Study Links Carbon Monoxide From Smoking to Lower Parkinson's Risk

Parkinson's disease has long been significantly less common among people who smoke cigarettes, a paradox that has puzzled researchers for decades and fuelled speculation that tobacco smoke somehow protects the brain. A new study adds fresh evidence for a specific explanation: carbon monoxide, one of the toxic gases produced when tobacco burns.

The analysis, led by researchers at the University of Oxford, drew on a prospective cohort of more than 500,000 adults in China and was published in the journal JAMA. After confirming that chronic smoking was associated with a lower risk of Parkinson's disease, the researchers looked for a link to carbon monoxide specifically, to help explain smoking's unexpected effect.

Carbon monoxide has anti-inflammatory effects and, in the brain, helps counteract cell stress, according to Toni Pitcher, a Parkinson's epidemiology researcher at the University of Otago in New Zealand who was not involved in the study. Supporting this idea, the researchers found that among people who had never smoked, those with higher carbon monoxide levels measured on their breath also had a reduced risk of Parkinson's disease.

The evidence has real limits. Most of the non-smokers in the analysis were women, leaving room for a sex bias in the results, and the study is observational, making it hard to establish cause and effect. Researchers also stress that any reduction in Parkinson's risk is vastly outweighed by the increased risk of a large number of other health problems linked to smoking. Smoking, they note, is not the solution it appears to be.

Study authors point to a registered clinical trial that is already testing the safety of low-dose carbon monoxide in people with Parkinson's disease.