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Cigarette smoking may accelerate cognitive decline in older adults. Smokers had declined four times that of nonsmokers. Higher cigarette pack-year exposure correlated with a significantly higher rate of decline. Study findings were published online in the January issue of the Journal of Alzheimer's Disease.

"For many years, published studies and popular media have perpetuated the myth that smoking is protective against the development of Alzheimer's disease.

The disease's impact on quality of life and health care costs continues to rise. It is therefore critical that we better understand its causes, in particular, the role of cigarette smoking," said Janine K. Cataldo, PhD, RN, assistant professor in the UCSF School of Nursing and lead author of the study.

According to the Alzheimer's Association, 5.3 million Americans currently have the disease, and that number will rise rapidly as the baby boom generation ages.

Alzheimer's disease also triples health care costs for Americans aged 65 and older, the organization states.

The UCSF team reviewed 43 published studies from 1984 to 2007. Authors of one-fourth of the studies had an affiliation with the tobacco industry.

The UCSF team determined that the average risk of a smoker developing Alzheimer's disease, based on studies without tobacco industry affiliation, was estimated to be 1.72, meaning that smoking nearly doubled the risk of Alzheimer's disease.

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In contrast, the team found that studies authored by individuals with tobacco industry affiliations, showed a risk factor of .86 (less than one), suggesting that smoking protects against Alzheimer's disease. When all studies were considered together, the risk factor for developing Alzheimer's disease from smoking was essentially neutral at a statistically insignificant 1.05.

Previous reviews of the association between smoking and Alzheimer's disease have not controlled for study design and author affiliation with the tobacco industry, according to Cataldo.

To determine if study authors had connections to the tobacco industry, the UCSF team analyzed 877 previously secret tobacco industry documents.