Heavy smoking during middle age can double the risk of Alzheimer's disease and dementia two decades later, researchers said on Monday.
Smoking already causes millions of deaths each year from cancer and heart disease.
"Our study suggests that heavy smoking in middle age increases the risk of both Alzheimer's disease and vascular dementia for men and women across different race groups," Rachel Whitmer, a research scientist with Kaiser Permanente in Oakland, California and colleagues wrote in the Archives of Internal Medicine.
They said smoking also causes cancer and heart disease. The new findings show it threatens public health in late life, when people are already more likely to develop dementia.
Whitmer's team analyzed data from 21,123 members of a health plan who took part in a survey when they were in their 50s and 60s.
About 25 percent of the group, 5,367 volunteers, were diagnosed with some form of dementia in the more than 20 years of follow up, including 1,136 people who were diagnosed with Alzheimer's disease.
Alzheimer's, the most common form of dementia, is a fatal brain disease in which people gradually lose their memories and their abilities to reason and care for themselves. It affects more than 26 million people globally.
People who smoked more than two packs of cigarettes a day had a higher risk of both Alzheimer's disease and vascular dementia.
"The increase in risk is not just for heavy smokers," Whitmer said in a telephone interview. "It's not if you smoke less you are in the clear, that is for sure."
She said compared with nonsmokers, those who smoked more than two packs a day had a 114 percent increased risk of dementia, a 157 percent increased risk of Alzheimer's disease and a 172 percent greater risk of vascular dementia.
Whitmer said it has been difficult to study the effects of smoking on brain health because heavy smokers often die from other conditions first.
"This is the first time someone has been able to look really over the long term," Whitmer said.
"We've known for some time that smoking is bad for your respective health," she said. "This really adds to our understanding that the brain is also susceptible.
The World Health Organization says 5 million people die every year from tobacco-related heart attacks, strokes and cancers. Another 430,000 adults die annually from breathing second-hand smoke.
A report last month said the worldwide costs of coping with dementia will reach $604 billion in 2010, more than one percent of global GDP output, and those costs will soar further as the number of sufferers triples by 2050.