A fresh look at past studies suggests kids who live with a smoker are more likely to wheeze or get asthma, providing more evidence for the link between second-hand smoke and breathing problems.
Researchers found that the biggest effect on wheeze and asthma symptoms was seen in babies and toddlers whose moms smoked while they were pregnant or soon after kids were born.
The findings don't prove that second-hand smoke led to asthma, but they add to other evidence that smoke exposure may trigger respiratory problems in youngsters, researchers said
"What this really clearly demonstrates is that the research and data documenting the adverse effects of tobacco smoke exposure on children's asthma is very strong," said Dr Harold J. Farber, who studies smoking exposure and asthma at Baylor College of Medicine and Texas Children's Hospital in Houston.
Asthma symptoms greater in toddlers
He said kids' lungs may be weaker when they're exposed to smoke in utero, and asthma drugs may not work as well in those children.
"Eliminating our children's tobacco exposure has got to be a critical public health priority," said Dr Farber, who wasn't involved in the report.
Researchers from the UK analysed more than 70 studies published between 1997 and 2011, all of which asked about smoking by parents or other household members and then tracked which kids were diagnosed with wheezing or asthma going forward.
Those studies showed that when moms smoked while they were pregnant, their kids were 28% to 52% more likely to wheeze, depending on the age at which they were assessed. The effect on asthma symptoms was greatest in babies and toddlers, who were 85% more likely to have asthma if they were exposed to smoke in the womb.
Smoking has more influence on wheezing
When moms or non-parent household members smoked, kids had up to a 70% higher chance of wheezing through age four, but the link to full-on asthma was less clear. The effect of maternal smokng was weakest in kids age five to 18.
There was limited data on how smoking by fathers affected kids' chances of wheezing or getting asthma, the authors reported in Paediatrics.
Dr Tricia McKeever from the University of Nottingham and her colleagues said their findings suggest that second-hand smoke has more of an influence on wheeze and asthma than researchers had previously estimated.
"Before, (second-hand smoke) was known as triggering an attack or exacerbating asthma," said Dr Geoffrey Fong, a tobacco researcher from the University of Waterloo in Canada.
Second hand smoke develops asthma
"This study shows that second-hand smoke may cause the development of asthma," and not just trigger attacks in kids who already have it, said Fong, who wasn't involved in the new study.
Dr Serpil Erzurum, an asthma researcher from the Cleveland Clinic, told Reuters Health the research provides more evidence that the development of asthma is based both on genetic predisposition and environmental exposure in the womb and early in life.
"This is very strong exposure that's still going on a daily basis despite all that we know about smoking," said Dr Erzurum, who wasn't tied to the new work.
The study is "a further reminder of the importance of smoke free laws and smoke-free homes to protect non-smokers, especially children," Fong told Reuters Health.
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