The latest study to find a link between smoking and lowered risk of Parkinson's disease says that the protective effect appears to be related to the number of years of smoking, not how many cigarettes a smoker consumes daily, Reuters reported March 10.

Researchers studied more than 305,000 men and women over 10 years and found that the more years the subjects smoked, the lower their risk of developing Parkinson's disease. Those who smoked at least a pack a day for less than 10 years reduced their risk by 4 percent, for example, but those who smoked 10-19 years had a 22-percent lower risk, and risk was reduced 41 percent among those who smoked for 30 years or more.

Parkinson's risk did not change based on how many cigarettes were smoked daily. Researchers warned against smoking to prevent Parkinson's, but said the research could help uncover the causes of the disease. They noted that smoking does not help control symptoms or extend survival among those who already have Parkinson's.

The study was published in the March 16, 2010 issue of the journal Neurology.