Can be used for giving a rough estimate of a person's vital lung capacity and, by slowly rebreathing air several times in a row, the practice of inhalation and exhalation. Since the same air is breathed, carbon dioxide, rather than oxygen, is transferred avoiding hyperventilation. This size is recommended for females and smaller frame males. Kit includes 90 degree plastic mouthpipe. Comes in 5.0 Liters & 6.0 Liters.