CDC Biosafety Levels (BSLs) are a set of graded protocols and precautions designed to contain microbes and other disease-causing agents in laboratory settings. There are four biosafety levels, ranging from BSL-1 to BSL-4, each corresponding to the degree of risk posed by the microbes being handled. These levels are determined based on several factors, including the severity of the infection, transmissibility, the type of work being conducted, and the route of exposure.