approved Parkinson's treatment, and the registered trial has not yet reported results demonstrating effectiveness.
Should people smoke or expose themselves to carbon monoxide to avoid Parkinson's?
No. Smoking causes cancer, cardiovascular disease, respiratory illness, and other serious harms. Carbon monoxide poisoning can cause permanent injury or death. Neither is a safe or medically supported Parkinson's prevention strategy.
The Paradox That Could Open a New Direction in Parkinson's Research
For decades, scientists have faced an uncomfortable contradiction.
Smoking is undeniably harmful.
Yet people who smoke have repeatedly shown lower rates of Parkinson's disease in epidemiological research.
Now, a major study involving more than half a million adults has identified a possible clue.
The association does not appear to be limited to tobacco users.
Even among people who have never smoked, higher measured levels of exhaled carbon monoxide were associated with lower Parkinson's risk.
That finding has prompted renewed interest in a gas more commonly associated with poisoning.
And earlier laboratory studies provide a plausible reason to continue investigating it.
At carefully controlled levels, carbon monoxide-related biological signaling may influence cellular stress responses and processes involved in neurodegeneration.
But an intriguing biological mechanism is not the same as a proven medical intervention.
The research has not established that carbon monoxide directly prevents Parkinson's.
It has not identified a safe environmental exposure level.
And it provides no justification for smoking or deliberately inhaling the gas.
What it does provide is a valuable scientific lead.
A possible explanation for a long-standing epidemiological mystery.
A direction for further laboratory experiments.
And a rationale for carefully monitored clinical trials.
The most important lesson is not that smoking might secretly be good for the brain. It is that studying an unexpected association can reveal biological mechanisms that scientists may eventually be able to target without reproducing the harmful exposure that first drew attention to them.
Sometimes, medical progress begins with a contradiction.
The challenge is discovering what the contradiction means—and turning that understanding into something genuinely safe and useful.
For Parkinson's researchers, the carbon monoxide hypothesis may represent one more step toward that goal.
Scientific Sources and Further Reading
- JAMA Neurology — Smoking, Exhaled Carbon Monoxide, and Risk of Parkinson Disease (2026). Original peer-reviewed research by Bueno Lopez and colleagues, published September 8, 2026.
- Original Study DOI — 10.1001/jamaneurol.2026.3037. Permanent scholarly reference.
- The BMJ — Parkinson's, Smoking, and Carbon Monoxide (September 2026). Independent medical reporting and context.
- npj Parkinson's Disease — Neuroprotection of Low-Dose Carbon Monoxide in Parkinson's Models (2024). Preclinical research exploring biological mechanisms.
- ClinicalTrials.gov — HBI-002 Low-Dose Carbon Monoxide Trial. Official clinical trial registration, including design, safety objectives, and recruitment information.
- CDC — Carbon Monoxide Poisoning. Public-health information about the dangers of carbon monoxide exposure.
