Parkinson’s and smoking: Exposure to carbon monoxide is linked to lower risk of disease

Carbon monoxide could help explain the long-observed link between smoking and a lower risk of developing Parkinson’s disease, according to a major new study. Researchers found that people with higher levels of carbon monoxide in their breath were less likely to develop Parkinson’s, including people who had never smoked.

The findings could provide an important clue as scientists attempt to understand why numerous previous studies have found smokers are less likely to develop Parkinson’s disease, despite smoking substantially increasing the risk of many other serious illnesses. Researchers analysed data from 512,701 adults in China who were followed for around 12 years.

During the study period, 1,131 people developed Parkinson’s disease, while 2,949 were diagnosed with other neurodegenerative conditions. Regular smokers were found to have around a 30% lower risk of developing Parkinson’s compared with people who had never regularly smoked. However, researchers wanted to understand what might be behind the association.

Previous research has investigated whether nicotine could have a protective effect, but clinical trials of nicotine treatment have failed to demonstrate a benefit in slowing the progression of Parkinson’s.

The new research instead focused on carbon monoxide – a toxic gas found in cigarette smoke but also produced naturally in small amounts by the human body. Among people who had never smoked, researchers found higher levels of carbon monoxide in exhaled breath were associated with a lower risk of Parkinson’s. People with exhaled carbon monoxide levels of at least 3 parts per million had approximately a 30% lower risk of developing the condition compared with those whose levels were below 3 parts per million.

The association was broadly dose-dependent, meaning Parkinson’s risk tended to fall as measured carbon monoxide levels increased. Importantly, researchers did not find the same association between carbon monoxide levels and other neurodegenerative diseases. The findings suggest carbon monoxide could potentially play a role in explaining the unusual relationship between smoking and Parkinson’s disease. However, researchers stressed that the study does not show that carbon monoxide prevents Parkinson’s, and it does not establish a cause-and-effect relationship.

It also provides no reason for people to start smoking or deliberately expose themselves to carbon monoxide. Smoking was associated with increased risks of lung cancer, heart disease, stroke and death during the same research.

Carbon monoxide itself can also be extremely dangerous at high concentrations because it prevents the blood from carrying sufficient oxygen around the body and can cause serious illness or death.

Instead, scientists are interested in whether carefully controlled, very low doses of carbon monoxide could have biological effects that might eventually be harnessed therapeutically without exposing patients to the dangers associated with smoking or carbon monoxide poisoning.

Laboratory research has previously suggested low concentrations of carbon monoxide may influence processes including inflammation, oxidative stress and cellular responses to injury – all of which are of interest in Parkinson’s research.

Clinical research into the possibility has already begun. A Phase 2a clinical trial is investigating an oral low-dose carbon monoxide drug in people with Parkinson’s disease to assess its safety, tolerability and effects on biological markers.

The latest study, published in JAMA Neurology, adds evidence to the theory that carbon monoxide rather than nicotine could be one of the factors behind the consistently lower rates of Parkinson’s observed among smokers.

Researchers say further studies will now be needed to understand exactly why higher carbon monoxide levels were associated with lower Parkinson’s risk and whether that discovery could eventually lead to new approaches to treating or preventing the disease.

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