THURSDAY, Dec. 20 (HealthDay News) -- Exposure to secondhand smoke in early infancy can boost a child's risk of developing allergies, Swedish researchers say.

A team at the Karolinska Institute, Stockholm, analyzed questionnaires filled out by the parents of more than 4,000 families.

The parents filled out the questionnaires when their children were ages two months, 12 months, 2 years and 4 years. In addition, the researchers collected blood samples from more than 2,500 children at the age of 4 to check for the presence of immunoglobulin E (IgE), which is released by the immune system in response to allergens. High levels of IgE indicate sensitization to allergens.

About 8 percent of the mother smoked throughout their pregnancy, and about 12 percent smoked during part of their pregnancy, but the researchers found no evidence that smoking during pregnancy affected a child's risk of becoming sensitized to certain allergens.

About 20 percent of parents smoked after their baby was born, and about 4 percent of the children were exposed to secondhand smoke from both parents.

Overall, 25 percent of the children had high IgE levels by the time they were 4 years old, with 15 percent allergic to inhaled allergens, 16 percent allergic to food allergens, and 7 percent allergic to both types of allergens.

Compared to children of nonsmokers, children exposed to secondhand smoke during early infancy were almost twice as likely to be allergic to inhaled allergens, such as pet dander, and about 50 percent more likely to have food allergies.

The study was published in the journal Thorax.

More information

The American Academy of Otolaryngology -- Head and Neck Surgery has more about secondhand smoke and children.

SOURCE: BMJ Specialist Journals, news release, Dec. 18, 2007