The study, published in Neurology, tracked nearly 7,000 people age 55 and older for an average of seven years, and found current smokers at the time of the study were 50 percent more likely to develop dementia than those who had never smoked or smokers who had quit.

However, the study showed smoking did not affect the risk of developing Alzheimer's disease for people who have the gene that increases the risk of Alzheimer's, called apolipoprotein E4, or APOEa4. For those without the APOEa4 gene, current smokers were nearly 70 percent more likely to develop Alzheimer's than non-smokers or past smokers.

"Smoking increases the risk of cerebrovascular disease, which is also tied to dementia," study author, Dr. Monique Breteler, of Erasmus Medical Center in Rotterdam, says in a statement. "Another mechanism could be through oxidative stress, which can damage cells in the blood vessels and lead to hardening of the arteries. Smokers experience greater oxidative stress than non-smokers, and increased oxidative stress is also seen in Alzheimer's disease."