aerosol dose from their own fur or that of other animals. To reduce variability, the Committee recommends that food should be removed during aerosol exposure.

In chambers without individual cages, animals can avoid high dose exposures by huddling together and covering their noses with their neighbors’ fur. To avoid this possibility, the Committee recommends the use of chambers with individual cages. A good example of this approach is a Hinners-type of exposure chamber, which allows for the exposure of a small number of animals in subdivided individual sections (Steinbach and others 2004; Moss and Cheng 1995b). Another disadvantage of the whole-body approach is that the desired level of exposure may take some time to stabilize in the chamber (Phalen and others 1984). When using a whole-body chamber, the Committee also recommends that the environmental temperature and humidity be regulated and the spatial and temporal distribution of the aerosolized material be uniform. Uniformity can be achieved by fitting the chamber with a cone or pyramid-shaped entry and exit and by either mixing the throughput air or by rotating the cages during exposure. The Rochester-type of chamber has a tangential inlet at the top that rotates the chamber air and provides good uniformity of exposure (Leach and others 1959). The Committee further recommends that samples for characterization of the exposure environment should be taken from the breathing zone nearest to the animal during an actual exposure. For more information regarding the use of whole-body exposure chambers, see Phalen and others (1984) and Moss and Cheng (1995b).

An alternative to whole-body exposure is head-only exposure, which requires that the head or neck region of the animal be firmly restrained. In contrast to the whole-body chamber, this type of exposure reduces the number of ways the aerosolized material can enter the animal and reduces variability in dosing. For this type of exposure, the Committee recommends that a good neck seal, which does not interfere with blood flow or ventilation, be utilized. In addition, environmental air temperature, humidity, and levels of carbon dioxide and oxygen need to be properly regulated. Other alternatives to whole-body exposures are nose-only or mouth-only exposure systems, which limit the entry of the aerosolized material to either the nose or oral cavity. An advantage to head-only, nose-only, or mouth-only exposure is that the amount of aerosolized material per animal is reduced compared to whole-body exposure and the concentration of the exposure material can be rapidly changed. Inhalation exposures by nose or mouth can be achieved using masks or cylindrical tubes (with a conical end to accommodate the head and one end open to the exposure environment) (Phalen 1984). The Committee points out that it is important to design and validate the tubes and masks properly so that exposure using these systems does not lead to stress on the part of the animal and altered respiration, which can affect responses to test agents.

Aerosol can also be delivered directly to the lungs via the mouth by introducing the aerosol through an endotracheal tube (Phalen and others 1984). This approach has been shown to eliminate losses in the upper airways and

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