London, Jan 10 : Scientists from China, Britain and the US have unearthed a link between passive smoking and the worsening risk of dementia, based on a study involving nearly 6,000 people in five provinces of China.

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.

Previous studies have shown a linkage between ETS and cognitive impairment, but this is the first to find a significant link with dementia syndromes, the journal Occupational and Environmental Medicine reports.

China is the largest consumer of tobacco in the world, with 350 million smokers.

Since 2006, the Chinese government has actively promoted the introduction of smoke-free environments in hospitals, schools, on public transport and in other public places, but implementation of these strategies has not been widespread.

Ruoling Chen, senior lecturer in public health from King's College, London, and colleagues, interviewed 5,921 people aged over 60 in rural and urban communities of Anhui, Guangdong, Heilongjiang, Shanghai and Shanxi to characterise their levels of ETS exposure and smoking habits and also assess levels of dementia syndromes.

They found that 10 percent of the group had severe dementia syndromes. This was significantly related to exposure level and duration of passive smoking.

The association with severe syndromes was found in people who had never smoked and in former and current smokers, according to a King's College statement.

Chen said: "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."

The World Health Organization (WHO) estimates that nearly 80 percent of the more than one billion smokers worldwide live in low-and middle-income countries, where the burden of tobacco-related illness and death is heaviest; but only 11 percent of the world's population are protected by comprehensive smoke-free laws.(IANS)