Pitfalls in the interpretation of long-term inhalation experiments

Age- and weight-matched groups of mice were enclosed in airtight chambers and exposed to clean, filtered air for 1 month. At the end of the exposure period, body, liver and spleen weights and plasma butyrylcholinesterase (BuChE) activity were measured. More than twice as many significant differences in these parameters occurred compared with the expected results if only random differences existed between the groups. Thus, isolation of animal groups for extended periods of time in inhalation experiments alone may lead to differences in various biological parameters. When testing the effects of unknown substances such differences may be mistaken for reactions to the test agent, which actually may have no effect.