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The risk of laboratory-acquired infections can be placed in clearer 
perspective when compared with the risk of loss -time or disabling injuries 
experienced in other occupations. For example, the rate of disabling 
injuries of all kinds at Ft. Detrick in 1964, including hospitalized 
laboratory-acquired illnesses, was 0.49 per million man-hours worked. 
This compares favorably with the experience for the same period in 
other industries as reported by the National Safety Council. The 
disabling injuries per million man-hours worked in 1974 for several 
industries were as follows: communication industry, 1.20 (lowest of 
all reported); automobile industry, 1.86; chemical industry, 3.12; 
Federal Government, 7.3; printing and publishing industry, 9.35; lumber 
industry, 17.76; air transport industry, 18.77; and mining industry, 
37. 78. This experience suggests that research involving human pathogens, 
where the hazard is recognized and safe practices and containment 
equipment are used, is a relatively safe occupation. 
It is important to emphasize the role of containment equipment in 
protecting the laboratory worker from potential exposures to micro- 
organisms containing recombinant DNA molecules. Analysis of 
comprehensive surveys of laboratory-acquired infections (3, 4, 5) 
demonstrates that fewer than 20 percent of known infections can be 
attributed to a documented accidental exposure. The knowledge that 
most microbiological practices create aerosols (6) suggests that 
inhalation of undetected aerosols may contribute significantly to 
occupational illness among laboratory workers who handle human 
pathogens. It is on this conclusion that physical containment equip- 
ment is required by the NIH Guidelines for control of potential aerosol 
exposures. Thus, as the assessed hazard level increases from Pi to 
P4, the NIH Guidelines require greater use of physical containment 
equipment. For example, procedures that have the potential for pro- 
ducing considerable aerosols are required to be isolated by physical 
containment equipment when these procedures involve microorganisms 
that require P2 containment. All procedures involving moderate-risk 
materials (P3), however, are required to be physically contained or 
isolated in such equipment, and all procedures involving high-risk 
materials (P4) are required to be contained in gas-tight biological 
safety cabinets. 
Open-front biological safety cabinets are used to contain organisms 
requiring P2 and P3 levels of physical containment. Personnel pro- 
tection against aerosol exposures is provided by the movement of air 
into the cabinet through the open front. This air flow has an average 
velocity of at least 75 feet per minute. These cabinets are designed 
so that fewer than 20 microorganisms will escape through the open 
8 8 
front when 1 x 10 (100, 000, 000) to 8 x 10 (800, 000, 000) micro- 
organisms are experimentally released within the cabinet. 
