Chronic obstructive pulmonary disease (COPD) is a progressive lung condition that makes it hard to breathe.

According to a new study, children who have smoking mothers may have an increased risk of developing COPD later in life. There are more than 11 million people who have COPD in the US, but the American Lung Association estimates that 24 million Americans are unaware of having it.

The study suggests children exposed to their mothers' second-hand smoking -- who will actively smoke in their adulthood -- will increase their risk by three times. The research is published in the journal Respirology.

COPD Explained

Identified by the American Lung Association as the third leading cause of death in the US, COPD is the collective term that includes chronic bronchitis and emphysema. While smoking is considered as its primary risk factor, non-smokers are also susceptible to acquiring the condition. COPD, fortunately, is a preventable and treatable condition.

Adult COPD Risk Starts in Childhood

The researchers tracked nearly 1,400 patients for five decades. They were given initial tests, where collected data included how much and how quickly they inhale and exhale. Over the course of the study, the patients were surveyed and had various testings to closely look at their lung capacities.

When researchers analyzed the surveys, they found that more than 9 percent had airflow obstruction. They found that participants who smoked and had mothers who smoked heavily (around 20 cigarettes each day) had 2.7 increased risk of suffering from it than those who were just exposed to maternal passive smoking. Non-smokers had reduced lung volumes as well.

Study author Jennifer Perret of the University of Melbourne in Australia and colleagues said that the results of the study "were not surprising" to them.

"Smoking in later life can result in deficits in lung function by middle age. So it was not unexpected to see that mothers' smoking . . . could also adversely influence the growing lungs of [their children]," she said, WebMD notes.

The study suggests that the combination of passive and active smoking could harm more than being exposed to second-hand smoke or smoking alone.

"Reduced lung function potential in childhood predisposes an individual to having reduced lung function as an adult," she added.

Maternal Smoking Does Not Directly Cause Increased COPD Risk

According to the researchers, their study only found an association or a risk factor. A mother's smoking habit does not directly cause their children increased COPD risk later in life. Medline Plus notes that the study suggested that boys are more susceptible to the negative effects of maternal smoking than girls. The authors state that this may be due to "biological differences."