Via MedicalXPress: "researchers [put] forward a theory for the first time based on research evidence that new air sacs, called alveoli, are constantly being formed. This contradicts information in most medical textbooks that explain that the tiny air sacs begin to develop before birth (around the 6th month of pregnancy) and continue to increase in number until the age of about 3 years. ... It was believed that there was no further increase in the number of alveoli beyond that age, and that the existing alveoli just expanded as the lungs grew bigger until final adult size was reached. Our study has challenged this by suggesting that new alveoli continue to be formed as the lungs grow. ... The researchers studied over 100 healthy volunteers aged between 7 and 21 years. Each volunteer [breathed] in hyperpolarised helium and held their breaths. ... The helium [behaves] like a magnetised gas. Within the scanner, we can measure how the magnetism decays, and this in turn depends on the size of the air sacs - alveoli - which contain the helium. ... We studied small children, whose lungs contain approximately one litre of air, and full-grown adults with lung volumes of around four litres. We found very little difference in the size of the alveoli across everyone we studied. If the size of the alveoli are hardly changing, this can only mean one thing - as our lungs increase in size, we must be growing new alveoli. ... This research has important implications. If we can continue to develop new alveoli beyond early childhood, going on through adolescence, there is the potential for lung repair following injury that was never realised before."