By Kathleen Doheny
TUESDAY, Aug. 19, 2014 (HealthDay News) -- Smoking while pregnant or around an infant has long been linked to development of asthma and allergies in young children. Now, researchers have found that the risk may persist into the teen years.
The study, which followed nearly 4,000 children in Sweden for 16 years, underscores the dangers of parental smoking, experts say.
"Exposure to second-hand smoke during pregnancy or infancy increases a child's risk of developing allergic disease even up to adolescence," said study researcher Jesse Thacher, a doctoral student at the Karolinska Institute in Stockholm.
For infants exposed to a parent's smoking, the risks of developing asthma and allergic rhinitis (stuffy or runny nose) in childhood or adolescence were 23 and 18 percent greater, respectively. The risk for eczema (inflamed, irritated skin) was 26 percent greater.
"Increased risks for asthma and rhinitis were seen primarily in early childhood, whereas those for eczema occurred later in life," Thacher said.
Previously, it wasn't clear whether the risks for asthma and allergies continued into the teen years, Thacher said.
The study, published online Aug. 18 in Pediatrics, only found an association between second-hand smoke and children's health problems, however. It wasn't designed to show a cause-and-effect relationship.
Thacher asked parents of children born from 1994 to 1996 about their smoking habits, other lifestyle information, and symptoms of allergic diseases in their children during the course of the study.
About 13 percent of the mothers smoked during pregnancy. Parental smoking during infancy was reported by more than 20 percent.
While the study included only Swedish children, ''it would be reasonable to expect comparable results between U.S. and Swedish children, because populations in these countries share similar lifestyle, and tobacco smoke should affect children in the same way," Thacher said.
Although fewer parents smoke around their children than in the past, it remains a problem in both countries, Thacher said.
"Rates of parental smoking in the U.S. vary by state, but it's around 17 percent," he said.
Allergies aren't the only childhood concern related to second-hand smoking. Second-hand smoke contains about 4,000 chemicals, according to the American Academy of Pediatrics. More than 50 of those chemicals are known to cause cancer.
Smoking during pregnancy has also been linked with miscarriage, premature birth, lower birth weight, sudden infant death syndrome (SIDS) and learning problems.
The findings provide new information and also reinforce other research, said Dr. Len Horovitz, a pulmonary specialist at Lenox Hill Hospital in New York City, who wasn't involved in the study.
"There's no safe smoking," he said, "and there doesn't seem to be much safe second-hand smoking either."
People are less aware of the dangers of second-hand smoke compared to direct smoking, Horovitz said. "I don't think the message about second-hand smoke exposure has been hammered home," he said.
One strength of the study, Horovitz said, is the large number of children and the long-term follow-up.
Exactly why second-hand smoke is linked with these health problems isn't known, Horovitz said. "We can surmise it is some sort of irritant or inflammatory process, but we don't really know the mechanism," he said.
Smoke exposure during pregnancy may contribute to asthma risk, for instance, by changes in lung growth or the health of the airways, the researchers wrote.