Breathe into a paper bag for 3 minutes six times a day. Seriously, it is an exercise used with people having diminished lung function, because it utilizes the natural carbon dioxide/oxygen sensor systems to trigger the 'breathe more, breathe deeper' reflexes. As for safety, we've used it for years, on doctor's orders, with my severely disabled sister as part of her physiotherapy and the doc credits it with her resistance to pneumonia - the killer of most people with her condition. When we started, her lung capacity was so low that she could breathe for three minutes into an air mask without opening the bag AT ALL - now she has normal or above normal lung capacity (without cardio - she can't walk or crawl)