A CDC booklet titled Classification of Etiological Agents on the Basis of Hazard served as a reference for assessing the relative risks of working with various biologic agents until an updated CDC/NIH document was published titled Biosafety in Microbiological and Biomedical Laboratories. The fourth edition of this manual is currently available on the CDC website (hup.7/www.cdcgov/od/ ohs/biosfty/biosfty.htm). In general, patient specimens pose a greater risk to laboratory workers than do microorganisms in culture, because the nature of etiologic agents in patient specimens is initially unknown. Biosafety Level 1 agents include those that have no known potential for infecting healthy people. These agents are used in laboratory teaching exercises for beginning-level students of microbiology. Level 1 agents include Bacillus subtilis and Mycobacterium gordome. Precautions for working with Level I agents include standard good laboratory technique as described previously.
Biosafety Level 2 agents are those most commonly being sought in clinical specimens, and they include all the common agents of infectious disease, as well as HIV and several more unusual pathogens. For handling dinical specimens suspected of harboring any of these pathogens, Biosafety Level 2 precautions are "sufficient. This level of safety includes the principles outlined previously plus limiting access to the laboratory during working procedures, training laboratory personnel in handling pathogenic agents, direction by competent supervisors, and performing aerosol-generating procedures in a BSC. Employers must offer hepatitis B vaccine to all employees determined to be at risk of exposure. Bacillus anthracis and Yersinia pestis, two organisms mentioned as possible bioterrorism agents, are actually listed as BSL-2 organisms.
Biosafety Level 3 procedures have been recommended for the handling of material suspected of harboring viruses unlikely to be encountered in a routine clinical laboratory, and for cultures growing Mycobacterium tuberculosis: the mold stages of systemic fungi, and for some other organisms when grown in quantities greater than that found in patient specimens. These precautions, in addition to those undertaken for Level 2, consist of laboratory design and engineering controls that contain potentially dangerous material by careful control of air movement and the requirement that personnel wear protective clothing and gloves, for instance. Persons working with Biosafety Level 3 agents should have baseline sera stored for comparison with acute sera that can be drawn in the event of unexplained illness. Francisella tularensis and Brucella spp. isolated from naturally occurring infections or infections following a bioterrorist event, are both BSL-3 pathogens. BSL-3 organisms are transmitted primarily by aerosols.
Biosafety Level 4 agents, which include only certain viruses of the arbovirus, arenavirus, or filo-virus groups, none of which are commonly found in the United States, require the use of maximum containment facilities. Personnel and all materials must be decontaminated before leaving the facility, and all procedures are performed under maximum containment (spedal protective clothing, Class m BSCs). Most of these facilities are public health or research laboratories. Potential bioterrorist agents such as smallpox require BSL-4 facilities. BSL-4 agents pose life-threatening risks, are transmitted via aerosols and do not have an available vacdne or therapy.
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