The dangers of secondhand smoke to babies are well documented, but new research claims the risks could be higher than previously thought.
Researchers found children with at least one parent who smokes had more than five times the level of a toxin in their bodies than those with nonsmoking mums and dads.
The toxin - cotinine - is a byproduct of nicotine and is used by experts as a measure of exposure to passive smoking.
Smoking is known to be a factor in cot death, which claims around 300 lives a year in the UK.
Scientists from Leicester and Warwick universities discovered that babies who have a mother who smokes had quadruple the amount of cotinine in their urine.
And having a father who smokes doubles the amount of the chemical in a child's body, the study published online in the Archives of Disease in Childhood found.
The authors said: "Our findings clearly show that by accumulating cotinine, babies become heavy passive smokers secondary to the active smoking of parents.
"Higher cotinine levels in colder times of year may be a reflection of the other key factors which influence exposure to passive smoking, such as poorer ventilation or a greater tendency for parents to smoke indoors in winter.
"Nicotine, of which cotinine is a byproduct, has recognisable cardiovascular stimulant effects.
"However, it is merely one of the several thousand constituents of tobacco smoke and may not be the most lethal."