9 
procedures without containment precautions, accidents with lyophilized tubes, 
animals excreting large amounts of infectious bacteria or virus, or pilot 
plant operations. 
In some laboratories , the secondary barriers contribute significantly 
to integrity of the experiment by reducing nonspecific or cross-infection 
of materials or animals. 
As far as biohazard outside the building is concerned, most secondary 
barriers are more for reasons of public relations than for anything else, 
except for pilot plants or other large-volume production, experimental 
aerosols, use of tick or insect vectors, and agents capable of spread to 
the animal or plant food supply. This view assumes that known infectious 
liquids, solids, animals, and animal wastes are decontaminated before disposal, 
as has long been standard practice in all microbiological laboratories. 
It is difficult to determine the importance of negative air balance 
within a building. In addition to those microepidemics in Table 1 in which 
the presence of a negatively balanced air-handling system might have reduced 
the number of infections, there are at least two episodes in which a defective 
air balance was the determining factor: (1) transmission of hog cholera in 
the large animal isolation facility of the National Animal Disease Laboratory 
at Ames, Iowa (52), (2) an unpublished account of a case of psittacosis in 
Building 539 at Fort Detrick in 1961, in an unvaccinated new technician who 
had never worked with this agent but who worked in the fourth laboratory room 
down the corridor from a room in which a Henderson aerosol exposure apparatus (54) 
was employed for the head-only aerosol challenge of mice and guinea pigs. 
The day of probable exposures was suitable to explain the day of onset of 
illness. It must be admitted that maintaining an uninterrupted flow of air 
from the corridor to all laboratory rooms is virtually impossible, because 
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