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If you are a nonsmoker exposed to sidestream tobacco smoke, you're at a significantly higher risk of developing dementia and other forms of cognitive impairment, according to new research in the UK and the US.
On Friday, the British Medical Journal published the research conducted at the University of Cambridge, Peninsula Medical School and University of Michigan on 5,000 non-smoking adults over the age of 50.
It is already known that an active smoker is at a significantly higher risk of dementia and that passive smoking can lead to poor cognitive performance in children and adolescents, as well as coronary heart disease, lung cancer, premature death, airway diseases such as asthma, chronic obstructive pulmonary disease and impaired lung function in adults.
But this new study, by Dr. David Llewellyn of the University of Cambridge and colleagues, is the first major one to conclude that second-hand smoke exposure could lead to irreversible dementia and other neurological problems.
They used data from the Health Survey for England in 1998, 1999 and 2001 and the English Longitudinal Study of Ageing. Saliva samples were tested for cotinine - a product of nicotine that can be found for about 25 hours after exposure to second-hand smoke.
Cotinine levels are an objective, short-term biomarker of second-hand smoke. Participants also provided a detailed smoking history.
The researchers used established neuropsychological tests to assess brain function and cognitive impairment - focusing on memory function, aptitude for numbers and verbal fluency (for example, naming as many animals as they could within a minute).
The tests' results were added together to provide a global cognitive function score. Participants whose scores were in the lowest 10 percent were defined as suffering from some level of cognitive impairment.
The link between second-hand smoke and cognitive impairment could be explained by the fact that heart disease increases the risk of developing dementia and second-hand smoke exposure is known to cause heart disease.
Dr. Mark Eisner from the University of California writes in an accompanying editorial that 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. Later in life, second-hand smoke may cause cardiovascular disease and stroke, which are themselves linked to cognitive decline. Until now, however, the suspicion that passive smoking is bad for the adult brain had not been scientifically confirmed," he wrote.
Eisner concludes by hoping that greater public awareness about the dangers of second-hand smoke, especially awareness about a much feared disease like Alzheimer's disease and other dementias, "would eventually translate into political action aimed at passing smoke-free legislation in regions of the world where public smoking is still permitted."
Scientific proof of health damage from passive smoking has persuaded legislators to ban smoking in public places and workplaces around Israel, and to increase fines and other means of deterrence.
Eisner noted in his editorial that "dementia has terrible consequences for quality of life, is greatly feared and is not easy to prevent. Consequently, publicizing the link between second-hand smoke and dementia may resonate powerfully with the public and increase awareness of the harms of passive smoking.
"Greater public awareness would eventually translate into political action aimed at passing smoke-free legislation in regions of the world where public smoking is still permitted."