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 secondhand 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 secondhand smoke caused kids to get asthma, but they add to other research suggesting 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.
He said kids' lungs may be weaker when they're exposed to smoke in the womb, 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," Farber, who wasn't involved in the report, told Reuters Health.
Researchers from the UK analyzed 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 per cent more likely to wheeze, depending on the age they were assessed. The effect on asthma symptoms was greatest in babies and toddlers, who were 85 per cent more likely to have asthma if they were exposed to smoke in the womb.
When moms or non-parent household members smoked, kids had up to a 70 per cent higher chance of wheezing through age four, but the link to full-on asthma was less clear. The effect of maternal smoking was weakest in kids age five to 18.
There was limited data on how smoking by dads affected kids' chances of wheezing or getting asthma, according to findings published Monday in Pediatrics.
Tricia McKeever from the University of Nottingham and her colleagues said their findings suggest that secondhand smoke has more of an influence on wheeze and asthma than researchers had previously estimated.
"Before, (secondhand smoke) was known as triggering an attack or exacerbating asthma," said Geoffrey Fong, a tobacco researcher from the University of Waterloo in Canada.
"This study shows that secondhand 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.
According to the Centers for Disease Control and Prevention, about nine per cent of US kids have asthma.
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 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.