Biological Safety Level
The Biological Safety Level (BSL) is used in the USA to describe a set of specifications for the precautions required to isolate potentially hazardous biological agents in a containment facility at a specified stringency, ranging from BSL-1, which indicates the least strict containment, to BSL-4, which is the containment required for the most hazardous human pathogens. A similar scale is used in the EU using the abbreviation P1-P4, where P indicates either Pathogen or Protection. For instance, a BSL-4 laboratory (P4 in Europe) is under reduced pressure relative to atmospheric pressure, the exhausted air is filtered, and all wastes including water and any hardware leaving the high-containment area are sterilized chemically or by heating. Trained personnel work inside the area in protective suits, overpressurized by air pumped in from outside through a dedicated system. The exterior of the suit is chemically disinfected when workers leave the area.