Chronic obstructive pulmonary disease, commonly called emphysema, kills more than 120,000 Americans every year, most of them smokers. So why haven’t you heard of it?

If ever there were a disease in dire need of a good publicist, COPD is it. Rates of other deadly diseases have been steadily decreasing, thanks in large part to widespread awareness. Heart disease and stroke continue to decline, as do the death rates for the four most common types of cancer (prostate, lung, breast and colorectal). The importance of fighting these killers has been knitted into the national conscience so tightly that it’s commonplace to be aware of health markers like cholesterol and blood pressure levels.  In these areas, health education has even impacted commerce and lifestyle—one can hardly sit in front of the television and enjoy a Danish without being urged to eat healthy, buy a calorie-free soda or at least consider medicine for controlling cholesterol.

But COPD? We know more about the CIA. So here’s an eye-opener: The first, second, and third causes of death in the U.S. are heart disease, cancer and stroke. COPD is No. 4.

COPD is an incurable lung condition in which air flow is compromised, making it difficult to breathe. According to estimates quoted by the American Lung Association, 11.4 million Americans had COPD in 2004; further evidence of impaired lung function in some 24 million adults suggest the 11.4 million number was short by half. More people are killed by chronic obstructive pulmonary disease than by every kind of accident combined (accidents are the No. 5 cause of death) or by diabetes (No. 6), and nearly twice as many die from COPD compared to Alzheimer’s (No. 7).

Since the rates of other leading causes of death are decreasing while COPD deaths increase, COPD is expected to be the No. 3 killer by the year 2020—just 12 years from now.

Silent Killer

Some diseases are accompanied by overt warning signs, others are not. Lucky survivors of heart disease, for instance, may first experience a tightening in the chest that provides not only a warning but a wake-up call to take better care. COPD is the silent type. The disease’s irreversible damage to the lungs can occur with no outward symptoms, and even when symptoms do exist they stand to be ignored.

“One of the reasons COPD is referred to as ‘the silent killer’ is that we don’t recognize the symptoms as being related to the lungs,” says Dr. Ronald Crystal, chief of pulmonary critical care at NewYork-Presbyterian Hospital/Weill Cornell Medical Center. “Very often people will say, ‘Yeah, I have a smoker’s cough’ or, ‘Yes, I’m getting out of shape.’ What they don’t recognize is that they’re losing lung function.”

Crystal notes that denial also plays a role. Feeling breathless can be a sign of aging or gaining weight—neither of which we like to acknowledge. But the shortness of breath that comes with climbing a flight of stairs or running to catch a bus is a symptom of COPD.

The Damage Done

Early stages of COPD are also characterized by silence. The lungs are gradually devastated with no initial impact on the patient’s breathing.

Pulmonologists generally divide COPD into two sub-categories: chronic bronchitis and emphysema, both conditions that affect the deepest and most fragile reaches of the respiratory system.

Think of the lungs as an inverted tree, where the windpipe is the trunk and the airways are branches that continually fork to smaller and smaller branches. When you get down to about the sixth fork, where the airways approach the width of a human hair, the branches are called small airways or bronchioles. At the end of each bronchiole is a clump of tiny sacs called alveoli—what Crystal calls the “business end” of the lung.

“That’s where gasses are exchanged,” he explains. “That’s where oxygen comes in from the air and where carbon dioxide, the waste gas of our metabolism, comes out. So the airways are the conduit of good air and bad air, if you will.”

The sacs are supposed to fill up with air when you breathe in and deflate when you breathe out. But in a case of chronic bronchitis, which can be brought on by smoking, the walls of the small airway become inflamed and swollen shut—thus the “obstruction” in chronic obstructive pulmonary disease. With emphysema, the delicate sacs are damaged so badly that they cannot inflate or deflate at all. So, both conditions shut down the respiratory system where it does its crucial work of bringing oxygen into the system and sending carbon dioxide back out. Most people with COPD have a mixture of both diseases.

If the disease progresses, the patient will have an increasingly difficult time breathing. Though an individual may only have a sense of not being able to get enough air, what’s actually happening is that he or she cannot exhale. Think about what it’s like to come up for air after being under water a little too long—you first exhale, hard, before gulping in the oxygen-rich air. A person with advanced COPD can not get that satisfying exhale.