Rationale

Biocontainment can be classified by the relative danger to the surrounding environment as biological safety levels (BSL). As of 2006, there are four safety levels. These are called BSL1 through BSL4, with one anomalous level BSL3-ag for agricultural hazards between BSL3 and BSL4. Higher numbers indicate a greater risk to the external environment. See biological hazard.
At the lowest level of biocontainment, the containment zone may only be a fume hood that utilizes HEPA filters. At the highest level the containment involves isolation of the organism by means of building systems, sealed rooms, sealed containers, personal isolation equipment similar to "space suits" and elaborate procedures for entering the room, and decontamination procedures for leaving the room. In most cases this also includes high levels of security for access to the facility, ensuring that only authorized personnel may be admitted to any area that may have some effect on the quality of the containment zone. This is considered a hot zone.