The levels

Biosafety Level 1 is suitable for work involving well-characterized agents not known to consistently cause disease in healthy adult humans, and of minimal potential hazard to laboratory personnel and the environment.[citation needed] Includes several kinds of bacteria and viruses including , Escherichia coli, Varicella (Chicken Pox), as well as some cell cultures and non-infectious bacteria. At this level precautions against the biohazardous materials in question are minimal, most likely involving gloves and some sort of facial protection. The laboratory is not necessarily separated from the general traffic patterns in the building. Work is generally conducted on open bench tops using standard microbiological practices. Usually, contaminated materials are left in open (but separately indicated) trash receptacles. Decontamination procedures for this level are similar in most respects to modern precautions against everyday viruses (i.e.: washing one's hands with anti-bacterial soap, washing all exposed surfaces of the lab with disinfectants, etc). In a lab environment, all materials used for cell and/or bacteria cultures are decontaminated via autoclave. Laboratory personnel have specific training in the procedures conducted in the laboratory and are supervised by a scientist with general training in microbiology or a related science.
Biosafety Level 2 is similar to Biosafety Level 1 and is suitable for work involving agents of moderate potential hazard to personnel and the environment. Includes various bacteria and viruses that cause only mild disease to humans, or are difficult to contract via aerosol in a lab setting, such as hepatitis A, B, and C, Influenza A, Lyme disease, Dengue fever, Salmonella, Mumps Bacillus subtilis, Measles, HIVScrapie.
It differs from BSL-1 in that
laboratory personnel have specific training in handling pathogenic agents and are directed by competent scientists;
access to the laboratory is limited when work is being conducted;
extreme precautions are taken with contaminated sharp items; and
certain procedures in which infectious aerosols or splashes may be created are conducted in biological safety cabinets or other physical containment equipment.
Biosafety Level 3 is applicable to clinical, diagnostic, teaching, research, or production facilities in which work is done with indigenous or exotic agents which may cause serious or potentially lethal disease as a result of exposure by the inhalation route. Includes various bacteria and viruses that can cause severe to fatal disease in humans, but for which vaccines or other treatment exist, such as Anthrax, West Nile virus, Venezuelan equine encephalitis, Eastern equine encephalitis, SARS, Tuberculosis, Typhus, Rift Valley fever, Rocky Mountain spotted feve, Yellow fever.
Laboratory personnel have specific training in handling pathogenic and potentially lethal agents, and are supervised by competent scientists who are experienced in working with these agents. This is considered a neutral or warm zone.
All procedures involving the manipulation of infectious materials are conducted within biological safety cabinets or other physical containment devices, or by personnel wearing appropriate personal protective clothing and equipment. The laboratory has special engineering and design features.
It is recognized, however, that some existing facilities may not have all the facility features recommended for Biosafety Level 3 (i.e., double-door access zone and sealed penetrations). In this circumstance, an acceptable level of safety for the conduct of routine procedures, (e.g., diagnostic procedures involving the propagation of an agent for identification, typing, susceptibility testing, etc.), may be achieved in a Biosafety Level 2 facility, providing
the exhaust air from the laboratory room is discharged to the outdoors,
the ventilation to the laboratory is balanced to provide directional airflow into the room,
access to the laboratory is restricted when work is in progress, and
the recommended Standard Microbiological Practices, Special Practices, and Safety Equipment for Biosafety Level 3 are rigorously followed.
The decision to implement this modification of Biosafety Level 3 recommendations are made only by the laboratory director.
Biosafety Level 4 is required for work with dangerous and exotic agents that pose a high individual risk of aerosol-transmitted laboratory infections, agents which cause severe to fatal disease in humans for which vaccines or other treatments are not available, such as Bolivian and Argentine hemorrhagic fevers, Smallpox (there is a vaccine), Marburg virus,Ebola virus, Hantaviruses, Lassa fever, Crimean-Congo hemorrhagic fever, and other various hemorrhagic diseases.When dealing with biological hazards at this level the use of a Hazmat suit and a self-contained oxygen supply is mandatory. The entrance and exit of a Level Four biolab will contain multiple showers, a vacuum room, an ultraviolet light room, and other safety precautions designed to destroy all traces of the biohazard. Multiple airlocks are employed and are electronically secured to prevent both doors opening at the same time. All air and water service going to and coming from a Biosafety Level 4 lab will undergo similar decontamination procedures to eliminate the possibility of an accidental release.
Agents with a close or identical antigenic relationship to Biosafety Level 4 agents are handled at this level until sufficient data are obtained either to confirm continued work at this level, or to work with them at a lower level.
Members of the laboratory staff have specific and thorough training in handling extremely hazardous infectious agents and they understand the primary and secondary containment functions of the standard and special practices, the containment equipment, and the laboratory design characteristics. They are supervised by competent scientists who are trained and experienced in working with these agents. Access to the laboratory is strictly controlled by the laboratory director.
The facility is either in a separate building or in a controlled area within a building, which is completely isolated from all other areas of the building. A specific facility operations manual is prepared or adopted. Building protocols for preventing contamination often uses negatively pressurized facilities, which, if compromised, would severely inhibit an outbreak of aerosol pathogens.
Within work areas of the facility, all activities are confined to Class III biological safety cabinets, or Class II biological safety cabinets used with one-piece positive pressure personnel suits ventilated by a life support system. The Biosafety Level 4 laboratory has special engineering and design features to prevent microorganisms from being disseminated into the environment. The laboratory is kept at negative air pressure, so that air flows into the room if the barrier is penetrated or breached. Furthermore, an airlock is used during personnel entry and exit.
Biosafety Level 5 does not exist, given that the protocols followed by a Biosafety Level 4 facility are as strict as technically feasible. In popular culture, this level has been referenced. However, the protocols used are usually so extreme they seem focused on preventing the object of study from being inadvertently contaminated by the researchers studying it than preventing the researchers from being inadvertently contaminated by what they are studying. The novel The Andromeda Strain by Michael Crichton, along with the 1971 film of the same name, directed by Robert Wise reference a fictional Level 5 facility. The novel Mount Dragon by Douglas Preston and Lincoln Child also references such a facility, and its measures likely have a similar purpose.