The effects of and tolerances for Carbon Dioxide in relation to recent developments in laboratory animal housing

During the last decades little focus has been on the deleterious effects of CO2. The majority of studies within the field were performed before 1970. However, as individually ventilated cage systems (IVS), un-ventilated filter-topped cages and other tightly sealed containment systems are increasing being used and are getting tighter allowing little or no air exchange between the cage and the surrounding environment, it is reasonable also to focus on the impact of CO2. The totally sealed IVS seems to be the ultimate solution; a system which might not be so far from being available on the market. Especially during failure of the individual cage ventilation this is problematic. At present no official limits for acceptable exposure in laboratory animals have been set, although some papers have discussed the item (Lipman, 1992; Serrano, 1971) In certain countries human exposures up to 5000 ppm (0.5%) during an eighthour working day as well as short term exposures up to 10000 ppm (1.0%) are accepted, and several papers recommend these levels for humans to be applied (Serrano, 1971; Lipman, 1992; Reeb et al., 1997). On the other hand, the effects of CO2 on animals, as investigated during the fifties and sixties, seem to be quite harmless and reversible. In contrast to e.g. ammonia no studies have so far been able to show any toxicological effects caused by CO2 (King et al., 1955; Schaefer et al., 1968).The present paper summarizes the effects of CO2 revealed in studies during the fifties and sixties in order to put these into a new perspective in relation to housing animals in new containment systems, such as the individually ventilated cage system, and discusses the physiological impact on the animal and human physiology caused by CO2 and how the effects of CO2 may be regarded as a common stressor. Finally, possibilities of dealing with the CO2 as a parameter and of setting limits for CO2 exposure in relation to animals are discussed