An international study by scientists in China, the UK and US found a link between passive smoking and syndromes of dementia.
The study of nearly 6,000 people in five provinces in China found that people exposed to passive smoking have a significantly increased risk of severe dementia syndromes.
Passive smoking, also known as 'second-hand' smoke or environmental tobacco smoke (ETS), is known to cause serious cardiovascular and respiratory diseases, including coronary heart disease and lung cancer.
However, until now it has been uncertain whether ETS increases the risk of dementia, mainly due to lack of research.
Previous studies have shown an association between ETS and cognitive impairment, but this is the first study to find a significant link with dementia syndromes.
China is the largest consumer of tobacco in the world, with 350 million smokers. Since 2006, the Chinese government has promoted the introduction of smoke-free environments in hospitals, schools, on public transport and in other public places, but implementation has not been widespread.
Recent data show that the prevalence of passive smoking is still high, with over 50 per cent of people exposed to environmental tobacco smoke on a daily basis.
China also has the highest number of dementia sufferers in the world, with increasing rates of new cases as the population ages.
Dr Ruoling Chen, senior lecturer in public health from King's College London, and colleagues interviewed 5,921 people aged over 60 in the rural and urban communities of Anhui, Guangdong, Heilongjiang, Shanghai and Shanxi to characterise their levels of ETS exposure, smoking habits and assess levels of dementia syndromes.
They found that 10 per cent of the group had severe dementia syndromes. This was significantly related to exposure level and duration of passive smoking.
The associations with severe syndromes were found in people who had never smoked and in former and current smokers.
The data from the Anhui cohort, which were collected at baseline in 2001-03 for dementia syndromes and in the follow up in 2007-08 for ETS exposure and dementia, further excluded the possibility that dementia syndromes caused people to be more exposed to environmental tobacco smoke.
"Passive smoking should be considered an important risk factor for severe dementia syndromes, as this study in China shows. Avoiding exposure to ETS may reduce the risk of severe dementia syndromes," Dr Ruoling Chen, also a visiting professor at Anhui Medical University said.
The study was published in the journal Occupational and Environmental Medicine.
