Biosafety Level 3 (BSL-3)

Biosafety Level 3 (BSL-3) is the third of the four graded laboratory-containment tiers defined by the CDC/NIH publication Biosafety in Microbiological and Biomedical Laboratories (BMBL, 6th edition, 2020). A protocol is assigned BSL-3 when it involves agents that can cause serious or potentially lethal disease through inhalation of infectious aerosols, but for which a licensed vaccine or effective treatment is routinely available. This page covers what BSL-3 specifically requires; for how BSL-3 relates to BSL-1, BSL-2, and BSL-4, and how a Biosafety Level is formally assigned, see the general Biosafety Level (BSL) entry — that context isn’t repeated here.

What makes a protocol BSL-3

BSL-3 sits between BSL-2’s moderate-hazard, contact-transmitted precautions and BSL-4’s maximum containment for high-consequence agents with no available countermeasure. What distinguishes BSL-3 from the level below it is the transmission route: BSL-3 agents are efficiently transmitted by inhalation of infectious aerosols, so containment has to control the air itself, not just contact and sharps exposure. What distinguishes BSL-3 from the level above it is the availability of a real countermeasure — a licensed vaccine, an effective treatment, or both — which is why BSL-3 agents, despite their severity, don’t require BSL-4’s suit laboratories or Class III cabinet lines. As with every biosafety level, BSL-3 is not defined by agent identity alone but by the combination of the agent’s Risk Group (most Risk Group 3 agents are handled at BSL-3, though an Institutional Biosafety Committee’s protocol-specific risk assessment can move a given protocol above or below that default), the practices followed, the PPE used, and the facility’s engineering controls.

Typical agents handled at BSL-3

Agents commonly assigned BSL-3 containment include Mycobacterium tuberculosis (the causative agent of tuberculosis), Coxiella burnetii (Q fever), Bacillus anthracis (anthrax, in forms and quantities that pose an inhalation risk), Francisella tularensis (tularemia), and Yersinia pestis (plague) — all agents capable of serious or lethal disease via inhalation, but all treatable with antibiotics or, in some cases, preventable with a vaccine. SARS-CoV-2 also illustrates the BSL-3 profile historically: CDC interim guidance issued in 2020 placed virus isolation and propagation work at BSL-3 with enhanced respiratory-protection practices, while routine diagnostic testing (which does not involve viral culture) was placed at BSL-2 — reflecting the same severity/aerosol-transmission-with-available-countermeasure logic that governs BSL-3 generally, once vaccines and treatments became available. CDC has since rescinded that interim elevated-BSL-3 guidance, so this is best read as a historical illustration of how the BSL-3 criteria were applied to a real emerging pathogen, not as SARS-CoV-2’s current, present-tense classification — an institution’s own IBC risk assessment governs what containment a given SARS-CoV-2 protocol requires today. This is a representative, not exhaustive, list; the actual containment for any specific agent is determined by an Institutional Biosafety Committee (IBC) risk assessment, not inferred from this list alone.

Required practices at BSL-3

BSL-3 builds on BSL-2 practice by adding controls aimed specifically at aerosol containment, on top of the contact/ingestion/sharps controls BSL-2 already requires:

• Controlled access. Laboratory access is restricted and controlled at all times, not just while work with the agent is actively in progress — typically through a controlled-access anteroom or double-door entry, rather than BSL-2’s simple restricted-access-during-work approach.
• All agent manipulation performed inside a biosafety cabinet. Unlike BSL-2, where only aerosol- or splash-generating steps require a biosafety cabinet, at BSL-3 every open manipulation of the agent is performed inside a certified Class II (or higher) biosafety cabinet (BSC) — none of the work is done on the open bench.
• Respiratory protection where indicated. Depending on the specific agent and procedure, personnel may be required to use respiratory protection (e.g., a fit-tested N95 respirator or a powered air-purifying respirator) for activities that carry residual aerosol risk outside primary containment.
• A written, agent-specific biosafety manual and standard operating procedures, with documented personnel training before independent work begins — a more formal training and competency-documentation expectation than at BSL-2.
• Baseline and, where indicated, periodic occupational health surveillance appropriate to the agent — for example, baseline and periodic tuberculosis skin/blood testing for personnel working with M. tuberculosis.
• Decontamination of all laboratory waste before disposal, typically by autoclaving within the BSL-3 suite itself rather than transport to a shared facility autoclave.

