NIH/CDC have defined 4 Biosafety Levels. Biosafety Levels (BSL) are levels of containment (1 lowest, 4 highest containment.)
BSL 1 - represents a basic level of containment that relies on standard microbiological practices with no special primary or secondary barriers recommended, other than a sink for hand washing.
BSL 2 - is suitable for work involving agents of moderate potential hazard to personnel and the environment.
BSL 3 - is suitable for work involving indigenous or exotic agents which may cause serious or potentially lethal disease as a result of exposure by inhalation.
BSL 4 - required for work with dangerous and exotic agents that pose a high risk of aerosol-transmitted laboratory infections and life-threatening disease.
For details on the criteria, practices, and facility requirements for each Biosafety go to
Biosafety Levels - Biosafety in Microbiological and Biomedical Laboratories (BMBL) 5th Edition