According to this report:

Lengthier smoking habits—but not more intense ones—seem to reduce the odds of developing Parkinson’s disease, according to a study in Neurology. The inverse association between smoking and Parkinson’s—the neurodegenerative disease characterized by difficulty in controlling movement and speech—was first reported half a century ago, but this is the first study to separate the number of years smoking from the number of cigarettes smoked per day. Researchers compared the smoking histories of 305,468 elderly subjects, 1,662 of whom had been diagnosed with the disease in the previous decade. Compared to the nonsmokers, subjects who had smoked at least a pack a day for one to nine years were only 4% less likely to develop Parkinson’s. But subjects who smoked as many cigarettes a day for more than 30 years had 41% shorter odds of developing the disease. The number of cigarettes a day, however, had no significant independent effect on Parkinson’s risk. The results suggest that any Parkinson’s-protective effects of tobacco reach saturation at low doses, the researchers said.