Exposure to second-hand smoke may increase the risk of developing dementia, according to a study by British and American researchers.

While scientists have long suspected a link between smoking and cognitive impairment, the study is the first large-scale attempt to link second-hand smoke to increased risk for cognitive impairment.

In a study in Thursday's online British Medical Journal, researchers with the universities of Cambridge and Michigan tested saliva samples of nearly 5,000 non-smokers over age 50. They were looking for cotinine — a product of nicotine that can be found in saliva for about a day after exposure to smoke.

Participants were also assessed for brain function and cognitive impairment.

The study found people with the highest cotinine levels had a 44 per cent increased risk of cognitive impairment, compared to people with the lowest cotinine levels.

The researchers argue the link between second-hand smoke and cognitive impairment could be explained, given that heart disease increases the risk of developing dementia and second-hand smoke exposure is known to cause heart disease.

In an accompanying editorial, Dr. Mark Eisner from the University of California said while the serious negative health effects of second-hand smoke like cancer and premature death have been established beyond doubt, there is still a lot to learn about the scale of illness caused by second-hand smoke.

"Emerging evidence suggests that parental smoking may impair childhood cognitive development," he writes. "Later in life, second-hand smoke may cause cardiovascular disease and stroke, which are themselves linked to cognitive decline."