Higher exhaled carbon monoxide was linked to lower Parkinson risk even in never-smokers

Design

Prospective nationwide cohort (China Kadoorie Biobank)

Population

512,701 adults aged 30 to 79 at recruitment, 10 regions of China

Primary outcome

Incident Parkinson disease over about 12 years

Effect

Regular smoking HR 0.70 (95% CI 0.62 to 0.79); never-smokers with exhaled CO 3 to 5 ppm HR 0.62 (0.53 to 0.73)

The China Kadoorie Biobank followed 512,701 adults for about 12 years; 1,131 developed Parkinson disease.

Regular smoking was associated with higher risks of lung cancer, heart disease, stroke and death, but a lower risk of Parkinson disease (HR 0.70, 95% CI 0.62 to 0.79). Among never-smokers, higher exhaled carbon monoxide was associated with lower Parkinson risk in a broadly dose-dependent way: HR 0.62 at 3 to 5 ppm and 0.65 at 11.5 ppm or more, against under 2 ppm. Carbon monoxide was not associated with other neurodegenerative diseases.

The authors suggest carbon monoxide may itself be protective, which would explain the long-observed smoking-Parkinson link. That is a hypothesis for the trials already under way, not a reason to change anything in clinic.

• Do not present smoking as protective against Parkinson disease; its other harms are far larger.
• Advise smoking cessation in people with or at risk of Parkinson disease as for anyone else.
• Be aware that carbon monoxide is being studied as a possible treatment; this is research-stage.
• When patients ask about the smoking-Parkinson link, explain it is an association with a possible mechanism, not a recommendation.

Why it matters

It offers a mechanism for one of the most persistent associations in Parkinson epidemiology.

Don't overread it

Observational; it cannot show carbon monoxide prevents Parkinson disease.

The statistics, in plain English

Hazard ratios of 0.62 to 0.68 suggest about a third lower risk at higher carbon monoxide levels. In an observational cohort this can reflect other differences between people, although adjustment for solid fuel use and passive smoking reduces that concern.

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