The team of researchers from Tasmania, Australia and Finland found that exposure to smoking in childhood thickens arteries' walls which, in turn, ups the risks of heart attack and stroke.
"The study shows that exposure to passive smoke in childhood causes a direct and irreversible damage to the structure of the arteries," said Seana Gall, research fellow at the Menzies Research Institute, Tasmania, and University of Tasmania.
The 3,776 children who participated in the research were aged between three and 18 at the start of the studies.
The researchers asked questions about parents' smoking habits and they used ultrasound to measure the thickness of the children's artery walls once they had reached adulthood.
The researchers found that carotid intima-media thickness, a measurement of the thickness of the innermost two layers of the arterial wall, in adulthood was 0.015 mm thicker in those exposed to both parents smoking than in those whose parents did not smoke.
The study published in the European Heart Journal, however, did not show an effect if only one parent smoked.
"We think that the effect was only apparent with both parents smoking because of the greater overall dose of smoke these children were exposed to," said Gall.