The team, from San Diego State University in California, found that chemicals emitted from burning cigarettes are absorbed into household surfaces and dust, subsequently contaminating the air that children breathe.

Even smokers who attempt to protect their children by smoking outside had homes with much higher ETS levels than the households of non-smokers.

The researchers visited 49 households belonging to parents of children who were less than 1 year old. Dust and surface wipe samples were collected from the living room and child's bedroom of each home, and nicotine monitors were placed in each of these rooms. In addition, hair and urine samples were taken from each child.

The team found that levels of ETS were up to seven times higher in the homes of parents who smoked outside the house than in homes where both parents were non-smokers. In comparison, people who smoked in the home had houses with ETS levels up to eight times higher than those recorded in the homes of non-smokers.

Writing in the journal Tobacco Control, the team concluded: "Infants of smokers are at risk of ETS exposure in their homes through dust, surfaces, and air. Smoking outside the home and away from the infant reduces but does not completely protect a smoker's home from ETS contamination and a smoker's infant from ETS exposure."

Lead researcher, Dr Georg Matt added: "We need to start thinking about second hand smoke as more than just tobacco smoke that can be inhaled when someone smokes. Second hand smoke consists of particles so small they can enter the deep lung, where they can cause havoc just because of their small size."

Exposure to ETS has been linked to an increased risk of asthma attacks and the development of a variety of other respiratory illnesses, including chronic obstructive pulmonary disease and lung cancer.