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Ventilation Design Handbook on Animal Research Facilities Using Static Microisolators 
Studies of rats and other rodents have found that approximately 50 percent of the passive 
water loss occurs via diffusion across the skin. Respiratory moisture production is highly 
affected by changes in body temperature and ambient temperature. An increase in 
breathing rate was found to elevate evaporative water loss in restrained rats, accounting 
for as much as 21 percent of the total heat loss at an ambient temperature of 31 °C. 
1.3.4 Ammonia and Carbon Dioxide Physiochemical Properties 
Ammonia and carbon dioxide are the major gaseous pollutants derived from animals and animal 
waste within the cage. Below are the physiochemical properties as well as current standards set 
by American Conference of Governmental Industrial Hygienists for ammonia and carbon 
dioxide. 
Ammonia (NH3): 
Ammonia is a colorless gas with a sharp, intensely irritating odor. It is lighter than air and easily 
liquefied by pressure, and it is very soluble in water, alcohol, and ether. Physiochemical 
properties include the following: 
Molecular weight: 
Specific gravity: 
Boiling point: 
Freezing point: 
Vapor pressure: 
17.03 
0.77 at 0 0 F; 0.6819 at boiling point 
-3.5 °C 
-77.7 °C 
liquid, 8.5 atmospheres at 20 °C 
Henderson and Haggard (1927) record temporary blindness and intolerable irritation from high 
concentrations. Osmond et al. (1968) describes severe eye damage and irritation of the glottis 
from exposure at high concentration. Schenker (1967) reports that toxic doses of ammonia 
acutely affect cerebral energy metabolism and this effect is localized at the base of the brain. 
Smyth (1956) found 1 ppm detected and identified by 10 subjects. Analysis of data obtained in 
plant surveys conducted by the Bureau of Industrial Hygiene, Detroit Department of Health, 
during 1965 to 1970 and National Institute For Occupational Safety and Health: Criteria For A 
Recommended Standard Occupational Exposure to Ammonia, 1974, identifies the limit of 
detection to be below 5 ppm and the complaint level to be 20 to 25 ppm. Patty’s Industrial 
Hygiene and Toxicology (1996) states that the odor threshold is between 5 to 53 ppm. As these 
last three references show, the actual level of initial odor detection varies significantly. 
The American Conference of Governmental Industrial Hygienists recommended a time-weighted 
average (TWA), threshold limit values (TLV) of 25 ppm (approximately 18 mg/m 3 ) to protect 
against irritation to eyes and the respiratory tract and minimize discomfort among workers. It also 
recommended that the short term exposure limit (STEL), however, be set at 35 ppm 
(approximately 27 mg/m 3 ). 
