How many stairs can you climb before feeling like your lungs will give out?

If you can’t climb stairs without feeling fatigued, or walk very far without huffing and puffing, it could be an early indication that your lungs are being compromised by chronic obstructive pulmonary disease, said Dr. Leonard Horowitz, pulmonary specialist at Lenox Hill Hospital in New York City.

COPD, a two-part inflammatory response in the lungs that causes difficulty breathing as a result of narrowing of the air passage, is the fourth-leading cause of death among adults in America.

“It's a horrible way to exist,” Horowitz told FoxNews.com. “There are some patients who get so bad that they have to be on ventilators… and the patients can be really miserable.”

And COPD's leading cause is preventable. Although chemical fumes, air pollution and other industrial exposures can cause the disease, smoking is responsible for 80 to 90 percent of cases, according to the Journal of the American Medical Association.

“Most cases ... are some mixture of emphysema and chronic bronchitis, not always equal parts,” Horowitz said. “When you get the two things combined, (it) creates thin airspaces where nothing can exchange.”

The chronic bronchitis arm of the disease is characterized by inflammation in the airways, which causes patients to cough up mucus or phlegm. The large amount of sputum clogs the airways that carry air to the lungs, causing excessive coughing.

A patient has emphysema when the alveoli, the air sacs that expand and shrink in the lung, become damaged. Emphysema can destroy the alveoli themselves, leaving fewer of them. This causes shortness of breath because it takes an emphysema patient longer to exhale.

“A normal person should be able to expel their entire lung capacity in 4 to 5 seconds,” Horowitz said. “Someone with COPD has an obstruction to expiration that can cause them to take 10, 12, 15 seconds to get the air out.”

COPD symptoms usually begin to show around the age of 60. Other symptoms include wheezing, chest tightness and fatigue. COPD is a progressive loss of airway function, so as it worsens, patients’ symptoms worsen, too. In its late stages, the symptoms can flare, and even become constant, when doing everyday activities.

Horowitz said that ultimately it isn’t COPD that kills 120,000 people a year. Usually the progressive disease needs an additional factor, such as pneumonia, to turn fatal.

Genetics also play a role in someone’s susceptibility to the disease.

“People are genetically more prone to lung damage with cigarette smoking in some families than in others,” Horowitz said. “There are some people that smoke their whole lives and have no evidence of lung damage, and some people that don’t smoke until they’re in their 40s and they start to have evidence of lung damage.”

The damage to your lungs is irreversible. But certain remedies can prevent further damage, and the most urgent remedy is to stop smoking, a point Horowitz strongly emphasized.

“I don't believe in the word ‘can't’ when it comes to smoking,” Horowitz said. “There is no such thing as ‘I can't stop smoking,’ it's that ‘I don't want to’ or ‘I am having difficulty and I need help doing it.’”

Lifestyle changes like exercise and diet can also improve the efficiency of breathing, even if they can’t reverse the damage already done.