Safety equipment and facility features at BSL-3

The defining engineering control at BSL-3 is directional, inward airflow: air moves from lower-risk to higher-risk areas of the facility and is not recirculated, so that any aerosol released inside the laboratory is drawn away from the exit rather than toward it. This is combined with:

• Sustained directional inward airflow, continuously monitored (commonly with a visual airflow indicator at the entrance) to confirm the pressure differential is maintained.
• A controlled, self-closing double-door entry (an anteroom or air lock) — personnel pass through two sets of doors rather than BSL-2’s single self-closing door, and the two doors are not normally opened simultaneously.
• HEPA-filtered exhaust air, discharged so it cannot re-enter the building’s air-handling system.
• Sealed penetrations in walls, floors, and ceilings, and surfaces that can withstand the disinfectants used for routine decontamination.
• A biosafety cabinet for all open manipulation of the agent, plus standard PPE (front-closing gowns or coveralls dedicated to the BSL-3 suite, gloves, and respiratory protection where the risk assessment indicates it).
• Dedicated, restricted-access laboratory space — separated from general building traffic, distinct from BSL-2’s requirement of a lockable door within a shared corridor.

What BSL-3 does not require, by contrast with BSL-4, is a dedicated isolated building, a positive-pressure supplied-air suit or Class III cabinet line for all work, redundant/backup air-handling systems, or a mandatory personnel decontamination shower on exit — those additional controls are reserved for agents with no available vaccine or treatment.

Who assigns BSL-3, and how it’s reviewed

For research involving recombinant or synthetic nucleic acid molecules, BSL-3 — like every biosafety level — is formally assigned by an institution’s Institutional Biosafety Committee (IBC) under the NIH Guidelines, as part of protocol registration and risk assessment, before laboratory work may begin. Many BSL-3 agents (including several listed above) also appear on the HHS/USDA Federal Select Agent Program list, which layers additional registration, personnel-suitability, and security requirements on top of IBC biosafety review, not instead of it. BSL-3 work with a select agent, or with any human pathogen the current BMBL recommends handling at BSL-3 or BSL-4, also falls within Dual Use Research of Concern (DURC) Category 1 oversight. The IBC’s registration and risk-assessment process, its minimum composition requirements, and how BSL relates to an agent’s underlying Risk Group are covered in full on the general Biosafety Level (BSL) page rather than repeated here. IBC review of laboratory biosafety is a separate compliance track from an Institutional Review Board (IRB)‘s review of human-subjects research ethics or a Scientific Review Committee‘s scientific-merit review — a single project can require sign-off from more than one of these committees, each reviewing a different dimension of the same work.

Worked examples

Example 1. A clinical microbiology and research laboratory that cultures Mycobacterium tuberculosis isolates operates under BSL-3: all manipulation of live cultures is performed inside a certified Class II biosafety cabinet, the suite is entered through a controlled double-door anteroom with continuously monitored directional inward airflow, personnel wear dedicated protective clothing and use respiratory protection for specific higher-risk procedures, and staff undergo baseline and periodic TB testing as part of the lab’s occupational health program under its IBC-approved biosafety manual.

Example 2. A public health reference laboratory investigating a suspected Yersinia pestis (plague) isolate handles the specimen at BSL-3 from the point its identity is suspected: work is confined to a certified biosafety cabinet within the BSL-3 suite, the isolate is also registered and tracked under the Federal Select Agent Program given Y. pestis‘s select-agent status, and laboratory waste is autoclaved within the suite before removal, consistent with both the lab’s IBC-approved protocol and its select-agent security plan.

Counter-example

A laboratory culturing Ebola virus is not a BSL-3 example, even though Ebola — like the agents typically handled at BSL-3 — causes serious disease via a route that includes aerosol exposure risk in some laboratory procedures: because no vaccine or treatment is routinely available for most exposed populations and the BMBL and virtually every IBC classify Ebola virus as requiring maximum containment, this work is assigned BSL-4, which adds a dedicated isolated facility, a positive-pressure suit laboratory or Class III biosafety-cabinet line for all agent manipulation, redundant HEPA-filtered air systems, and a mandatory decontamination shower on exit — controls that go beyond what BSL-3 requires. Disease severity and inhalation risk alone don’t determine the level; whether an effective countermeasure exists is what separates BSL-3 from BSL-4.

Related terms

• Biosafety Level (BSL) — the full four-tier system (BSL-1 through BSL-4) and how a level is assigned
• Biosafety Level 2 (BSL-2) — the moderate-hazard, contact/ingestion-focused tier immediately below BSL-3
• Biosafety Level 4 (BSL-4) — maximum containment, for high-consequence agents with no available vaccine or treatment
• IRB (Institutional Review Board)
• Scientific Review Committee
• Laboratory Safety (OSHA Chemical Hygiene Plan) — the parallel OSHA framework for chemical, rather than biological, laboratory hazards

For the broader compliance context this term sits within, see the Integrity & Compliance cluster page.