New US research has found that smoking while pregnant could be even worse for children with asthma than exposing them to secondhand smoke in childhood. 

Carried out by researchers from the Icahn School of Medicine at Mount Sinai, New York, the team looked at 2,070 children aged six to 11 years to assess the relationship between lung function and type of secondhand smoke exposure.

The researchers asked parents to self-report on their children's exposure to tobacco smoke during pregnancy and measured the children's current tobacco smoke exposure using the levels of cotinine in the blood. Cotinine is a major metabolite of nicotine and a strong biomarker of exposure to tobacco smoke.

The team then measured the lung function of the children using a spirometry test.

Results showed that nearly 10 percent of both children with and without asthma had reduced lung function.

Current exposure to tobacco smoke was found to be independently associated with airflow obstruction in the lungs of participants, although the extent of the association was small. 

However, exposure to tobacco smoke during pregnancy was associated with a 2.5 times increase in the chance of having airflow obstruction in children with asthma.

"This study implicates maternal smoking in pregnancy as the period of secondhand exposure that is more strongly associated with worse lung function in asthmatic children," commented the study's lead investigator Dr. Stacey-Ann Whittaker Brown, MD. "Maternal smoking in pregnancy may set children with asthma on a trajectory of poor lung function in later childhood, and other studies suggest this effect may be lifelong."

"Childhood asthma is a significant source of morbidity for US children; those with poor lung function have an even greater burden of disease," explained Dr. Whittaker Brown, "As we learn more about improving asthma outcomes in children, it is important to find out not only what environmental exposures are implicated in poor lung function, but also when those exposures are most harmful."

According to the American Lung Association, asthma is the most common chronic condition among children, currently affecting an estimated 6.2 million U.S. children under 18 years.

Secondhand smoke is one of the main triggers for the condition, with the American Lung Association warning that exposure to tobacco smoke can cause serious harm to children, and estimating that around 400,000 to one million children with asthma have their condition worsened by exposure to secondhand smoke.

The results can be found published online in the journal CHEST®.