At least one in four heavy, long-term smokers who do not quit will develop the deadly lung disease COPD, Danish scientists said on Tuesday.

Chronic obstructive pulmonary disease includes chronic bronchitis and emphysema and is a leading cause of death worldwide. It begins with a cough and leads to fatigue, shortness of breath and difficulty breathing as the lungs are destroyed.

Scientists at the Hvidovre Hospital in Denmark who monitored the health of 8,000 people during a 25-year study found that a quarter of smokers suffered from COPD.

“The absolute risk of developing COPD among continuous smokers is at least 25 percent, which is larger than was previously estimated,” said Dr. Peter Lange, a member of the research team.

The people monitored in the study included smokers and non-smokers. The researchers checked their lung function at the start of the study and regularly over the 25-year period.

The results, which are published in the journal Thorax, showed that persistent smokers were six times more likely to develop COPD than non-smokers.

If smokers gave up the habit soon after the start of the study, their risk of COPD dropped. None of the smokers who quit developed severe COPD.

During the study 2,900 people died, including 109 from COPD.

“Our finding is quite simple: the longer people smoke, the higher the risk of developing COPD,” said Lange.

Deaths from COPD are increasing in most countries. The World Health Organization (WHO) estimates it will become the third-biggest cause of death worldwide by 2020.

COPD afflicts people over 40 who have been smoking for many years. It contributes to other disorders such as pneumonia, heart disease and stroke. There is no cure for COPD but treatment can relieve the symptoms and therapies to slow its progression are being tested.