Children as young as seven are physically affected by their parents smoking, a study has found.
Those subjected to second-hand smoke at home had elevated levels of cotinine - a by-product of nicotine - in their systems.
The levels were particularly high in children whose mothers smoked 10 or more cigarettes a day.
Cotinine levels in the seven-year-olds were four times higher than in children of non-smoking mothers, research found.
And by the time the children had reached 15, the levels had risen to five times higher.
The elevated levels at ages seven and 15 are comparable to those in teenagers who are infrequent smokers.
Passive smoking is known to increase the risk of heart disease, lung cancer and respiratory disease.
Researchers at the University of Bristol say the research provides new evidence of the need to reduce smoking in private homes and cars.
Alex Stilby, lead author of the research, published in the journal Nicotine and Tobacco Research, said children and teenagers were affected by second-hand smoke.
He said: "We have found that the children of mothers who smoke have elevated cotinine levels, indicating clear evidence of passive smoking exposure.
"This provides a strong public health message about the risks to children if there are adults smoking in the home. Our research shows that the risks apply to older children just as much as to younger ones."
The research involved 3000 children when they were aged seven and 2000 when they reached 15 who were taking part in the Children of the 90s study.
It found elevated levels of cotinine in the blood of children at both ages were strongly related to whether the mother smoked and if so, how heavily - providing "clear evidence" of passive smoking.
A study by the Royal College of Physicians showed that about 17,000 children in the UK are admitted to hospital every year because of illnesses caused by second-hand smoke.
Cotinine levels of non-smoking 15-year-olds were five times higher if their mother smoked 10 or more cigarettes a day, compared with children of non-smokers.
Seven-year-old children has levels that were four times higher.
The levels of passive smoking exposure at aged seven and 15 are comparable to levels of exposure seen at age 15 in infrequent smokers who have cigarettes less than once a week.
Professor Marcus Munafo, the senior academic on the paper, said: "At the age of seven it is highly unlikely that children have started smoking, so the presence of cotinine in their blood at this age provides clear and conclusive evidence of the risks to young children from adults smoking at home.
"Cotinine is a measure of nicotine exposure. You can get it from other sources of nicotine, like patches and gum, but again it is unlikely that a seven-year-old will be using those so therefore the only place they could have got it from is tobacco smoke.
"You can't get those levels of exposure from simply walking past someone who smokes.
"The best thing to do is to stop smoking and there are services around the country to provide great support.
"If someone doesn't feel able to stop, then doing small things like not smoking inside can reduce exposure to their family."
Professor David Lomas, chair of the Medical Research Council's Population and Systems Medicine Board, who part-funded the study, added: "'We already know that passive smoking can harm children and has a cumulative, damaging impact on their lung development.
"What large, well-conducted studies like this offer us is clear, measurable evidence that parents smoking around their children presents a tangible risk to their health that shouldn't be underestimated."
Association of Maternal Smoking with Child Cotinine Levels is published in the journal Nicotine and Tobacco Research.
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