WEDNESDAY, May 18, 2016 (HealthDay News) -- Many smokers have symptoms of chronic obstructive pulmonary disease (COPD) even before they've been diagnosed with the lung condition, according to a new study.

Symptoms of COPD include shortness of breath, coughing, difficulty exercising and history of asthma. COPD is the third leading cause of death in the United States, researchers said. Smoking is a major cause of the disease.

"We found that a significant number of current and former smokers who don't meet the typical criteria for COPD [based on a breathing test] otherwise look and behave like patients who do carry a diagnosis of COPD," said study co-lead author Dr. Meilan Han, medical director of the University of Michigan Women's Respiratory Health Program.

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The study included more than 2,700 people. It included current or former smokers and people who never smoked.

"We found significant respiratory symptoms in half of the current or former smokers with technically 'normal' breathing tests. We also found that these individuals needed to seek medical attention for breathing flare-ups with similar frequency as some patients who actually meet criteria for a diagnosis of COPD," Han said in a university news release.

But Han noted that it isn't clear if all of these people will eventually be diagnosed with COPD. More study is needed, she said.

"Given how common smoking is, everyone likely knows someone who suffers the condition we describe here. As these patients may require medical treatment and in some cases even hospitalization, the impact of this condition is very real. This study is just the first step in trying to better identify these patients so we can develop targeted treatments," Han said.

The study was published recently in the New England Journal of Medicine.

More information

The U.S. National Heart, Lung, and Blood Institute has more about COPD.