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It's the first study to link secondhand smoke to this form of mental deterioration, said lead researcher Thaddeus Haight, a research statistician at the University of California, Berkeley.
"There have been studies that have shown that exposure to secondhand smoke is related to subclinical cardiovascular disease and clinical cardiovascular disease," Haight noted.
"There also have been studies showing that atherosclerosis, hardening of the arteries, is related to an increased risk of dementia."
Dementia increase
The current study completes the loop by showing that inhaling someone else's smoke increases the incidence of dementia, he said.
"This study attempts to look at the relationship between cardiovascular disease and dementia and also looks at the independent, direct effects of tobacco on the nervous system," Haight said.
"There is an alternative pathway other than cardiovascular disease with potential neurotoxic effects. Secondhand smoke could affect the neurodegenerative process behind dementia and may lower the threshold for dementia-like symptoms."
Haight was scheduled to present the findings this week at the annual meeting of the American Academy of Neurology, in Boston.
The findings
in their study, the Berkeley team evaluated data on more than 3,600 participants enrolled in a long-running study of cardiovascular health.
They compared 985 never-smokers with no cardiovascular disease and no dementia to 495 people who reported an average of 28 years of lifetime exposure to another person's smoking.
A six-year evaluation revealed that elderly people exposed to secondhand smoke for 30 years or more were about 30 percent more likely to develop dementia than those without such exposure, the researchers said.
"We're now looking at how smoke affected dementia directly and are also attempting to separate out the effects of secondhand smoke that occur through clinical vascular disease," Haight said.
Doubled risk
Analysis of the data showed that the combination of long-term exposure to secondhand smoke and the presence of cardiovascular disease nearly doubled the risk of dementia.
Long-term exposure to secondhand smoke alone increased the risk of dementia by about a third.
The study also found a greater incidence of dementia in people who were not diagnosed with cardiovascular disease but who had detectable abnormalities of their carotid arteries, the main arteries to the brain, on ultrasound images.
People exposed to secondhand smoke who had those abnormalities (such as narrowed carotid arteries) were 2.5 times more likely to develop dementia as those with no carotid abnormalities and no secondhand smoke exposure.
Reduce smoke exposure
The study results provide support for efforts to reduce exposure to secondhand smoke, such as bans on smoking in restaurants and bars, Haight said.
"It's reasonable to suppose that anything that is bad for your heart is bad for your brain, so it is no great surprise that secondhand smoke could be responsible for development of carotid artery disease and dementias of all kinds," said Bill Thies, vice president for medical and scientific affairs at the Alzheimer's Association.
The exact degree of danger is somewhat unclear, because the data in the report are not complete, but the danger does exist, Thies said.
"This study is an important addition to the overwhelming evidence of serious health harms from secondhand smoke," said Matthew L. Myers, president of the Campaign for Tobacco-Free Kids.
"It underscores the need for all states to pass comprehensive smoke-free laws covering all workplaces and public places."
(HealthDayNews, May 2007)
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