Carbon Dioxide. Carbon dioxide is a normal constituent of exhaled breath and can be used as a screening technique to evaluate whether adequate quantities of fresh air are being introduced into an occupied space. Carbon dioxide concentrations are normally higher indoors than the generally constant ambient (outdoor) CO2 concentration which typically ranges from 300 to 350 ppm. When indoor CO2 concentrations exceed 1000 ppm in areas where the only known source is exhaled breath, inadequate ventilation is suspected, and widespread complaints can be anticipated. Carbon dioxide concentrations at this level do not present a health hazard but suggest that other indoor contaminants may also be elevated. In combination, these may contribute to health complaints such as headache, fatigue, and eye/throat irritation. Assuming that there are no significant uncontrolled emission sources in the building, along with outside air being brought in by good quality HVAC systems, and that temperature and relative humidity are at comfortable levels, then complaints related to Indoor Air Quality should be minimal if interior CO2 concentrations are maintained below 600 ppm.