An alarming new study shows that mothers who smoke are putting their children at severe risk of developing chronic obstructive pulmonary disease (COPD) as adults — but why is that the case?

The study found that children have a much higher risk of developing COPD as adults based on research that followed the participants for decades, as we reported recently. The risk was triple for children who grew up in a household where the mother smoke more than 20 cigarettes per day. The research doesn’t prove that COPD is related to smoking in the household, but the association is strong, according to a statement from researchers.

COPD is often related to smoking, which is believed to account for 80 percent of COPD deaths, although a quarter of Americans with COPD have never smoked cigarettes, according to a Centers for Disease Control and Prevention report.

Inhaling smoke during childhood is especially bad because lungs grow and develop slowly, and therefore kids are more vulnerable to developing it early and seeing symptoms develop later in life, the CDC states.

Symptoms can include a nagging cough, shortness of breath, and wheezing. When it gets severe, the disease includes difficulty catching your breath, blue or gray lips, a fast heartbeat, swelling in the feet, and weight loss.

“We now appreciate that the risk of COPD can be accumulated from sources other than personal smoking, where the negative consequences of a mother’s smoking may continue through to later adult life,” Dr. Jennifer Perret, lead author of the Respirology study, said in a statement. “Our findings further strengthen the current recommendation for smoking abstinence, especially for pregnant women and young mothers.”