https://consteril.com/biosafety-levels-difference/

A specialized research laboratory that deals with potentially deadly infectious agents like Ebola would be designated as BSL-4—the highest and most stringent level.

The Centers for Disease Control and Prevention (CDC) sets BSL lab levels as a way of exhibiting specific controls for the containment of microbes and biological agents. Each BSL lab level builds upon on the previous level—thereby creating layer upon layer of constraints and barriers. These lab levels are determined by the following

• Risks related to containment
• Severity of infection
• Transmissibility
• Nature of the work conducted
• Origin of the microbe
• Agent in question
• Route of exposure