Key points from article :
6 cohort studies were done in the US and included over 25,000 adults.
Participants took 2 spirometry tests, an average of 7 years apart.
Lung function decreases almost as fast in people smoking 5 or fewer cigarettes a day, as 30 or more.
Lung function declined faster for smokers compared with people who had never smoked.
Study was published in the peer-reviewed journal The Lancet Respiratory Medicine.
