Lead researcher Renee D. Goodwin of the Mailman School of Public Health at Columbia University in New York identified 3,065 male twin pairs, who had lived together in childhood, and who had both served on active military duty during the Vietnam War.

The study found that among all twins -- identical twins who share the same genetic material and fraternal twins who share half of the same genetic material -- who suffered from the most PTSD symptoms were 2.3 times as likely to have asthma compared with those who suffered from the least PTSD symptoms.

"If there had been a strong genetic component to the link between asthma and PTSD, the results between these two types of twins would have been different, but we didn't find substantial differences between the two," Goodwin said in a statement.

The findings suggest people with asthma who experience a traumatic event might benefit from seeking professional help, because they could be more vulnerable to developing PTSD, Goodwin said.

The study is published in the the American Journal of Respiratory and Critical Care Medicine.