A new study shows that children who regularly breathe in second-hand smoke as infants are almost twice as likely to develop allergies.
The findings come from research headed by the Institute of Environmental Medicine, in Stockholm, Sweden, which involved more than 4000 families.
Parents were interviewed about their children's allergies and asked to fill in questionnaires when their child was aged two months, 12 months, two years and four years.
The questionnaires included questions about various environmental factors to which the children had been exposed before and after birth. These factors included cigarette smoke, pet dander (animal hair and dead skin) and certain foods with high allergenic potential such as wheat, cow's milk and peanuts.
Information on other factors that can affect allergy incidence was also gathered. Data on the outdoor air pollution the children were exposed to was extrapolated from levels of vehicle and industry pollutants known to be present where each child lived. Socioeconomic factors were also taken into account with parents asked to complete employment and education histories. In addition the parents were asked about their own history of allergic diseases.
To confirm the presence of allergies blood samples were taken from more than 2500 children at the age of four. The researchers looked for IgE antibodies which are at the heart of allergic reactions.
When the immune system detects an allergen to which it has become sensitised it releases these antibodies which attach to the allergen and trigger the release of immune chemicals such as histamine. It is high levels of histamine that are primarily responsible for the inflammation and typical symptoms associated with allergies.
The results of the study which are published in the journal Thorax, show that one in 12 mothers smoked throughout pregnancy and one in eight smoked during part of their pregnancy. There was no evidence however to suggest that this affected a child's risk of developing allergies.
The parent's responses to the questionnaires also revealed that one parent of one in five children smoked after the birth, with around one in 20 kids exposed to smoke from both parents. It was this exposure that seems to have had a significant effect on the occurance of allergies in the children studied.
Out of the 2500 children who had blood samples taken it was found that one in four had high IgE levels by the time they were four years old. Of hese 15% were allergic to inhaled allergens (animal hair, pollen etc), 16% were allergic to food allergens and 7% to both types.
The authors concluded that once other factors were taken into account, children exposed to second-hand smoke as babies were almost twice as likely to be allergic to both inhaled allergens and foods than those who lived in a smoke-free environment.
There have been a number of smaller studies that have aimed to look at whether exposue to cigarette smoke increases the risk of allergies in children but esults have not provided a clear picture. This is the first study to sepeate the effects of pre-natal and post-natal parental smoking.