Match biological agents, practices and facilities to biosafety levels 1 to 4.
Awareness level only. This lesson explains the ideas so you recognise them. It does not qualify you to do this work. Before you handle chemicals, biological materials or lab equipment for real, you need the training your employer or institution requires, and you must work under supervision until they sign you off.
Why it matters
Biological technicians, biochemists and medical laboratory technologists work with microorganisms, cell lines and human samples that can cause infection. You cannot see the hazard, so labs rely on agreed levels of containment. In the US, the reference is Biosafety in Microbiological and Biomedical Laboratories (BMBL), published by the CDC and NIH (currently the 6th edition). It defines four biosafety levels (BSL-1 to BSL-4), each adding practices, safety equipment and facility features on top of the level below.
Risk groups vs biosafety levels
These are related but not the same:
- A risk group (RG1 to RG4) classifies an agent by how dangerous it is: whether it causes disease in healthy adults, how serious, whether it spreads easily, and whether treatment or vaccines exist.
- A biosafety level is the set of containment conditions chosen for a particular piece of work, based on a risk assessment of the agent and the procedure.
Usually RG2 agents are handled at BSL-2, but a procedure that makes aerosols or uses large volumes can call for more containment. The institution's biosafety officer and Institutional Biosafety Committee make that call, not the individual.
The four levels
| Level | Typical agents | Key practices and equipment | Facility |
|---|---|---|---|
| BSL-1 | agents not known to cause disease in healthy adults, e.g. non-pathogenic E. coli K-12 strains, Bacillus subtilis | standard microbiological practices: handwashing, no eating or drinking, no mouth pipetting, decontaminate work surfaces; lab coat, gloves, eye protection as needed | open bench, sink, door |
| BSL-2 | moderate hazard, spread by contact, ingestion or needle-stick, e.g. Staphylococcus aureus, Salmonella, hepatitis B virus, most human blood and tissue work | BSL-1 plus restricted access, biohazard signs, sharps precautions, biosafety cabinet for aerosol-generating work, documented training | self-closing doors, eyewash, autoclave available |
| BSL-3 | serious or potentially lethal disease by inhalation, e.g. Mycobacterium tuberculosis | BSL-2 plus all open work in a biosafety cabinet, respiratory protection as required, medical surveillance, strict access control | controlled access through two self-closing doors, directional inward airflow, exhaust not recirculated |
| BSL-4 | dangerous and exotic agents with high risk of life-threatening disease and no vaccine or treatment, e.g. Ebola virus | BSL-3 plus clothing change, shower on exit, Class III cabinet or positive-pressure suit | separate building or isolated zone, dedicated air and decontamination systems |
Only a small number of BSL-4 labs exist worldwide. As a beginner you will work at BSL-1 or BSL-2.
Worked example: matching work to a level
- A teaching lab grows a non-pathogenic E. coli strain for a plasmid experiment. BSL-1: standard practices on an open bench.
- A hospital lab tests patient blood samples. Human blood can carry bloodborne pathogens, so it is handled under BSL-2 practices with Universal (Standard) Precautions, and US employers must also follow OSHA's Bloodborne Pathogens Standard (29 CFR 1910.1030), which includes annual training and offering the hepatitis B vaccine.
- A research lab grows M. tuberculosis cultures. Transmission is by inhalation, so BSL-3.
- A BSL-2 agent is to be spun in large volumes, which creates aerosols. The risk assessment may require sealed rotors opened in a biosafety cabinet, or enhanced practices; the biosafety officer decides.
The biohazard symbol
The three-ringed biohazard symbol marks rooms, freezers, waste and equipment containing biological hazards. At BSL-2 and above, the door sign also lists the agents, the level, required PPE and an emergency contact.
Standard microbiological practices (every level)
- Wash hands after work and before leaving.
- No eating, drinking, applying cosmetics or handling contact lenses in the lab.
- Never mouth-pipette; use mechanical pipetting devices.
- Minimise splashes and aerosols.
- Decontaminate work surfaces after work and after any spill.
- Decontaminate waste before disposal (often by autoclave).
- Handle sharps with extreme care.
Common mistakes
- Treating "BSL-1" as "no rules". Standard practices still apply.
- Confusing risk group and biosafety level, or picking a level yourself instead of following the institution's risk assessment.
- Forgetting human samples are BSL-2 work even when the patient seems healthy.
- Working outside a biosafety cabinet when a procedure makes aerosols.
Quick recap
- BMBL (CDC/NIH) defines BSL-1 to BSL-4; each level adds practices, equipment and facility features.
- Risk group describes the agent; biosafety level describes the containment chosen after a risk assessment.
- Most entry-level work is BSL-1 or BSL-2; human blood and tissue are handled as BSL-2.
Official source: CDC and NIH, BMBL, https://www.cdc.gov/labs/bmbl/index.html (Checked on 2026-10-23.)