Volume I - Section I - Introduction 
Page I - 5 
microenvironment than in the macroenvironment (Besch 1980; Gamble and Clough 1976; 
Murakami 1971; Serrano 1971; Flynn 1959). Microenvironmental conditions can induce 
changes in metabolic and physiologic processes and/or alterations in disease susceptibility 
(Schoeb and others 1982; Broderson and others 1976; Vesell and others 1976). 
Temperature and Humidity: Regulation of body temperature within normal variation is 
necessary for the well-being of homeotherms. Generally, exposure of unadapted animals 
to temperatures above 85 °F (29.4 °C) or below 40 °F (4.4 °C) without access to shelter 
or other protective mechanisms might produce clinical effects (Gordon 1990) that could 
be life-threatening. Animals can adapt to extremes by behavioral, physiologic, and 
morphologic mechanisms, but such adaptation takes time and might alter protocol 
outcomes or otherwise affect performance (Gordon 1993; Garrard and others 1974; 
Pennycuik 1967). Environmental temperature and relative humidity can depend on 
husbandry and housing design and can differ considerably between primary and 
secondary enclosures. Factors that contribute to variation in temperature and humidity 
include housing material and construction, use of filter tops, number of animals per cage, 
forced ventilation of the enclosures, frequency of bedding changes, and bedding type. 
Recommended dry-bulb temperatures for mice, rats, hamsters, gerbils, and guinea pigs 
are 18 to 26 °C (64 to 79 °F). However, some conditions might require increased 
environmental temperatures. These conditions include postoperative recovery, 
maintenance of chicks for the first few days after hatching, housing of some hairless 
rodents, and housing of neonates that have been separated from their mothers. 
Ventilation: The purposes of ventilation are to supply adequate oxygen; remove thermal 
loads from animals, lights, and equipment; dilute gaseous and particulate contaminants; 
adjust the moisture content of room air; and, where appropriate, create static-pressure 
differentials between adjoining spaces. Establishing a room ventilation rate, however, 
does not ensure the adequacy of the ventilation of an animal's primary enclosure and 
hence does not guarantee the quality of the microenvironment. The degree to which air 
movement (drafts) causes discomfort or biologic consequences has not been established 
for most species. The volume and physical characteristics of the air supplied to a room 
and its diffusion pattern influence the ventilation of an animal's primary enclosure and so 
are important determinants of its microenvironment. The relationship of the type and 
location of supply air diffusers and exhaust vents to the number, arrangement, location, 
and type of primary enclosures in a room or other secondary enclosure affects how well 
the primary enclosures are ventilated and should therefore be considered. The use of 
computer modeling for assessing those factors in relation to heat loading and air diffusion 
patterns can be helpful in optimizing ventilation of primary and secondary enclosures. 
The guideline of 10 to 15 fresh-air changes per hour has been used for secondary 
enclosures for many years and is considered an acceptable general standard. Although it is 
effective in many animal housing settings, the guideline does not take into account the 
range of possible heat loads; the species, size, and number of animals involved; the type 
