Does smoking protect against Parkinson’s? A study of half a million people points to carbon monoxide, revealing a surprising clue in cigarette smoke

In a medical paradox, people who smoke cigarettes seem to be less prone to getting Parkinson’s disease, even though smoking is one of the top factors contributing to preventable diseases and deaths. A new study has shown that this phenomenon may not have anything to do with nicotine, but with yet another chemical compound contained in smoke, carbon monoxide.

Scientists from the University of Oxford conducted a research involving more than half a million Chinese adults and discovered that increased concentrations of carbon monoxide in breath were linked to a reduced chance of developing Parkinson’s disease, including people who have never smoked before. Their research, led by Clara Bueno Lopez and colleagues, was published on September 8, 2026 in JAMA Neurology.

Why smoking and Parkinson’s have puzzled scientists

Parkinson’s is a progressive neurodegenerative disease characterized by the death of brain cells responsible for the production of dopamine. This condition manifests in symptoms such as tremors, rigidity, slowness of movements and balance disorders and it is the second most common neurodegenerative disease worldwide.

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However, there is substantial evidence indicating an inverse association between smoking and Parkinson’s disease. An accompanying JAMA Neurology editorial notes that decades of research have identified smoking as one of the most consistently observed factors associated with lower Parkinson’s risk.

Despite the belief that nicotine could be the responsible compound due to its interactions with brain receptors, a recent randomized clinical trial has concluded that skin-applied nicotine patches were not able to slow the progression of early, untreated Parkinson’s disease, weakening the case for nicotine as the key protective compound.

What the new study found

The study involved analysis of data from 512,701 adults in the China Kadoorie Biobank. Over a period of around 12 years, 1,131 individuals in the study developed Parkinson’s disease.

The regular smokers were found to have less chance of getting Parkinson’s with the hazard ratio being 0.70 which means that their risk was 30% lower than nonsmokers. This was even after adjusting for various factors. However, smoking was still associated with high risks of developing lung cancer, heart disease, stroke and premature death.

The more interesting observation came from individuals who never smoked. As the level of carbon monoxide in the breath increased, the risk of developing Parkinson’s decreased among them. The association persisted even when other factors like secondhand smoke and solid fuel use were taken into consideration.

Could carbon monoxide protect brain cells?

It is well known that carbon monoxide is a toxic gas that can cause damage since it inhibits the delivery of oxygen in the blood. But the body also itself produces small amounts of carbon monoxide as the gas takes part in biological signaling.

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Laboratory research suggests that carbon monoxide may affect inflammation and stress, which are factors that play a role in neurodegeneration. Nevertheless, the results obtained by scientists do not show that carbon monoxide prevents Parkinson’s disease. It demonstrates only a connection that should be studied further.

The finding could lead to safer Parkinson’s treatments

The authors state that the findings bolster the rationale for studying low-dose, controlled carbon monoxide as a possible treatment rather than using smoking as a strategy for lowering the risk of developing Parkinson’s.

A low-dose carbon monoxide drug candidate named HBI-002 is currently undergoing testing in humans with Parkinson’s disease, according to the paper. Its purpose is to see if the exposure can be done in a controlled way and if it has any impact biologically or clinically.

What’s important is that the researchers stress that the findings do not justify smoking. On the contrary, their research revealed just the opposite, since when they looked at the major smoking outcomes, they found that smoking was highly associated with severe disease and death.

The next stage would be replicating the findings in other populations and conducting laboratory and clinical studies to prove if carbon monoxide has a causal relationship. Once proven, scientists could possibly isolate a biological benefit from the lethal mixture of toxins contained in cigarettes.