Regular smokers had a 30 per cent lower risk of Parkinson's disease in a study of more than 500,000 people, while never-smokers with higher exhaled carbon monoxide levels showed a similar association. The finding is raising new questions about why.

The result does not mean smoking prevents Parkinson's. Instead, researchers are examining whether something in tobacco smoke, including carbon monoxide, could help explain a link seen in studies spanning decades.

That distinction matters because smoking remains a major cause of preventable disease and death, despite the unusual Parkinson's association reported across observational studies for years.

The latest research points to carbon monoxide as one possible clue, while earlier studies have explored tobacco compounds and nicotine. Understanding the biology could eventually help researchers identify targets for treatment or prevention.

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Carbon Monoxide Offers a Clue

Researchers from the University of Oxford and institutions in China analysed 512,701 adults in the China Kadoorie Biobank. The study was published online in JAMA Neurology on September 8, 2026.

Among never-smokers, those with exhaled carbon monoxide levels of at least 3 parts per million had a 30 per cent lower Parkinson's risk. The study was observational and cannot prove carbon monoxide prevents disease.

Researchers adjusted for several lifestyle and environmental factors. The findings show an association, not proof of protection.

Dr Michael Okun, a neurologist and movement-disorders specialist at the University of Florida, said carbon monoxide is usually considered a poison.

The finding adds to decades of research linking smoking with lower Parkinson's risk. A 65-year follow-up of 30,000 male British doctors, published in Neurology in 2020, found a lower risk among smokers.

That study recorded 283 deaths from Parkinson's disease and reported that the inverse association strengthened with greater smoking exposure. It also found that the association weakened as time since quitting increased.

Tobacco Compounds Under Study

Research has also examined tobacco without cigarette smoke. A 2017 study in the American Journal of Epidemiology followed 20,175 people in Sweden and found lower Parkinson's risk among male users of Swedish moist snuff, or snus.

A 2005 study of 95,981 never-smoking men found an inverse association between smokeless-tobacco use and Parkinson's disease mortality. Researchers reported an age-adjusted risk of 0.22 compared with never-users.

Researchers are therefore examining whether a tobacco component, rather than smoking itself, could be involved.

Nicotine is another proposed explanation. Experimental studies have suggested that nicotine can affect dopamine signalling and may protect dopamine-producing neurons from damage.

However, human trials have not established nicotine as a neuroprotective treatment. The American Parkinson Disease Association notes that nicotine-patch trials have not produced convincing evidence for such a benefit.

Weighing Risk Against Reward

Parkinson's disease is a progressive neurodegenerative disorder. Common symptoms include tremor, slowed movement, stiffness and problems with gait and balance.

About 1.1 million people in the US are estimated to live with Parkinson's, with the number expected to reach 1.2 million by 2030, according to the Parkinson's Foundation.

That burden contrasts sharply with smoking's harms. The US Centers for Disease Control and Prevention says cigarette smoking and secondhand smoke cause more than 480,000 deaths in the US each year.

The new research does not suggest that people should smoke. Instead, it offers a possible clue about why smoking has repeatedly been associated with lower Parkinson's risk.

The JAMA Neurology researchers said the findings provide epidemiological evidence supporting a possible role for carbon monoxide in Parkinson's disease. Further research is needed to determine whether the association reflects a biological effect or other factors.

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