Shielding Effects in Small Animal Exposure Systems

The shielding effects of cages used in biological studies of the effects of exposure to 60 Hz electric fields on small animals can reduce the apparent dosage to which the animals are subjected. This paper describes an experiment performed to determine the degree of cage shielding resulting from the combined effects of soiling of the cage walls by excretions from the animals occupying them and the relative humidity in which the system is operated. Data obtained indicates the level of cage shielding can become significant after a short period of animal occupation unless either the relative humidity is kept below 40 percent or the cages are frequently cleaned. Accuracy in biological studies of the effects of exposure to 60 Hz electric fields on small animals requires that the strength of the electric field, and the resulting current induced in the bodies of the exposed animals, be accurately known and maintained at a relatively constant level for determination of exposure dosage. Reductions in field strength and the current induced in the animals´ bodies, referred to as the short circuit current (Sc), which result from the shielding effect of cages and adjacent animals have been measured and factored into exposure studies [1]. These measurements, however, were made using clean cages. This paper describes an experiment performed to measure the shielding effect of cages as soiling of the surfaces by animal excretions progressed with the length of time the cages had been occupied and at relative humidity levels of 30 to 90 percent.