More than half of kids who were part of a new study from California tested positive for secondhand smoke exposure -- despite only a handful of their parents admitting to lighting up.
Parents may think kids are only exposed if they're around someone actively smoking a cigarette, researchers said, or are unaware of where else their children might be breathing in smoke.
They said a blood test may help identify and reduce smoke exposure.
"What the test does is allow the doctor -- in consultation with the parent -- to figure out the source of exposure and then to eliminate it," said Dr. Jonathan Winickoff, an associate professor ofpediatrics at Massachusetts General Hospital for Children in Boston.
Winickoff, who co-wrote an editorial accompanying the study, told Reuters Health the test can also identify if a child is being exposed to smoke without the parent realizing it, such as by living in an apartment building where smoking is allowed.
Secondhand smoke exposure in kids has been tied to -- among other things -- sudden infant death syndrome, respiratory problems, ear infections and asthma.
Researchers at the University of California, San Francisco tested 496 blood samples left over from children -- most age one to four -- to determine how many of them were exposed to secondhand smoke.
The blood samples were initially taken at San Francisco General Hospital to test for lead exposure between November 2009 and March 2010. Lead exposure testing is common and part of standard pediatric care, according to the authors.
The researchers tested the leftover blood for cotinine, a chemical produced by the body after it is exposed to nicotine.
Overall, 55 percent of the blood samples had a measurable amount of cotinine, which meant those kids were exposed to smoke within the last three to four days.
Only 13 percents of parents, however, admitted their child had been exposed to secondhand smoke.
"I think parents do not understand the various sources of potential exposure," said Dr. Neal Benowitz, one of the study's co-authors from UCSF.
The researchers pointed out in their Archives of Pediatrics & Adolescent Medicine report that some parents may also believe a child has to be around someone who is smoking to be exposed -- but that is not the case.
For example, kids can still get the effects of secondhand smoke if they spend time in a room where someone recently smoked.
Benowitz and his colleagues said testing children for cotinine could ultimately prevent diseases brought on by secondhand smoke exposure by helping to detect the source.
"Once you know there is exposure then you can talk to the parent," said Benowitz, who added that a doctor can ask if anyone inside the child's home or daycare smokes to identify the source.
Testing for cotinine is currently not readily available to the general public. Benowitz said it is also fairly expensive at about $90 to $100 per test, but that the price might come down as it becomes more widely used.
Winickoff said the best approach would be to integrate cotinine testing with routine lead testing.
As for what parents can do right now, Dr. Harold J. Farber, who studies smoking exposure and asthma at Baylor College of Medicine and Texas Children's Hospital in Houston, said they should keep their child away from places where people are smoking and places that allow smoking.
Farber told Reuters Health that it's also important for parents to stop smoking themselves.
"You can't be a parent who smokes and not expose your child. Keeping it away from your child isn't enough," he said.