Passive smoking associated with decreased chance of PD
 

Gallo et al. report [1]:

"A total of 715 incident PD cases were ascertained in a cohort of 220494 individuals from NeuroEPIC4PD, a prospective European population-based study... Hazard ratios (HRs) for smoking <20 years were 0.84, ... 20-29 years 0.73 ... and >30 years 0.54 ... compared with never smokers. ... The inverse association between passive smoking and PD, HR 0.70 ... ruled out the effect of unmeasured confounding."

Reference:

[1] "Exploring causality of the association between smoking and Parkinson's disease"
Valentina Gallo Paolo Vineis Mariagrazia Cancellieri Paolo Chiodini Roger A Barker Carol Brayne Neil Pearce Roel Vermeulen Salvatore Panico Bas Bueno-de-Mesquita et al.
International Journal of Epidemiology, November, 2018
Exploring causality of the association between smoking and Parkinson’s disease | International Journal of Epidemiology | Oxford Academic

John