Fume hoods are not just part of the furniture, but are important items of chemical safety equipment with a dynamic mechanical function. A fume hood is the apparatus on which we depend to protect us from exposure to toxic and hazardous chemical vapours, gases and fumes. They are much more than boxes on a duct. They require technical finesse to allow a gentle air flow to provide good fume containment.

Good fume containment depends on the quality of air flow through the fume chamber.

Good air flow begins in the laboratory, by eliminating cross draughts and differential pressure. The average fume hood face velocity, on which we depend for containment of fumes and our protection from harmful fume exposure, is relatively gentle.

Makeup air should enter the laboratory passively and without resistance, allowing the fume hood exhaust system to manage the air flow.

The entry profile of the fume hood should be curved, like a bell-mouth.
• The curved side entry profile should blend tangentially with the inner walls of the fume chamber.
• Because the sill needs to have a step for spill retention, a hollow aerofoil is required across the full width of the sill to enhance air flow across the work top.
• A vertical rising sash is preferred The lower edge of the sash requires a generous radius across its full width.
• All of these features help to avoid sudden changes in direction of airflow, which cause turbulence. They work together to promote near laminar flow into the fume hood.

A baffle is required across the rear of the fume chamber, to create a diffused and balanced air distribution across the entire work zone. The baffle should cover the entire back wall and part of the roof, and should have multiple openings to ensure complete air balance.

In vertical cross section, the airflow should fan out from the sash opening, up and back through the fume chamber. All air flow pathways should be linear, with as few eddies as possible.

It is not possible to have a single linear air flow with more than one air inlet. Bypass fume hoods typically have high turbulence behind the sash, because two separate air streams from the working opening and the bypass merge together in this zone. While baffles can to some extent control the shape of this turbulence, it is still a zone close to the operator where reverse flow is commonplace. A Variable air volume (VAV) control system allows the fume hood to have a single air inlet, with the air flow regulated in proportion to the size of opening. The mixing zone is eliminated, and air flow conditions inside the fume hood are much improved.

Fume hoods are more expensive that laboratory furniture. This is because they are important items of chemical safety equipment with a dynamic mechanical function. They are subjected to aggressive working conditions with corrosive fumes and liquid spills. Often their housekeeping is neglected. It is therefore important that fume hoods are made to be robust and durable

There are many fume hoods on the market which are mass produced from cheap materials. The materials themselves, and the methods of their assembly, are often unsuitable for an aggressive chemical environment. A good example is a painted metal cabinet resting on top of a bench. Capillary effects draw liquids from chemical spills, splashes, and condensate, into the lap join, with rapid onset of corrosion, which subsequently bursts through the paint finish. An unmeasured effect of degradation is how it diminishes operators responsible behaviour for housekeeping and safety. This highlights the importance of making fume hoods with quality methods, and low maintenance synthetic materials.

Steady laminar airflow is at the root of laboratory fume hood safety. It is the means used to control chemical fumes gases and vapours. Therefore it is important to monitor airflow status continuously.

There are other hazards present in fume hoods too, especially when electricity and flammable gases or liquids are in use. We need to limit the risk of ignition. Safety sequence control systems manage these risks by disabling ignition sources and energy supplies during start up and shut down phases, and automatically when alarm states occur.