New York: As little as one hour of exposure to tobacco smoke per week can increase the risk of having respiratory symptoms, such as shortness of breath and a dry cough at night among teenagers, warns a study. "There is no safe level of second-hand smoke exposure," said lead author of the study Ashley Merianos, Assistant Professor at the University of Cincinnati in the US.

"Even a small amount of exposure can lead to more emergency department visits and health problems for teens. That includes not just respiratory symptoms, but lower overall health," Merianos said.