Children exposed to passive smoking are likely to do worse at school than their peers, research suggests.
Millions of children are exposed to passive smoking
Exposure to even low levels of tobacco smoke in the home was linked to lower test results for reading and maths.
The greater the exposure, the worse the decline was, the US Children's Environmental Health Center team found among nearly 4,400 children.
The findings support calls to ban smoking in public places, they told Environmental Health Perspectives.
Although they did not look at the effects over time as the children grew up, the researchers took into account other individual differences that might have skewed the results, such as parental education.
The study was based on data gathered between1988 and 1994 to give a snapshot of the health of people in the US at that time.
To measure exposure to environmental tobacco smoke, the scientists measured levels of cotinine, a substance produced when nicotine is broken down by the body.
Cotinine can be measured in blood, urine, saliva and hair.
Children aged between six and 16 were only included in the analysis if their blood cotinine levels were at or below 15 ng/ml - a level consistent with environmental tobacco smoke exposure - and if they denied using any tobacco products in the previous five days.
Dr Kimberly Yolton's team then looked at the children's cognitive and academic abilities in relation to skills such as maths, reading, logic and reasoning.
There was, on average, a one-point decline in reading scores for each unit increase in cotinine at levels above 1 ng/ml.
Furthermore, there was a five-point decline for each unit increase in cotinine at levels below 1 ng/ml, suggesting that even low levels of exposure to tobacco can impair brain function.
Similarly, passive smoking was linked with nearly a two-point decline in a standardized math test.
Dr Yolton said: "These declines may not be clinically meaningful for an individual child, but they have huge implications for our society because millions of children are exposed to environmental tobacco smoke."
She said more studies looking at the effects of passive smoking over time as children grow would be useful, but said she was confident that their findings stood.
She said they added further incentive for countries to set public health standards to protect children from exposure to environmental tobacco smoke.
Last week, the UK government launched a series of hard-hitting TV advertisements to encourage smokers to quit, particularly those who are parents.
In November, it said smoking would be banned in the majority of public places within the next four years.
But campaigners are worried this will not protect children against exposure to large amounts of smoke in their own homes.
Amanda Sandford, research manager at Action on Smoking and Health, said: "This shocking study strengthens the case for protecting children from second-hand smoke in all indoor environments.
"Banning smoking in all public places where children have access should be a priority, but all adults should refrain from smoking when children are present and that includes the home environment."
Dr Lawrence Whalley of the University of Aberdeen, who has led research showing that smokers' cognitive ability decreases with time, said the current study findings were very important.
"This fits in with what we know about the effects of tobacco smoke, even though it is not conclusive.
"Smoking is bad for children.
"Smoking in pregnancy causes low weight babies who do not fare as well in later years.
"It also causes respiratory diseases."