BSL-3-a is a three-level Laboratory of microbial biosafety and non-air-borne biological factors, commonly known as P3 laboratory. Although its research object can be prevented and controlled, it belongs to high biological hazards (technical specifications described as high individual hazards, low group hazards), such as SARS virus. This kind of biosafety laboratory can be built in the same building as other ordinary laboratories, but it should be divided independently, strictly restrict the flow of people and logistics, generally choose the area far from the external wall of the building, especially the core operating room should be located in the protective zone. BSL-3-a's protective zone includes buffer and core main operating room; auxiliary zone has more clothes, monitoring, washing and disinfection, etc. Access passage should be separated, chemical shower disinfection room and external protective clothing replacement room should be arranged close to the exit of core main operating room.
The P3 laboratory is designed and constructed for 10,000 or 100,000 cleanliness with 15-25 breathing times per hour, non-unidirectional flow, minimum negative pressure-30 PA relative to atmospheric pressure, minimum pressure difference of 10 Pa relative to outdoor direction, noise below 60 dB (A), annual indoor temperature 18-27 C, annual relative humidity 30-70%, negative pressure exhaust from the room and local ventilation of biosafety cabinet. Jointly undertake, generally design interlocking automatic control to ensure constant exhaust air volume, that is, constant negative pressure, exhaust air discharged after medium-effect filtration; P3 biosafety laboratory and adjacent buffer rooms have strict air tightness requirements for maintenance structure, can use 50 mm thick special color steel plate + tempered glass double-layer closed observation window and, the protective area of the envelope structure is prohibited from setting repair openings, The transfer window with fumigation and disinfection function is set between adjacent rooms, which is an important difference from the common microbial cleanroom.