Study links carbon monoxide levels to lower risk of Parkinson's in non-smokers

A woman crosses an avenue in Beijing, China, on a day of heavy pollution. A study involving more than 500,000 Chinese adults found an association between higher levels of exhaled carbon monoxide and a lower risk of Parkinson's. — Photo: AP Photo/Andy Wong

People who have never smoked, but who had higher levels of carbon monoxide in their exhaled breath, had a lower risk of developing Parkinson's disease over approximately 12 years, according to a study involving more than 500,000 adults in China.

The finding draws attention because for decades studies have observed something seemingly contradictory: smokers usually have a lower risk of Parkinson's, even though cigarettes increase the risk of cancer, cardiovascular disease, stroke, and death.

The new analysis suggests that this relationship may involve carbon monoxide, one of the gases released in tobacco combustion, rather than cigarettes themselves.

"Among never-smokers, higher levels of exhaled carbon monoxide were associated with a lower risk of Parkinson's," the authors stated in the study.

Published in the prestigious journal JAMA Neurology this Tuesday (8th), the study followed 512,701 participants aged 30 to 79 across ten regions in China. Over the period, 1,131 cases of Parkinson's were recorded.

Among never-smokers, carbon monoxide levels of 3 parts per million or higher in exhaled breath were associated with an approximately 30% lower risk of the disease compared to levels below that value.

The relationship continued to hold even after adjusting for factors such as secondhand smoke exposure and the use of coal or wood at home.

"The novelty is that the authors directly measured exhaled carbon monoxide and found that, even among people who have never smoked, higher levels are associated with a lower risk of Parkinson's," said José A. Morales-García, a researcher at the Complutense University of Madrid, in a commentary for the Science Media Centre España.

The study is observational. This means it identifies an association between higher levels of the gas and a lower risk of Parkinson's, but it does not allow the conclusion that carbon monoxide is the cause of this reduction.

Nor does it mean that smoking brings any benefit. In the same study, habitual smoking was associated with a higher risk of lung cancer, heart disease, stroke, and death from all causes.

"These results in no way mean that smoking is beneficial or that one should smoke to prevent Parkinson's," added Morales-García.

Carbon monoxide is primarily known as a toxic gas and can cause severe poisoning at high concentrations. In small amounts, however, it is also produced naturally by the body and takes part in cellular processes.

Laboratory and animal research had already raised the hypothesis that low doses of the gas might have effects on nerve cells.

The study also has limitations. The participants were all from China, where factors such as smoking habits, pollution, and exposure to domestic fuels may differ from those observed in other populations.

For this reason, experts themselves argue that the association should be tested in other groups before broader conclusions are drawn.

Morales-García further states that the results could reinforce research into the use of very low, controlled doses of carbon monoxide in Parkinson's studies, but emphasizes that this possibility is still far from medical practice.

"It is a promising avenue, but still experimental and without any immediate clinical application," said the researcher.