Children with at least one parent who smoked were found to have more than five times the level of a toxin than those who did not.
The toxin – cotinine – is a by-product of nicotine and is used by experts as a measure of exposure to second-hand smoke.
Smoking is known to be a risk factor for cot death, which claims around 300 babies’ lives a year in the UK.
Researchers from the University of Leicester and the University of Warwick found that babies who had a mother who smoked had quadruple the amount of cotinine in their urine. Meanwhile, having a father who smoked doubled the amount of cotinine in the urine.
The researchers found that colder rooms were also associated with higher levels of cotinine and that babies with higher levels were also more likely to sleep in a bed with their parents, which is another known risk factor for cot death.
The authors suggested that these babies could be sleeping close to clothing or other objects contaminated with smoke particles. Babies of smoking parents were also more likely to weigh less and were from poorer families.
The authors said: “Our findings clearly show that by accumulating cotinine, babies become heavy passive smokers secondary to the active smoking of parents.”
Nearly 40% of under-5s are believed to be exposed to tobacco smoke at home, and smoke may be responsible for up to 6,000 deaths of young children a year in the US alone.