Smokers have a lower risk of Parkinson’s disease, and new research may explain why

Story By Staff Reporter

Smoking is among the leading causes of illness and early death. Even so, some studies suggest smokers are about half as likely as non-smokers to develop Parkinson’s disease.

A puzzle that has lasted 50 years

Researchers first spotted this link more than 50 years ago. Scientists in the UK and US saw that smokers died at much higher rates overall, yet fewer of them died of Parkinson’s. Many later studies have found the same pattern.

Nobody knows why. Researchers have suggested several explanations, but none has proved right.

Cigarette smoke contains thousands of chemicals and gases, so scientists have tested many of them. Nicotine drew special interest because it acts on the brain. But trials of nicotine in people with Parkinson’s found no sign that it slows the disease. The hunt for the compound behind the lower risk goes on.

Half a million adults

My colleagues and I recently ran a study that offers new evidence for carbon monoxide (CO). Laboratory and animal research hints that small doses of CO may protect the brain. Until now, though, no large human study had paired CO measurements with later Parkinson’s diagnoses.

We used data from the China Kadoorie Biobank, which includes half a million adults from diverse backgrounds across China. When people joined the study, researchers measured the CO in their breath. This reflects their recent exposure to the gas. The team then followed participants over time and recorded who developed conditions such as Parkinson’s.

Like earlier studies, we found that regular smokers had roughly a 30% lower risk of Parkinson’s. Smokers also breathed out more CO.

The result among people who had never smoked was more striking. Those with higher exhaled CO had a lower risk of Parkinson’s. In this group, exhaled CO showed no link with other brain diseases such as dementia. It showed no link with lung cancer or other smoking-related illnesses either.

Our findings challenge the idea that smoking itself protects against Parkinson’s. Instead, they suggest CO may explain part of the lower risk in smokers. CO might also protect people whether or not they smoke. Our results do not support taking up smoking or putting off quitting, because smoking clearly harms health.

Why do some people breathe out more CO?

Some lifelong non-smokers also have raised CO in their breath. It often comes from their surroundings. Carbon-based fuels such as wood, coal and petrol release CO when they burn incompletely. Polluted outdoor air, indoor air from cooking or heating with solid fuels, and other people’s tobacco smoke can all raise the CO you breathe out.

We tried to adjust for these influences. Higher breath CO still went hand in hand with a lower risk of Parkinson’s after we accounted for region, season, solid fuel use and second-hand smoke. However, we cannot say where the remaining CO came from.

CO also comes from inside the body, because cells constantly make tiny amounts. Large amounts of CO are poisonous. At these low levels, though, CO does useful work. It shields cells from damage, dampens inflammation and supports the immune system. These effects could help explain the lower risk.

What comes next?

Our study only observed people, so it cannot prove that CO protects against Parkinson’s. CO might simply mark people who are less prone to the disease. Still, the results add weight to the idea that CO may protect the brain’s nerve cells.

Researchers now need large, well-designed human trials to learn whether CO could treat Parkinson’s. One trial of low-dose CO in people with the disease is already under way. Its findings will show how CO relates to Parkinson’s and whether it could one day treat or prevent the disease.