<P> In addition, the facility which houses the BSL - 3 laboratory must have certain features to ensure appropriate containment . The entrance to the laboratory must be separated from areas of the building with unrestricted traffic flow . Additionally, the laboratory must be behind two sets of self - closing doors (to reduce the risk of aerosols escaping). The construction of the laboratory is such that it can be easily cleaned . Carpets are not permitted, and any seams in the floors, walls, and ceilings are sealed to allow for easy cleaning and decontamination . Additionally, windows must be sealed, and a ventilation system installed which forces air to flow from the "clean" areas of the lab to the areas where infectious agents are handled . Air from the laboratory must be filtered before it can be recirculated . </P> <P> Biosafety level 3 is commonly used for research and diagnostic work involving various microbes which can be transmitted by aerosols and / or cause severe disease . These include Francisella tularensis, Mycobacterium tuberculosis, Chlamydia psittaci, Venezuelan equine encephalitis virus, Eastern equine encephalitis virus, SARS coronavirus, Coxiella burnetii, Rift Valley fever virus, Rickettsia rickettsii, several species of Brucella, chikungunya, yellow fever virus, and West Nile virus . </P> <P> Biosafety level 4 (BSL - 4) is the highest level of biosafety precautions, and is appropriate for work with agents that could easily be aerosol - transmitted within the laboratory and cause severe to fatal disease in humans for which there are no available vaccines or treatments . BSL - 4 laboratories are generally set up to be either cabinet laboratories or protective - suit laboratories . In cabinet laboratories, all work must be done within a class III biosafety cabinet . Materials leaving the cabinet must be decontaminated by passing through an autoclave or a tank of disinfectant . The cabinets themselves are required to have seamless edges to allow for easy cleaning . Additionally the cabinet and all materials within must be free of sharp edges in order to reduce the risk of damage to the gloves . In a protective - suit laboratory, all work must be done in a class II biosafety cabinet by personnel wearing a positive pressure suit . In order to exit the BSL - 4 laboratory, personnel must pass through a chemical shower for decontamination, then a room for removing the positive - pressure suit, followed by a personal shower . Entry into the BSL - 4 laboratory is restricted to trained and authorized individuals, and all persons entering and exiting the laboratory must be recorded . </P> <P> As with BSL - 3 laboratories, BSL - 4 laboratories must be separated from areas that receive unrestricted traffic . Additionally airflow is tightly controlled to ensure that air always flows from "clean" areas of the lab to areas where work with infectious agents is being performed . The entrance to the BSL - 4 lab must also employ airlocks to minimize the possibility that aerosols from the lab could be removed from the lab . All laboratory waste, including filtered air, water, and trash must also be decontaminated before it can leave the facility . </P>

Which bsl level is for lethal agents for which there are no vaccines or other countermeasures