Understanding BSL Classifications and Pathogens

What Does BSL Mean?

“BSL” stands for Biosafety Level – a tiered system of containment safeguards used in laboratory settings to minimise the risk of exposure to infectious agents. These controls are designed to protect laboratory personnel, the community, and the environment, and vary depending on the microbiological agent in use (gov.uk).

In the UK, BSLs are more commonly referred to as Containment Levels (CL1–CL4), defined by the Advisory Committee on Dangerous Pathogens (ACDP) and regulated by the Health and Safety Executive (HSE). In practice, the UK uses CL3 and CL4 terminology when referring to work with high-risk human pathogens.

For animal pathogens, including zoonotic diseases, the UK applies a parallel classification known as Animal Containment Levels (ACL1–ACL4) under the Specified Animal Pathogens Order (SAPO) 2008, regulated by DEFRA and the Animal and Plant Health Agency (APHA).

Each system – whether CL, ACL, or BSL – represents a progressive increase in safety measures involving microbiological practices, safety equipment, and facility engineering. Internationally, the same structured approach is reflected by the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC), which use the terms BSL-1 to BSL-4.

Overview of BSL Classifications (1 to 4)

Each BSL level builds on the last, adding specific controls for laboratory practices, safety equipment, and facility design.

BSL–1

As the lowest of the four, biosafety level 1 applies to laboratory settings in which personnel work with low-risk microbes that pose little to no threat of infection in healthy adults. An example of a microbe typically worked with at BSL-1 is a non-pathogenic strain of E. coli.

BSL–2

This biosafety level covers laboratories that work with agents associated with human diseases (i.e. pathogenic or infectious organisms) that pose a moderate health hazard. Examples of agents worked with in BSL-2 include equine encephalitis viruses, HIV, and Staphylococcus aureus.

BSL–3

Building upon the two prior levels, BSL-3 laboratories work with microbes that are either indigenous or exotic and can cause serious or potentially lethal disease through inhalation. Examples include yellow fever, West Nile virus, and the bacteria that causes tuberculosis (Containment Level 3 Lab).

BSL–4

BSL-4 labs are rare. As the highest level of biological safety, a BSL-4 lab involves work with highly dangerous and exotic microbes. These infections are frequently fatal and come without treatment or vaccines – such as Ebola and Marburg viruses.

Why BSL Standards Matter in Laboratory Design

Whether in the form of BSLs, Containment Levels, or Animal Containment Levels, these frameworks require laboratories to adopt progressively more robust safety measures. BSL-3 and BSL-4 (CL3 and CL4 in the UK) laboratories must meet specific regulatory standards to protect workers and the public from exposure to dangerous pathogens.

• Negative air pressure to ensure air flows inward
• Sealable interiors for gaseous disinfection (fumigation)
• Independent HVAC systems with HEPA-filtered exhaust
• Interlocking doors and restricted access systems

UK ACDP guidance (HSE containment guidance) specifies that CL3 facilities must have continuous, sealed walls and ceilings, and all pipe or cable penetrations must be gas tight. These design principles are not optional – they are legal requirements to mitigate biosafety risks and protect public health.

PBSC’s Role in Supporting High-Containment Labs

PBSC Ltd is a UK-based manufacturer of specialist equipment engineered for use in high-containment environments such as BSL-3 and BSL-4 laboratories. Our products are used globally in facilities where safety, performance, and compliance with stringent biosafety standards are essential.

PBSC solutions are designed to support containment at the building envelope level – facilitating secure personnel and material transfer while maintaining environmental integrity. We provide not only high-specification equipment but also comprehensive services, including system installation, validation, and ongoing preventative maintenance.

• Sealed Access Doors
  • APR Inflatable Seal Door Set
  • Mechanical Seal Door
• Vision Panels
• Transfer Equipment
  • Barrier Hatch
  • High-Containment Chemical Dunk Tank
  • Decontamination Hatch
• Decontamination Chamber

All PBSC high-containment systems are designed to integrate into facility-wide biosafety protocols and support long-term operational reliability in critical environments.

For more information on Understanding BSL Classifications and Pathogens talk to PBSC Limited