A new study from Columbia University Mailman School of Public Health has discovered a significant link between birth order and development of asthma symptoms.

The study revealed that children with older siblings are more likely to suffer from asthmatic problems than the oldest or only children.

The researchers found that kids with at least two older siblings were 50 pct more likely than other children to have gone to an emergency department or been hospitalized overnight for breathing problems.

"Our findings support the hypothesis that having older siblings increases a child's risk of exposure to infectious agents before age two years, and in turn increases the child's risk for wheezing," said Matthew Perzanowski, lead author, PhD and assistant professor of Environmental Health Sciences at the Mailman School of Public Health.

"Some studies have found that having older siblings increases the risk of wheeze in babies and toddlers.

"Our findings are novel in that we found that among the four-year-olds in this study, the pattern was the same as has been observed in younger children elsewhere," he added.

The children in the study were selected from the participants of Head Start programs in New York City.

"Previous findings of the opposite association between asthma and birth order among older children and adults have served as the basis for what is called the hygiene hypothesis, the idea that exposure to infectious agents at a very young age reduces the risk of asthma in the long term," said Inge Goldstein, senior author of the study and senior lecturer in the Mailman School's Department of Epidemiology.

"Only by continuing to follow these children can we determine whether and how birth order predicts diagnosed asthma and asthma that persists throughout childhood," Goldstein added.

The findings revealed that wheezing was higher among boys than girls (32 pct vs 21 pct) and higher among children with an asthmatic parent than other children (53pct vs 22pct).

The associations of birth order with respiratory symptoms were statistically significant only among those children who were not allergic and among those without an asthmatic parent.

"But even if the patterns of association change as the children grow, we have learned from this study that four year-olds with older siblings are more likely than other four year-olds to experience respiratory symptoms that burden them and their families and impose the costs of care on them and the community," said Judith Jacobson lead researcher and associate professor of clinical Epidemiology at the Mailman School.

The study is pre-published online in the journal Clinical and Experimental Allergy.