Explain what a chemical fume hood and a biosafety cabinet each protect, and the habits that keep them working.
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
Fume hoods and biosafety cabinets are the most important engineering controls in a lab: they keep hazards away from your lungs without relying on PPE. They look similar (a box with a glass sash and airflow), but they protect against different things and are not interchangeable. Using the wrong one, or using the right one badly, can expose you and everyone in the room.
The chemical fume hood
A chemical fume hood pulls room air in through the front opening, past your work, and exhausts it out of the building through ductwork and a fan on the roof.
- Protects: you (and the lab) from chemical vapours, gases, dusts and small splashes (the sash acts as a shield).
- Does not protect: your sample from contamination. Air is unfiltered room air.
- Typical face velocity: around 80 to 120 feet per minute (about 0.4 to 0.6 m/s) at the working sash height; your institution tests and certifies each hood, often yearly, and marks the approved sash height.
Using a fume hood well
- Check that it is working before starting: look at the airflow monitor or alarm, or a tissue strip at the sash. If it alarms or looks wrong, do not use it; report it.
- Work at least 6 inches (15 cm) inside the sash, where capture is reliable.
- Keep the sash at or below the marked height, and lower it when you are not reaching in. The sash protects your face.
- Do not clutter it. Bottles and equipment block airflow to the rear baffle slots. Raise large equipment on blocks so air flows underneath.
- Do not store chemicals in the working hood. Use ventilated storage cabinets.
- Move slowly. Fast arm movements and people walking past create turbulence that pulls vapour out.
- Never put your head inside the hood.
The biological safety cabinet (BSC)
A biosafety cabinet uses HEPA filters (which trap at least 99.97% of particles at 0.3 µm) to clean the air. The most common type, Class II, gives three kinds of protection:
- Personnel: room air is drawn in at the front grille, so aerosols do not escape toward you.
- Product: HEPA-filtered air flows down over the work surface, protecting cultures from contamination.
- Environment: exhaust air passes through a HEPA filter.
| Class | Protects | Use |
|---|---|---|
| Class I | personnel, environment | rarely used now |
| Class II (A2, B1, B2) | personnel, product, environment | most BSL-2 and BSL-3 work |
| Class III | maximum: a sealed glove box | BSL-4 and the highest-risk work |
Important: HEPA filters trap particles, not chemical vapours. Many Class II cabinets (type A2) recirculate most of their air back into the room or cabinet, so they are not for volatile or toxic chemicals unless your biosafety officer has approved a ducted type for that purpose.
Using a BSC well
- Let it run for a few minutes before work (follow your SOP), and check the airflow indicator.
- Disinfect the work surface before and after (often 70% ethanol or another approved disinfectant).
- Load everything you need before starting, so you do not move in and out.
- Never block the front or rear grilles with paper, arms or equipment.
- Work from clean to dirty, in the middle to back of the work surface.
- No open flames: they disrupt airflow and can damage the filter. Use sterile disposable loops or electric incinerators.
- UV lamps are not a substitute for chemical disinfection.
Clean benches are not either one
A laminar flow clean bench blows HEPA-filtered air toward you. It protects the product only. Never use one for biological hazards or chemicals.
Worked example: which one?
| Task | Use | Why |
|---|---|---|
| Diluting concentrated ammonia | fume hood | vapour hazard to you; sample sterility does not matter |
| Passaging a human cell line | Class II BSC | protect you from potential agents and the cells from contamination |
| Preparing sterile media without hazards | BSC or clean bench | product protection is the goal |
| Using chloroform to extract DNA from a BSL-2 sample | ask your biosafety officer | both hazards: may need a ducted BSC or a split procedure |
Common mistakes
- Using a BSC for volatile chemicals.
- Using a fume hood for sterile cell culture.
- Raising the sash fully to make reaching easier.
- Covering the front grille of a BSC with absorbent paper.
- Ignoring an alarm or an out-of-date certification sticker.
Quick recap
- Fume hood: exhausts chemical vapours outside; protects you, not your sample.
- Class II BSC: HEPA-filtered; protects you, your sample and the environment from biological aerosols, not from chemical vapours.
- Keep work deep inside, sashes low, grilles clear, and check the airflow indicator every time.
Official source: OSHA Laboratories, https://www.osha.gov/laboratories (Checked on 2026-10-23.)