New Delhi. Breathing in second-hand smoke has now been shown for the first time to raise the risk for Alzheimer's disease and other forms of dementia, said Dr KK Aggarwal, President Heart Care Foundation of India.

There have been studies that have shown that exposure to secondhand smoke is related to subclinical cardiovascular disease and clinical cardiovascular disease. There also have been studies showing that atherosclerosis, hardening of the arteries, is related to an increased risk of dementia.

A new study completes the loop by showing that inhaling someone else's smoke increases the incidence of dementia. Also there is an alternative pathway other than cardiovascular disease with potential neurotoxic effects. Secondhand smoke could affect the neurodegenerative process behind dementia and may lower the threshold for dementia-like symptoms.

A six-year evaluation revealed that elderly people exposed to high lifetime exposure to secondhand smoke for 30 years or more were about 30 percent more likely to develop dementia than those without such exposure. Analysis of the data showed that the combination of long-term exposure to secondhand smoke and the presence of cardiovascular disease nearly doubled the risk of dementia. Long-term exposure to secondhand smoke alone increased the risk of dementia by about a third.

The study also found a greater incidence of dementia in people who were not diagnosed with cardiovascular disease but who had detectable abnormalities of their carotid arteries, the main arteries to the brain, on ultrasound images. People exposed to secondhand smoke who had those abnormalities (such as narrowed carotid arteries) were 2.5 times more likely to develop dementia as those with no carotid abnormalities and no secondhand smoke exposure.