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The capability for preventing exposures to experimental materials 
is greatest with the use of class III biological safety cabinets. These 
cabinets are required for all experiments requiring P4-level physical 
containment. Class III cabinets are gas-tight, stainless steel, venti- 
lated enclosures, usually of a modular design, with attached arm -length 
rubber gloves. The cabinets are ventilated at a rate of 10-20 air changes 
per hour, with the supply and exhaust air passing through HEPA (high- 
efficiency particulate air) filters. Provision for exhaust air incineration 
is usually provided, and all liquid wastes from the cabinets pass through 
a secondary sewage sterilization system. The cabinets are usually 
equipped with compressed air, vacuum, hot and cold water, ultraviolet 
lights, and any desired array of back- or bottom -mounted refrigerators, 
incubators, deep freezers, centrifuge cabinets, necropsy cabinets, and 
animal -holding cabinets. 
"Gas-tight" means that the cabinets were pressurized to three inches 
water-guage pressure with dichlorodifluoromethane, and all joints, 
junctures, attachments, and the rubber gloves were tested with a halogen 
detector. The NIH Guidelines require that under these test conditions 
there shall be no leak rate greater than .01 oz. of gas per year. When 
used to contain microorganisms, the class III cabinets are operated 
under a reduced pressure of from 0.5 to 1.0 in. of water guage. This 
operating condition provides an additional safety factor. 
Gloves are either 15 or 30 mil. thick. They are produced by a 
multiple -dip cement process and electrically tested at 5, 000 or 10, 000 
volts, depending upon thickness, to detect any defect. 
Class III cabinets provide, under ordinary circumstances, an impene- 
trable barrier between the inside of the cabinet and the laboratory 
environment. However, it is recognized that human error and accidents 
can compromise the integrity of class III cabinets. Attached rubber 
gloves have been torn, improper techniques in changing gloves have been 
reported, contaminated materials have been improperly removed from 
the cabinets, and fires and explosions have occurred in these cabinet 
systems. Scientists and technicians have injected themselves through 
the gloves when using a syringe and needle, and animal caretakers have 
received animal bites through the gloves. Although these situations 
have been observed, they occur infrequently. This is exemplified by the 
fact that in Building 1412B at Ft. Detrick, where P4 conditions were 
used for 10 years, there was only one laboratory-acquired infection. 
This was caused by a needle puncture through an attached glove. 
