Exposure to Second-Hand Smoke Linked to Lower Risk of Developing Parkinson’s, Study Suggests

Second-hand smoke may have a neuroprotective effect, as exposure to this type of indoor pollutant seems to be associated with a decreased risk of developing Parkinson’s disease, according to a recent study.

Conversely, exposure to certain air pollutants — like nitrogen dioxide and ozone — might contribute to a higher risk of developing the disease.

The study, “The impact of long-term exposure to ambient air pollution and second-hand smoke on the onset of Parkinson disease: a review and meta-analysis,” was published in Public Health.

Air pollution is composed of a variety of particulate air pollutants, volatile organic compounds, gaseous air pollutants, and airborne metals. Exposure to polluted air has been consistently associated with adverse effects in respiratory and cardiovascular diseases, but little is known about the effects of such exposure in neurodegenerative disorders such as Parkinson’s.

Although the exact causes behind Parkinson’s disease are not fully understood, it is thought to be induced by “a complicated interplay of environmental and genetic factors,” according to these researchers.

As such, “further investigation of the modifiable risk factors of [Parkinson’s disease] is of imperative significance and expected to have broad implication for the primordial prevention of this disease,” they said.

Second-hand smoke is a type of indoor air pollution. Evidence indicates that active smokers have a 50% lower chance of developing Parkinson’s in comparison with non-smokers. “However, whether this negative correlation is causal and persists among persons regularly exposed to SS [second-hand smoke] remains undetermined,” the researchers said.

To learn more, the scientists searched five medicine-related databases for any observational or epidemiological evidence on the relationship between long-term exposure to air pollution and second-hand smoke and Parkinson’s susceptibility.

The studied air pollutants included: particulate matter with an aerodynamic diameter of less than 2.5 μm (PM_2.5), such as combustion particles, organic compounds, and metals; particulate matter with less 10 μm in diameter (PM₁₀), including dust, pollen, and mold; nitrogen dioxides (NO₂); ozone (O₃); and carbon monoxide.

The researchers combined the results of 21 studies, involving a total 222,051 Parkinson’s patients, and performed a statistical review known as a meta-analysis.

A marginally significant higher risk of developing Parkinson’s disease was observed in those exposed to PM_2.5, NO₂, and O₃. Although carbon monoxide was found to be positively associated with Parkinson’s susceptibility, statistical significance was not attained.

“Second-hand smoke conferred reduced risk of Parkinson disease, regardless of exposure occasions [at home/at work/in children] and timing,” the researchers said. This suggests that second-hand smoke might have a neuroprotective effect in those who are susceptible to developing Parkinson’s at some point in their lives.

Some possible explanations exist as to why cigarette smoke may be associated with a lower risk of Parkinson’s. One theory is that some tobacco smoke compounds contain properties that inhibit monoamine oxidase (MAO), an enzyme that plays a key role in the activation of MPTP, a well-known Parkinson’s-inducing neurotoxin, and that is involved in the degradation process of dopamine released by nerve cells.

“Other hypotheses include the direct neuroprotective effect of nicotine by stimulating dopamine release, upregulating nicotinic receptors, and inhibiting alpha-synuclein fibrillation, thereby suppressing and relieving parkinsonian symptoms,” the researchers said.

Given the many harmful effects of air pollutants, public and environmental health strategies that reduce outdoor air pollution levels could help lower the burden of Parkinson’s disease.

Despite the potential neuroprotective effect of second-hand smoke, caution is advised when interpreting these results, as more large-scale studies are necessary to fully understand such an association.