Guide for the Care and Use of Laboratory Animals - Office of ...

46 GUIdE FOR ThE CARE ANd USE OF LAbORATORy ANIMALSand should therefore be considered. The use of computer modeling forassessing those factors in relation to heat loading, air diffusion patterns, andparticulate movement may be helpful in optimizing ventilation of microandmacroenvironments (Hughes and Reynolds 1995).Direct exposure of animals to air moving at high velocity (drafts) shouldbe avoided as the speed of air to which animals are exposed affects the rateat which heat and moisture are removed from an animal. For example, air at20°C moving at 60 linear feet per minute (18.3 m/min) has a cooling effectof approximately 7°C (Weihe 1971). Drafts can be particularly problematicfor neonatal homeotherms (which may be hairless and have poorly developedmechanisms for thermoregulatory control), for mutants lacking fur, andfor semiaquatic amphibians that can desiccate.Provision of 10 to 15 fresh air changes per hour in animal housingrooms is an acceptable guideline to maintain macroenvironmental air qualityby constant volume systems and may also ensure microenvironmentalair quality. Although this range is effective in many animal housing settings,it does not take into account the range of possible heat loads; the species,size, and number of animals involved; the type of primary enclosure andbedding; the frequency of cage changing; the room dimensions; or the efficiencyof air distribution both in the macroenvironment and between themacro- and microenvironments. In some situations, the use of such a broadguideline might overventilate a macroenvironment containing few animals,thereby wasting energy, or underventilate a microenvironment containingmany animals, allowing heat, moisture, and pollutants to accumulate.Modern heating, ventilation, and air conditioning (HVAC) systems (e.g.,variable air volume, or VAV, systems) allow ventilation rates to be set inaccordance with heat load and other variables. These systems offer considerableadvantages with respect to flexibility and energy conservation,but should always provide a minimum amount of air exchange, as recommendedfor general use laboratories (Bell 2008; DiBerardinis et al. 2009).Individually ventilated cages (IVCs) and other types of specialized primaryenclosures, that either directly ventilate the enclosure using filteredroom air or are ventilated independently of the room, can effectively addressanimals’ ventilation requirements without the need to increase macroenvironmentalventilation. However, cautions mentioned above regarding highvelocityair should be considered (Baumans et al. 2002; Krohn et al. 2003).Nevertheless, the macroenvironment should be ventilated sufficiently toaddress heat loads, particulates, odors, and waste gases released from primaryenclosures (Lipman 1993).If ventilated primary enclosures have adequate filtration to address contaminationrisks, air exhausted from the microenvironment may be returnedto the room in which animals are housed, although it is generally prefer-