Secondhand smoke can pose serious health problems, especially for children, whose bodies and brains are still developing. Smoke-free laws have reduced exposure to secondhand smoke in bars and restaurants, but what about protecting kids at home?

As a new study shows, the cardiac effects of secondhand smoke in childhood can last well into adulthood – which means that the children of smokers may be set up for heart problems at an age when they’re too young to even have a say in the matter.

The children of smokers are already at greater heart risk for a number of reasons.

Researchers followed 3,700 families in Australia, querying the parents about whether they smoked and their other lifestyle factors. The children were between 3 and 18 years old when the study began, and the researchers examined them once they reached adulthood to look at how healthy their hearts were.

The team measured the thickness of the inner two layers of the carotid artery wall. Thickening of these layers is linked to heart risk.

Children whose parents both smoked had a small but significant increase in the thickness of their artery walls as adults. The kids from families in which only one of the parents smoked didn’t have this thickening of the arteries.

The result held true after other heart risk factors, like children's own smoking habits, physical activity, body weight, alcohol consumption, blood pressure and cholesterol level in adulthood, were accounted for.

Thickening of the carotid artery wall is only one measure of smoking's effects; there are many others, the authors point out. The idea that exposure to passive smoke in childhood could have a measurable physical effect so many years later is what’s important in the current research results.

“Our study shows that exposure to passive smoke in childhood causes direct and irreversible damage to the structure of the arteries,” said study author Seana Gall of the University of Tasmania in a news release. “Parents, or even those thinking about becoming parents, should quit smoking. This will not only restore their own health but also protect the health of their children into the future.”

The children of smokers are already at greater heart risk for a number of reasons. They were more likely, as adults, to be smokers themselves or overweight than those whose parents didn't smoke.

One peculiarity of the current study is that only children of families in which both parents smoked had thickened arteries as adults. The authors aren’t sure why this is, but they think it might be because children in families where both parents smoke are likely to be exposed to secondhand smoke more often.

“We can speculate that the smoking behaviour of someone in a house with a single adult smoking is different,” said Gall. “For example, the parent that smokes might do so outside away from the family, therefore reducing the level of passive smoking. However, as we don't have this type of data, this is only a hypothesis.”

What’s clear is that secondhand smoke is not good for anyone, particularly for children, whose organs — like their heart and brain — are still developing. This makes them particularly susceptible to chemical exposures.

So if you or your partner smokes, as the authors urge, quit smoking now: Whether you have a child or not, the beneficial effects of stopping are huge and immediate.

The study is published in the European Heart Journal.