The biosafety level framework, as defined by the WHO Laboratory Biosafety Manual and the CDC/NIH Biosafety in Microbiological and Biomedical Laboratories (BMBL), assigns organisms to risk groups based on their capacity to cause human disease, the severity of that disease, the availability of treatment or preventive measures, and the risk of community spread. These risk group assignments are then mapped to biosafety levels (BSL-1 through BSL-4) that define the facility design, equipment, practices, and protective equipment required for safe work.
Understanding the framework also means understanding where the nuances are. The biosafety level required is not determined solely by the organism. It is also determined by what you are doing with it. Routine culture of a Risk Group 2 organism on a bench may be acceptable under BSL-2 practices. Generating aerosols of the same organism, through centrifugation without a sealed rotor, sonication, or high-speed homogenisation, may require BSL-3 practices even with the same organism. The procedure changes the risk.
This page covers the full biosafety level classification system, the risk groups and how organisms are assigned to them, the specific facility and practice requirements at each level, how to determine which level applies to your procedure, the types of biological safety cabinets available and what each protects, and the decontamination requirements that govern how contaminated waste is handled.
Risk Groups: How Organisms Are Classified
The WHO classifies infectious agents into four risk groups, each defined by the combination of individual risk to the exposed person and community risk in terms of spread.
Risk Group 1 organisms are those not known to consistently cause disease in immunocompetent, healthy adult humans. They are well-characterised and unlikely to pose any significant hazard to laboratory workers or the community. Examples: non-pathogenic Escherichia coli K-12, Bacillus subtilis (non-toxigenic strains), Saccharomyces cerevisiae, Micrococcus luteus, and most environmental non-pathogenic organisms.
Risk Group 2 organisms can cause human disease but are unlikely to spread widely in the community and are susceptible to effective treatment or preventive measures. Most common clinical pathogens fall into Risk Group 2: Staphylococcus aureus, Streptococcus pyogenes, Salmonella (non-typhoidal serovars), Hepatitis B virus, Influenza A virus (non-pandemic strains), HIV (in a culture setting), Escherichia coli pathogenic strains, Candida albicans, and many others.
Risk Group 3 organisms can cause serious or potentially fatal disease in humans through the respiratory route (or other routes) and may spread in the community, but effective treatment or preventive measures are available. These organisms require significantly higher containment because accidental aerosol exposure can lead to serious infection. Examples: Mycobacterium tuberculosis, SARS-CoV-2, Brucella species, Coxiella burnetii (Q fever), West Nile virus, Yellow fever virus, and HIV when work involves concentration of large volumes.
Risk Group 4 organisms cause severe and often fatal disease in humans for which no effective treatment or vaccines are available, and which represent a high risk of community spread. Only a small number of specialised facilities worldwide are authorised to work with these organisms. Examples: Ebola virus, Marburg virus, Lassa fever virus, Crimean-Congo haemorrhagic fever virus, Nipah virus, and variola virus (smallpox).
Biosafety Level 1: The Starting Point
BSL-1 is appropriate for work with well-characterised agents not known to consistently cause disease in healthy adults. Standard microbiological practices are sufficient, with no special containment equipment required beyond a standard laboratory bench and a functional sink for handwashing.
Required practices at BSL-1: wearing a laboratory coat that covers street clothing, wearing gloves for all procedures that may involve contact with biological material, wearing eye protection when splashes or sprays are possible, washing hands before leaving the laboratory, decontaminating work surfaces after any spill and at the end of each work session, autoclaving or chemically disinfecting all biological waste before disposal. Eating, drinking, applying cosmetics, handling contact lenses, and storing food or drink in the laboratory are prohibited. Mouth pipetting is absolutely prohibited.
BSL-1 is the appropriate level for undergraduate teaching laboratories using well-characterised non-pathogenic strains, and for basic research using standard molecular biology tools such as E. coli K-12.
Biosafety Level 2: The Standard Clinical and Research Level
BSL-2 is appropriate for work with agents that can cause human disease but where the risk of severe disease is moderate and effective treatment is available. Most clinical microbiology diagnostic work and most general microbiology research is performed at BSL-2. BSL-2 builds on BSL-1 with additional requirements that address the specific risks of working with known pathogens.
