Children and young adults exposed to secondhand smoke have decreased sensitivity to common cough-inducing irritants, a new study says.

The study results might explain how normal healthy kids who live with parents who smoke, develop bronchitis, pneumonia and other respiratory diseases.

"Cough protects our lungs from potentially damaging environmental threats, such as chemicals and dust. Living with a parent who smokes weakens this reflex, one of the most vital of the human body," said Julie Mennella, PhD, a developmental biologist at Monell Chemical Senses Center.

According to estimates about 60 percent of U.S. children between the ages of three and 11 as well as 18 million youth aged 12 to 19 years old are exposed to tobacco smoke on a regular basis. This is higher than secondhand smoke exposure in non-smoking adults.

The study had a small sample size, about 38 children and teenagers between the ages of 10 and 17. Out of the participants, 17 were exposed to secondhand smoke at home while 21 were not.

All the participants were asked to inhale increasing concentrations of capsaicin from a nebulizer. Capsaicin is found in chilies and is an irritant that induces cough.

The concentration of capsaicin was increased until the participant coughed twice. This concentration of capsaicin was marked as the participants' cough threshold.

Researchers found that the cough threshold of children who were exposed to secondhand smoke was higher than that of children who weren't exposed to smoke, meaning that it took higher levels of irritants to elicit coughing in these children because they had lower sensitivity.

Having low sensitivity to irritants can pose a serious risk. One, children will be exposed to dangerous substances but the body doesn't initiate a defense attack like coughing to throw out the irritant. Second, children will be able to endure the smoke from the first puff of cigarette that they take, making them want to continue smoking.

"This study suggests that even if an exposed child is not coughing, his or her respiratory health may still be affected by secondhand smoke," said Paul Wise, PhD, sensory scientist at Monell.

The study was published in the journal Tobacco and Nicotine Research.