Despite very few of their parents admitting they smoked, more than half of the children whose blood was selected for a study on smoke exposure tested positive for secondhand cigarette fumes.
Researchers at the University of California in San Francisco tested 496 blood samples left over from children aged one to four to see how many of them had been affected.
The samples were initially taken to look for lead exposure, and the medics then tested the leftover samples for cotinine - the chemical produced when someone has been exposed to nicotine.
They found that 55 per cent of the sample had 'measurable' amounts of cotinine, meaning the children had been exposed to smoke within the previous three to four days.
Despite this, only 13 per cent of mums and dads 'fessed up to their kids being exposed to smoke.
Medics say many parents may misguidedly believe that children are only considered 'exposed' to smoke if they are near someone who is actually smoking a cigarette, and they are unaware of where else their children might be breathing in smoke.
Dr Jonathan Winickoff, an associate professor of pediatrics at Massachusetts General Hospital for Children in Boston told Reuters:
"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."
Winickoff said the test can also identify if a child is being exposed to smoke without the parent realising, for example, if they live in a building - such as a block of flats - where smoking is allowed.
Exposure to secondhand smoke has been linked to asthma and other respiratory problems, ear infections and even sudden infant death syndrome.
Publishing the report in the Archives of Pediatrics & Adolescent Medicine, the authors of the study say that children can also endure the effects of secondhand smoke if they spend time in a room where someone recently smoked.
One of the authors, Dr. Neal Benowitz said he thought 'parents do not understand the various sources of potential exposure'.
He added that testing children for cotinine could help 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," he said.
What do you think about this?
Do you actively prevent your children from coming into contact with secondhand smoke?