Additional BSL-2 requirements beyond BSL-1: all work that may generate aerosols or splashes must be performed in a Class II Biological Safety Cabinet (BSC). This is the most important BSL-2 requirement, because aerosol generation from procedures like mixing, vortexing, centrifugation with non-sealed rotors, or pipetting can create infectious droplets that are inhaled. When working outside the BSC, face protection (goggles or face shield) is added where splash risk exists. Access to the laboratory is restricted while work is in progress. A biohazard sign must be posted on the laboratory door when Risk Group 2 organisms are in use.
Sharps use is minimised and managed carefully: needles and syringes are not recapped by hand, sharps are disposed of directly into puncture-resistant sharps containers, and needlestick and sharps injury protocols must be clearly established and posted.
Biosafety Level 3: Serious Pathogens, Serious Containment
BSL-3 is designed for work with agents that may cause serious or potentially fatal disease through respiratory transmission, where aerosol exposure is the primary concern. The facility design at BSL-3 adds multiple layers of engineering control that provide secondary containment: the building itself becomes a barrier between the organisms and the outside environment.
BSL-3 facility requirements include: directional inward airflow maintained by supply and exhaust air handling systems (so any air movement is into the laboratory rather than out), air that is exhausted to the outside through HEPA filters and never recirculated to other building areas, a double-door entry that prevents direct access to the laboratory from the corridor (either a change room or an airlock), surfaces that can be sealed for decontamination (sealed bench surfaces, sealed floors, coved corners). The facility must be separated from general building traffic.
PPE at BSL-3 includes: solid-front gowns or coveralls rather than front-opening lab coats (to prevent skin exposure in the event of a spill), double gloves for all procedures, fit-tested N95 respirators or powered air-purifying respirators (PAPRs) when working outside the BSC, face shields when splash risk exists. All work with the infectious agent is performed within a Class II BSC or equivalent primary containment.
Biosafety Level 4: The Highest Containment
BSL-4 is reserved for agents causing life-threatening disease with no effective treatment or vaccine available, and for which aerosol transmission is a significant risk. BSL-4 facilities are completely isolated from the rest of the building, maintain constant negative air pressure relative to all adjacent spaces, and have dedicated HEPA-filtered air supply and exhaust systems. Entry requires changing out of street clothing and into dedicated laboratory clothing, often followed by a chemical shower upon exit.
Two approaches exist at BSL-4. In a cabinet laboratory, all manipulations are performed inside Class III BSCs (gas-tight, totally enclosed glove boxes) while the operator works in standard BSL-3 PPE in the surrounding laboratory. In a suit laboratory, operators wear positive-pressure suits that completely enclose the body and are supplied with filtered, breathable air from the facility air system or from self-contained breathing apparatus. The entire laboratory environment is the primary containment. Exiting requires a chemical decontamination shower while wearing the suit.
Only approximately 50 BSL-4 facilities exist worldwide, concentrated in the USA, Europe, Australia, Canada, and a small number of other countries. Access to work with BSL-4 agents is tightly regulated through government and institutional biosafety authorities.
Biological Safety Cabinets: The Critical Primary Containment Tool
The BSC is the single most important piece of primary containment equipment in any microbiology laboratory working above BSL-1.
A Class I BSC provides inward airflow protection for the operator by drawing room air into the cabinet through the open front. Exhaust air passes through HEPA filters before being released to the environment. The sample inside the cabinet is not protected from airborne contamination from the room.
A Class II Type A2 BSC, the most widely used type, provides protection for the operator (inward airflow from the room), the sample (HEPA-filtered downward airflow over the work surface), and the environment (HEPA-filtered exhaust). This is the standard BSC for BSL-2 and BSL-3 work.
A Class III BSC is a completely enclosed gas-tight glove box. All manipulations are performed through attached rubber gloves from outside. Supply and exhaust air pass through double HEPA filters. Class III BSCs are used for BSL-4 work in cabinet laboratories.
BSCs must be certified annually by a qualified engineer. They must be decontaminated before filter changes or servicing, typically using vaporised hydrogen peroxide (VHP) or formaldehyde gas.
Decontamination at Each Biosafety Level
All levels: work surfaces are decontaminated with 70 per cent ethanol or 10 per cent bleach (sodium hypochlorite) at the beginning and end of each session and immediately after any spill. All biological waste is autoclaved before disposal.
BSL-2 and above: liquid waste containing infectious agents is chemically treated (bleach added to a minimum final concentration of 0.5 per cent hypochlorite) before disposal to drain, or collected for autoclaving. Solid waste is autoclaved.
