Aversiveness of carbon dioxide.

We are grateful to Dr Ronald Wood for providing in his letter in the July issue (Wood 2005) his expert views and methodologies that could potentially be used in future to test the behavioural effects of airborne contaminants and irritants. Dr Wood seems to accept in his letter that carbon dioxide is aversive but he is unsure how aversive, and believes that it can be safely and humanely used in rodents. We, on the other hand, based on existing evidence, do not accept that concentrations that are capable of inducing unconsciousness can be humanely (i.e. to stop any animal suffering) used. The main tenet of Dr Wood’s criticism of the work conducted by Leach et al. (2002) is that it is unable to differentiate between horizontal movements as a result of its depressant effect on the central nervous system (CNS) from those resulting from aversion to carbon dioxide. However, a CNS response of this type would probably take a minimum of 7–10 s from inhalation of 80% carbon dioxide in air to produce such horizontal locomotory movement (Kohler et al. 1999), whereas we found that the withdrawal time (the time taken to leave a chamber containing carbon dioxide) occurred in less than 2 s at low-to-moderate concentrations (26–36%) and less than 1 s at high concentrations (>51%). This is further borne out by the dwelling times observed (time spent in the chamber over a 3-min period), with rats spending less than 2 s at low and moderate concentrations and less than 1 s at high concentrations (mice spent less than 6 s at medium and high concentrations). Therefore, with these types of response, there is insufficient time for carbon dioxide to stimulate a horizontal locomotory reaction due to CNS depression, rather an aversion. Further, this notion that a CNS depressant gas, like carbon dioxide, induces animals to leave an atmosphere containing it, simply due to an increase in random horizontal movements by an animal, does not seem reasonable for a variety of reasons, including: