Washington (AFP)

The disease is called chronic obstructive pulmonary disease (COPD): it mainly affects smokers but a third of the patients have never smoked, a mystery clarified by a large study funded by American public funds.

The chronic disease is linked to the too small size of the respiratory tract, which has not developed enough, according to the study published Tuesday in the scientific journal Jama.

COPD is the fourth leading cause of death in the United States. The disease gradually blocks the airways, leading to coughing, sputum and shortness of breath. One in 10 adults suffers from it beyond the age of 40.

Smoking and air pollution have long been observed to be the main causes, but fewer smokers and better air quality have not reduced the incidence of COPD as much than expected.

The research team analyzed the lung scans of 6,500 adults, smokers and non-smokers, with or without COPD.

"We were struck that people who had smaller airways than expected were at a much higher risk of developing COPD, compared to people with normal or larger airways," AFP said. lead author, Benjamin Smith, of Columbia University Irving Medical Center.

As for those who had smoked for decades without ever developing the disease, "their airways were much wider than expected, relative to the size of their lungs."

"This suggests that these people, who have wider airways, have a reserve to resist the harmful effects of tobacco," said the doctor.

Smoking remains an important risk factor, demonstrated for decades.

But the team concludes that "the gap between the airways and the size of the lungs" seems to explain the variations in risk of developing the disease.

- Mysterious causes -

It is not known why the airways develop too much or not enough in some people. It is a avenue for future research, and Benjamin Smith has obtained funding to explore possible genetic causes.

Another possibility is a developmental problem during childhood, perhaps from the womb until the end of growth. Is airway growth affected by mother's smoking, air pollution or respiratory microbes?

The hope is to understand the root causes, to intervene from childhood - specialists draw the parallel with rickets, common until the 20th century when we understood that vitamin D and calcium were essential for bone development.

In the shorter term, these new works emphasize that chronic obstructive pulmonary disease covers several medical realities, with treatments that work more or less well depending on the causes.

Doctors have known, for example, for a long time that bronchodilators, used by patients to relax the muscles of the lungs and reduce respiratory discomfort, were much more effective in some people than others, which is probably linked to the size of the airways. .

© 2020 AFP