Nitrogen dioxide pollution exposure influences risk for Parkinson’s disease

An individual’s exposure to nitrogen dioxide air pollution strongly correlated with risk for Parkinson’s disease, according to findings from a retrospective cohort study of more than 78,000 adults published in JAMA Neurology.

“The association of NO_² exposure with public health is often attributed to the exposure to traffic-related emissions, such as ultrafine particles. However, some studies have suggested that NO_² may directly exert toxic effects on the brain,” Sungyang Jo, MD, of the department of neurology at Asan Medical Center, University of Ulsan College of Medicine, in Seoul and colleagues wrote. “The direct toxic effect of NO_² is in line with the results of a number of epidemiologic studies showing correlations between NO_² and neurological disorders, such as PD, cognitive decline, stroke and amyotrophic lateral sclerosis.”

Jo and colleagues investigated the relationship between air pollution and the risk for incident PD among adults in Seoul.

The large cohort included 78,830 adults registered in the Korean National Health Insurance Service database. For inclusion in the study, individuals had to be above 40 years without PD and have lived in Seoul between January 2002 and December 2006. Each participant had an annual follow up from January 2007 to December 2015, for a total of 757,704 total person-years of follow-up.

The researchers estimated participants’ long-term air pollution exposure over an average of 5 years using their residential addresses at the district level. They analyzed exposure to particulate matters, nitrogen dioxide, ozone, sulfur dioxide and carbon monoxide and applied adjustments for demographic, socioeconomic and comorbidity factors.

More than half of the study participants were women (52.1%) and the mean age of the cohort overall was 54.4 years. During the study period, 338 participants developed incident PD (0.4%). The researchers reported an incidence rate for PD of 44.6 cases per 100,000 person years.

Jo and colleagues found that exposure to nitrogen dioxide correlated with an increased risk for PD (HR for highest vs. lowest quartile, = 1.41; 95% CI, 1.02-1.95). This association persisted after adjusting for age, sex and insurance type (model 2), model 2 factors plus comorbidities (model 3), and model 3 factors plus lifestyle (model 4). Meanwhile, the other air pollutants analyzed did not exhibit any significant association with PD risk.

In addressing the limitations of the study, Jo and colleagues noted that they did not measure indoor pollutant concentrations or occupational exposure, which may have impacted their findings. However, the use of a study population from a metropolitan city meant that “exposure to pesticide would be low,” according to the study results.

“This finding suggests the role of air pollutants in PD development, advocating for the need to implement a targeted public health policy,” they wrote. “This study was one of the few studies that evaluated the association between air pollutants and PD risk in Asian countries, where the high levels of air pollution are a major health threat.”

In a related editorial, E. Ray Dorsey, MD, of the University of Rochester Medical Center, Michael S. Okun, MD, of the University of Florida, Gainesville, and Caroline M. Tanner, MD, PhD, of the University of California, San Francisco, discussed the challenges of linking air pollution with PD. The complexities regarding Jo and colleagues’ study — which the authors acknowledge applies to many facets of PD — include a reliance on public data, difficulty accounting for interactions and confounding factors, a long time period between exposure and outcome and general difficulty with diagnosing PD. However, “the study adds to the growing evidence suggesting an association between air pollution and PD,” according to the authors.

“The absence of consistent findings across studies should not obscure the possibility that air pollution is partially fueling the rise of PD,” they wrote. “PD is now the world’s fastest growing brain disease, and up to half of affected individuals reside in Asia, making this linkage, if true, critically important.”