Smoking is one of the biggest causes of ill health and early death. Yet smokers seem to have a lower risk of developing Parkinson’s disease than non-smokers – by around half according to some studies.

This surprising link was first reported more than 50 years ago, when researchers in the UK and the US noticed that smokers were dying at much higher rates overall, but were less likely to die from Parkinson’s. Many studies since then have found the same pattern.

Why smoking is linked to a lower risk of Parkinson’s is still unclear. Researchers have proposed several explanations, but there is no definitive answer yet.

Cigarette smoke contains thousands of chemicals and gases, so researchers have looked at many of them for clues. Nicotine has attracted particular interest because it affects the brain, but trials testing nicotine in people with Parkinson’s have found no evidence that it slows the disease. The search for the molecule that appears to lower the risk of Parkinson’s in smokers continues.

Half a million adults

A study my colleagues and I recently conducted provides new evidence that carbon monoxide (CO) may play a role. Laboratory and animal studies have........

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