Many current and former smokers who do not meet spirometric criteria for COPD have significant respiratory disease, a research reveals.

The study of  8,872 patients aged between 45 and 80 and who smoked at least a pack of cigarettes a day for 10 years found that half of the patients were considered disease-free based on spirometry alone.

However when their CT scans were assessed the researchers discovered that 42% of them had emphysema or airway thickening, 23% had significant shortness of breath, and 15% walked less than 350 metres in six minutes.

The patients also reported worse quality of life compared to patients who had never smoked, with a quarter of the smokers having scores that exceeded a threshold considered clinically significant.

“It appears that smoking progressively takes its toll with advancing age, even when spirometry remains within the population norms,” the study authors wrote in JAMA Internal Medicine.

“We believe that our results highlight the importance of smoking prevention and cessation as a primary strategy to prevent lung disease and other long-term effects of smoking,” concluded the study authors led by Elizabeth A. Regan from National Jewish Health in Denver.