A new research has found that breathing in second-hand tobacco smoke in early infancy boosts the risk of developing allergies.
The findings of the research, carried out on more than 4000 families, are based on the parental survey responses about their children's allergies, the different kinds of environment they had been exposed to before and after birth.
Among them, parental smoking, pet dander (animal hair and dead skin), and foodstuffs, were included.
The parents were required to fill out questionnaires when their children were 2 months and 12 months old and again when they were 2 and 4 years old.
A blood sample was taken from over 2500 children at the age of 4 to look for the presence of immunoglobulin E, or IgE for short.
IgE is an immune system chemical released in response to allergic substances or allergens. High levels indicate 'sensitisation' to allergens.
The survey found that one in 12 mothers (8pct) smoked throughout pregnancy, and one in eight (12pct) smoked during part of their pregnancy
But there was no evidence that smoking while pregnant affected a child's risk of becoming sensitised to certain allergens.
But one parent of one in five children smoked after their baby was born. And around one in 20 children (4pct) were exposed to tobacco smoke from both parents.
One in four children had high IgE levels by the time they were 4 years old, with 15pct allergic to inhaled allergens, 16pct to food allergens, and 7pct to both types.
Children exposed to second-hand tobacco smoke in early infancy were almost twice as likely to be allergic to inhaled allergens, such as pet dander, as those who had not been so exposed.
And they were almost 50pct more likely to be allergic to foodstuffs.
It was found that only children whose parents were not affected by allergies were affected.
The research has been published ahead of print in the journal Thorax.