London, July 12 (IANS): Passive smoking in childhood or during teenage years may increase risk of chronic obstructive pulmonary disease (COPD), a progressive ailment that makes it hard to breathe, says a study.

The results appeared in the New England Journal of Medicine showed that suboptimal development of lung function in childhood and adolescence is a major determinant of COPD risk in older age.

COPD is one of most common causes of death in the world today with active smoking accounting for approximately 85 percent of all cases.

"COPD can be developed in different ways, so achieving normal growth in lung function in early adulthood is an important factor in terms of future risk," said Peter Lange, professor at the department of public health, University of Copenhagen in Denmark.

For decades, the development of COPD has been ascribed to accelerated decline of lung function from a normal level achieved in young adulthood.

But the study said that not all at risk patients show accelerated lung function decline.

The scientists revealed two major trajectories of lung function leading to COPD: the fast decline trajectory, where lung function declines very rapidly from a normal level, and the alternative trajectory where suboptimal development of lung function during childhood and adolescence is the major determinant of COPD in older age.

In two of the cohorts, the longest follow-up period was approximately 25 years.

The potential longer-term benefits of this study will include a better understanding of the development of COPD, which could play an important part in the prevention of new cases.

The maximally achieved level of lung function in childhood and early adulthood is an important determinant of future COPD risk.

"Every effort should be undertaken to achieve normal growth of lung function including non-smoking during teenage years, treatment of asthma in childhood and reducing exposure to agents such as passive smoking," Lange said.