What Is a Fume Cupboard For?

Every school and university chemistry lab has one: a glass-fronted cabinet with a sliding window, a steady hum of fans, and a sign saying keep sash lowered. It’s called a fume cupboard in British English and a fume hood in American English, and it’s one of the most important pieces of safety equipment in chemistry. Yet many students use one without knowing how it works or what it can and can’t do.

What it does

A fume cupboard is a ventilated enclosure that captures hazardous gases, vapours, fumes and dusts at the source and carries them away, so they’re not breathed in by the person working at it or anyone else in the room.

It does three jobs at once:

1. Containment: keeps harmful substances inside the cabinet.
2. Removal: extracts them through ducting, usually to the outside, high above the building.
3. Protection: the sash (front window) forms a physical shield against splashes and, to a limited extent, small explosions or fires.

How it works

A fan, usually on the roof, pulls air out of the cabinet through a duct. To replace that air, room air flows in through the front opening. This steady inward flow means any gas released inside is swept away from the user and up the duct, rather than drifting out into the lab.

Face velocity

The speed of air entering the front opening is called the face velocity. It needs to be:

• high enough to stop fumes escaping (typically around 0.4–0.5 m/s for many conventional fume cupboards, though standards differ between countries and designs)
• not so high that it creates turbulence, which can actually pull fumes back out around the user’s body

Many fume cupboards have an airflow indicator or alarm that sounds if the flow falls too low. If it’s alarming, don’t use it.

Baffles

Inside, slotted panels at the back (baffles) distribute the extraction so that both heavy vapours near the work surface and lighter ones near the top are drawn away evenly.

The sash

The sliding front window controls the size of the opening. A lower sash means:

• a smaller opening, so higher face velocity and better containment
• more of your body shielded by the glass

Work with the sash as low as practical, ideally at or below the marked maximum working height, and close it fully when you’re not working inside.

When must you use one?

Use a fume cupboard whenever an activity might release a hazardous substance into the air, for example:

• toxic or corrosive gases: chlorine, sulfur dioxide, nitrogen dioxide, hydrogen chloride, ammonia (see lab safety rules)
• volatile organic solvents, especially toxic or highly flammable ones
• concentrated acids that fume, such as concentrated hydrochloric or nitric acid
• bromine and iodine vapour
• reactions with unpleasant smells (such as preparing esters or sulfur compounds)
• fine toxic powders that could become airborne

The risk assessment for each practical states whether a fume cupboard is required. See risk assessment in chemistry and how to read a safety data sheet.

How to use one correctly

1. Check it’s working: the airflow indicator shows normal flow; you can hear the fan. A thin strip of tissue held at the opening should be drawn gently inwards.
2. Clear the space: remove anything not needed for the current work. Clutter disrupts airflow.
3. Work at least 15 cm (6 inches) inside the front edge. Fumes released right at the opening can escape.
4. Keep the sash low, below the marked working height, with your arms reaching under it.
5. Raise equipment slightly on blocks where possible so air can flow underneath and to the baffles.
6. Don’t block the baffles at the back with bottles or boxes.
7. Move slowly. Fast arm movements and people walking past create air currents that can pull fumes out.
8. Never put your head inside the cupboard.
9. When finished, clean up, remove your equipment and close the sash.

What it can’t do

A fume cupboard is a powerful control, but not a magic one.

• It doesn’t replace eye protection or gloves. Splashes can still reach your hands and face.
• It isn’t a storage cabinet. Storing chemicals inside reduces airflow and adds fuel if there’s a fire. See chemical storage.
• It won’t contain a large explosion. The sash offers some protection against minor events, but scale and risk assessment matter more.
• A standard fume cupboard doesn’t neutralise anything. It moves hazards away; it doesn’t destroy them (except some specialist filtered types).
• It won’t work if it’s switched off or faulty. Never assume it’s working; check.
• It’s not a place for waste disposal by evaporation. Evaporating solvents to get rid of them simply releases them to the atmosphere. See disposing of chemical waste.

Types of fume cupboard

Type	Key feature	Typical use
Ducted	Extracts air to the outside through a duct	Most labs; widest range of chemicals
Recirculating (filtered)	Passes air through filters (often activated carbon) and returns it to the room	Schools or rooms where ducting is impractical; only for specific, compatible chemicals, and filters must be changed on schedule
Specialised	e.g. perchloric acid hoods with wash-down systems, or radioisotope hoods	Specific hazardous work

Filtered cupboards are only safe with chemicals the filter is designed to capture. They’re not a general substitute for ducted extraction.

Fume cupboards are also different from biological safety cabinets and laminar flow cabinets, which protect samples or people from microorganisms, not chemical vapours.

What to do if something goes wrong inside

• Small spill: keep the sash low and the extraction running; clean up with the correct spill kit, wearing gloves and eye protection.
• Small fire: close the sash fully if safe to do so. This limits oxygen and contains the fire. Tell the teacher or supervisor immediately and follow the fire procedure.
• Airflow alarm: stop work, close containers, lower the sash and report the fault. Don’t restart until it has been checked.

Maintenance and testing

Fume cupboards are tested regularly by trained people, typically at least once a year and after any repair. Tests measure face velocity at several points and sometimes use smoke to visualise airflow. A label on the cupboard usually shows the date of the last test. If it’s overdue, or the cupboard seems weak, report it.

Key takeaways

• A fume cupboard captures hazardous gases, vapours and dusts and extracts them away from the user.
• Air flows in through the front at a controlled face velocity; the sash controls the opening and shields the user.
• Use one whenever toxic, corrosive, flammable or smelly substances might become airborne.
• Work 15 cm inside, keep the sash low, avoid clutter and move slowly.
• It doesn’t replace PPE, isn’t for storage or evaporating waste, and must be checked and tested regularly.