BSL-3: pass-through autoclaves within the facility allow autoclaving of waste without removing it from the containment zone. All waste, both solid and liquid, is decontaminated within the zone before removal.
BSL-4: all materials leaving the facility are double-bagged, autoclaved, or chemically decontaminated within the facility before removal. The suit laboratory or cabinet laboratory environment itself is sealed and decontaminated with VHP between sessions when decontamination of the space itself is required.
Frequently Asked Questions
What determines the biosafety level for a specific experiment?
The required BSL depends on two things working together: the risk group of the organism being used, and the procedures being performed. A Risk Group 2 organism might require only BSL-2 for routine culture but BSL-3 practices for aerosol-generating procedures. The risk assessment must consider the organism, the volume, the concentration, the procedure, and any relevant host factors such as working with immunocompromised animal models.
What is the difference between a risk group and a biosafety level?
Risk groups classify organisms based on their inherent hazard properties. Biosafety levels classify the containment facilities and practices required for safe work. They are related but not identical: the same organism may require different BSL practices depending on what procedure is being performed.
Why is a BSL-3 room under negative pressure?
Negative pressure ensures that air flows from lower-risk areas outside the laboratory into the higher-risk laboratory space, rather than outward. If there is an accidental release of infectious material inside a BSL-3 laboratory, the negative pressure prevents aerosols from escaping into corridors or other areas of the building. All exhaust air is HEPA-filtered before release to the environment.
What is a positive pressure suit?
A positive pressure suit, worn in BSL-4 suit laboratories, is a one-piece encapsulating garment that completely surrounds the operator's body including the head, and is supplied with filtered breathable air from a hose connected to the facility air system or from a built-in air supply. The suit is maintained at a pressure slightly above the surrounding room pressure, so any leak in the suit results in air flowing out rather than potentially infectious room air flowing in.
What organisms are worked with at BSL-4?
BSL-4 is required for organisms in WHO/CDC Risk Group 4: primarily the haemorrhagic fever viruses including Ebola virus, Marburg virus, Lassa fever virus, Crimean-Congo haemorrhagic fever virus, and Nipah virus. Variola virus (smallpox) is also a BSL-4 agent but is maintained in only two WHO-authorised repositories worldwide (CDC Atlanta and VECTOR in Russia).
What is a Class II Type A2 BSC?
A Class II Type A2 BSC is the most widely used biological safety cabinet. It provides inward airflow from the room to protect the operator, HEPA-filtered downward airflow over the work surface to protect the sample from environmental contamination, and HEPA-filtered exhaust to protect the environment. Approximately 70 per cent of the air is recirculated through internal HEPA filters over the work surface, and 30 per cent is exhausted to the room or to ducting outside.
What PPE is required at BSL-2?
At BSL-2, PPE includes a laboratory coat (preferably back-fastening to reduce front contamination risk), disposable nitrile gloves, and safety glasses or goggles. When working in the BSC or when performing procedures with splash risk, a face shield is added. Surgical masks are added when working with organisms that have limited respiratory transmission potential and when BSC use is not possible. Dedicated laboratory shoes are recommended.
What should you do after a needlestick in a BSL-2 lab?
Remove the contaminated glove immediately. Allow the wound to bleed freely for 1 to 2 minutes under running water. Wash the site thoroughly with soap and running water for at least 10 minutes. Apply antiseptic. Remove yourself from the laboratory and seek immediate medical attention. Complete an accident report. Report to the laboratory supervisor and biosafety officer immediately. Post-exposure prophylaxis or monitoring may be required depending on the specific organism.
Who oversees biosafety in a research institution?
The Institutional Biosafety Committee (IBC) at university and research settings is responsible for reviewing and approving research involving biological agents. The biosafety officer (BSO) manages day-to-day biosafety compliance, trains staff, conducts inspections, and serves as a resource for biosafety questions. At the national level, biosafety is regulated by the relevant government authority (CDC and NIH in the USA, APHA in the UK, OGTR in Australia).
What is the risk assessment process for a new procedure?
A biosafety risk assessment systematically evaluates: the hazardous properties of the organism (risk group, transmission route, infectious dose, available treatment), the procedure to be performed (does it generate aerosols, involve large volumes, include centrifugation or sonication?), the exposure risk (who is exposed, how, for how long?), and the consequences of exposure (severity of disease, availability of treatment). Based on this assessment, the appropriate containment level and mitigation measures are determined and documented before the work begins.