These examples illustrate the utility of carbon dioxide measurements as a diagnostic tool for indoor air quality. Carbon dioxide is sometimes said to be an unreliable indicator because of uneven mixing. However, even though mixing does occur slowly in very still air, in nearly all cases there is enough air movement that mixing is not a misleading factor for diagnostic purposes. Measurements do have to be recorded with enough context for proper interpretation. For example, to distinguish between the effects of natural and mechanical ventilation. Or to understand how the results are affected by the number of people in the building and what they are doing. Or how they relate to the nature and layout of the HVAC system and how it is adjusted or intended to be used. Carbon dioxide measurements say little about how healthy an indoor environment is. But they can tell a great deal about how comfortable it is and, with good observation, can give important clues as to where to look for factors that may be affecting health. The interaction of subjective and objective elements in the interpretation of CO2 measurements makes them a fascinating and powerful tool in the hands of a careful investigator.