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Alveolar air is best obtained by asking the person to breath out into a long tube. The air first exhaled is really coming from the conducting airways, not in contact with capillary blood, and from a tap in the tube close to the mouth a sample of air is taken toward the end of exhalation. A more convenient way is to have the person breathe out through a short tube and capping the tube (both ends) at the end of exhalation. The tube can then be shipped for analysis. Exhaling into a bag is OK, there is some dilution from the conducting airways air, i.e., dead space volume. The proper term then is "mixed exhaled air". See Stewart, R. D., Hake, C. L. and Peterson, J. E. Use of breath analysis to monitor trichloroethylene exposures. Arch. Environ. Health, 29, 6-13, 1974.