disease, and it recommends guidelines for handling specific agents. The four levels of control, referred to as biosafety levels 1 through 4, describe microbiological practices, safety equipment, and features of laboratory facilities for the corresponding level of risk associated with handling a particular agent. The selection of a biosafety level is influenced by several characteristics of the infectious agent, the most important of which are the severity of the disease, the documented mode of transmission of the infectious agent, the availability of protective immunization or effective therapy, and the relative risk of exposure created by manipulations used in handling the agent.

Biosafety level 1 is the basic level of protection appropriate only for agents that are not known to cause disease in normal, healthy humans. Biosafety level 2 is appropriate for handling a broad spectrum of moderate-risk agents that cause human disease by ingestion or through percutaneous or mucous membrane exposure. Hepatitis B virus, human immunodeficiency virus (HIV), and salmonellae and toxoplasma spp. are representative of agents assigned to this biosafety level. Extreme precaution with needles or sharp instruments is emphasized at this level. A higher level of control may be indicated when some of these agents, especially HIV, are grown and concentrated.

Biosafety level 3 is appropriate for agents with a potential for respiratory transmission and for agents that may cause serious and potentially lethal infections. Emphasis is placed on the control of aerosols by containing all manipulations. At this level, the facility is designed to control access to the laboratory and includes a specialized ventilation system, such as a biological safety cabinet, that minimizes the release of infectious aerosols from the laboratory. The bacterium Mycobacterium tuberculosis is an example of an agent for which this higher level of control is appropriate. Exotic agents that pose a high individual risk of life-threatening disease by the aerosol route and for which no treatment is available are restricted to high containment laboratories that meet biosafety level 4 standards. Worker protection in these laboratories is provided by the use of physically sealed glove boxes or fully enclosed barrier suits that supply breathing air.

Several authoritative reference works are available that provide excellent guidance for the safe handling of infectious microorganisms in the laboratory, one of which is Biosafety in the Laboratory—Prudent Practices for the Handling and Disposal of Infectious Materials (NRC, 1989). Standard microbiological practices described in these references are consistent with the prudent practices used for the safe handling of chemicals.

Practices that are most helpful for preventing laboratory-acquired infections are as follows:

  • Wear protective gloves and a laboratory coat or gown.

  • Wash hands after infectious material is handled, after gloves are removed, and before leaving the laboratory.

  • Perform procedures carefully to reduce the possibility of creating splashes or aerosols.

  • Contain in biological safety cabinets operations that generate aerosols.

  • Use mechanical pipetting devices.

  • Promptly decontaminate work surfaces after spills of infectious materials and when procedures are completed.

  • Never eat, drink, smoke, handle contact lenses, apply cosmetics, or take or apply medicine in the laboratory.

  • Wear eye protection.

  • Take special care when using "sharps," that is, syringes, needles, Pasteur pipets, capillary tubes, scalpels, and other sharp instruments.

  • Keep laboratory doors closed when experiments are in progress.

  • Use secondary leak-proof containers to move or transfer cultures.

  • Decontaminate infectious waste before disposal.

5.E.2 Radioactive Materials

Prudent practices for working with radioactive materials are similar to those needed to reduce the risk of exposure to toxic chemicals (section 5.C has similar information) and to biohazards:

  • Know the characteristics of the radioisotopes that are being used, including half-life, types and energies of emitted radiations, the potential for exposure, how to detect contamination, and the annual limit on intake.

  • Protect against exposure to airborne and ingestible radioactive materials.

  • Never eat, drink, smoke, handle contact lenses, apply cosmetics, or take or apply medicine in the laboratory, and keep food, drinks, cosmetics, and tobacco products out of the laboratory entirely so that they cannot become contaminated.

  • Do not pipet by mouth.

  • Provide for safe disposal of waste radionuclides and their solutions.

  • Use protective equipment to minimize exposures.

  • Use equipment that can be manipulated remotely, as well as shielding, glove boxes, and personal protective equipment, including gloves, clothing, and respirators, as appropriate.

  • Plan experiments so as to minimize exposure by reducing the time of exposure, using shielding against exposure, increasing your distance from the radiation,

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