Fume hood
A fume hood or fume cupboard is a large piece of scientific equipment common to chemistry laboratories designed to limit a person's exposure to hazardous and/or unpleasant fumes.  Fume hoods were originally manufactured from timber, but now epoxy coated mild steel is the main construction material.  Two main types of unit exist, ducted and recirculating.  With the ducted type, old asbestos vent pipe has been superseded on health grounds, typically with PVC or polypropylene.  The principle is the same for all units; air is drawn in from the front of the cabinet by a fan, and either expelled outside the building or made safe through filtration and fed back into the room.

# Construction and location
Fume hoods (fume cupboards) are generally available in 5 different widths; 1000 mm, 1200 mm, 1500 mm, 1800 mm and 2000 mm. The depth varies between 700 mm and 900 mm, and the height between 1900 mm and 2400 mm.  These can accommodate from one to three operators. They are generally set back against the walls and are often fitted with infills above, to cover up the extract ductwork. Because of their shape they are generally dim inside, so many have internal lights with gas-proof covers. The front is a movable sash, usually in glass, able to move up and down on a counterbalance mechanism. On educational versions, the sides of the unit are often also glass, so that several pupils can gather around a fume hood at once. Alarm control panels are common, see below.

# Recirculating fume hoods
Mainly for educational use, or where the design of a building does not permit the fitting of external ductwork, these units generally have a fan mounted on the top (soffit) of the hood, or beneath the worktop.  Air is sucked through the front opening of the hood and through a filter, before passing through the fan and being fed back into the workplace.

## Pre-filtration
The first stage of filtration consists of a physical barrier, typically of open cell foam, which prevents large particles from passing through.  A filter of this type is generally inexpensive, and would last for approximately six months, dependent on usage.

## Main filtration
After pre-filtration, the fumes are sucked through a layer of activated charcoal which absorbs the majority of chemicals that pass through it. Ammonia and carbon monoxide will, however, pass through most carbon filters. Additional specific filtration techniques can be added to combat chemicals that would otherwise be pumped back into the room. A main filter will generally last for approximately two years, dependent on usage.

### Pros
- Ductwork not required.
- Heated air is not removed from the workplace.
- Contaminated air is not pumped into the atmosphere.

### Cons
- Filters must be regularly maintained and replaced.
- Greater risk of chemical exposure than with ducted equivalents.
- The extract fan is near the operator, so noise may be an issue.

# Ducted fume hoods
Most fume hoods for industrial purposes are ducted.  A large variety of ducted fume hoods exist.  Air is removed from the workspace and dispersed into the atmosphere.
The fume hood is only one piece of the lab ventilation system.  As the recirculation of lab air to the rest of the facility is not permitted, air handling units serving the non-laboratory areas are kept segregated from the laboratory units. As a means of improving indoor air quality, some laboratories also utilise single-pass air handling systems, where air that is heated or cooled is used only once prior to discharge. Many laboratories continue to utilise return air systems to the laboratory areas to minimise energy and running costs, while still providing adequate ventilation rates for acceptable working conditions. The fume cupboards serve to evacuate hazardous levels of contaminant.
To reduce lab ventilation costs, variable air volume systems are employed, which reduce the volume of the air exhausted as the fume hood sash is closed.  This product is often enhanced by an automatic sash closing device, which will close the fume hood sash when the user leaves the fume hood face.  The result is that the hoods are operating at the minimum exhaust volume all of the time that no one is actually working in front of them.
Since a six foot constant volume hood uses as much energy as three average homes in America, the reduction of minimization of exhaust volume is particularly beneficial in reducing facility energy costs as well as minimizing the impact on the facility infrastructure.  Particular attention must be paid to the discharge location, so as not to risk public safety, or to pull the exhaust air back into the building supply air system.

### Pros
- Fumes are completely eradicated from the workplace.
- Low maintenance.
- Quiet operation, due to the extract fan being some distance from the operator.

### Cons
- Unsightly ductwork.
- Heated air is removed from the workplace.
- Fumes are dispersed into the atmosphere, rather than being treated.

## Specific types

### Low flow / high performance
In recent years, laboratory fume hood manufacturers have developed and introduced energy-efficient low-flow / high-performance fume hoods, designed to maintain or improve operator protection while reducing expensive HVAC operating costs.

### Perchloric acid
These units feature a waterwash system in the ductwork.  Because perchloric acid fumes settle, and form explosive crystals, it is vital that the ductwork is cleaned internally with a series of sprays.

### Waterwash
These fume hoods have an internal wash system that cleans the interior of the unit, to prevent a build-up of dangerous chemicals.

### Scrubber
This type of fume hood absorbs the fumes through a chamber filled with plastic shapes, which are doused with water.  The chemicals are washed into a sump, which is often filled with a neutralising liquid.  The fumes are then dispersed, or disposed of, in the conventional manner.

# Use
To determine whether a chemical is likely to require a fume hood for safe usage, its MSDS should be consulted. If there is any doubt, a hood should be used.
An operating and maintenance manual should be provided with a new fume hood, which will detail full usage instructions for a new user.
If you already know about the hood you are going to use, begin by making sure no one else is using it or has left things in it. If it's free collect what you need to be in the hood (reagents and/or the experimental apparatus if the products will give off noxious fumes).
If the light in the hood is too dim to see (It probably will be; even if there's enough light in the room, you will be working in your own shadow) then turn on the internal light.
Stand (or sit if there is a stool) where you will be likely to be when you are working, then lower the glass front as far as you can without making it impossible to get your arms under it and work around.

# Control panels
Most fume hoods are fitted with a mains-powered control panel. Typically, they perform one or more of the following functions:
- Warn of low air flow.
- Warn of too large an opening at the front of the unit.  Known as a "high sash" alarm, this is caused by the sliding glass at the front of the unit being raised higher than is considered safe, due to the resulting air velocity drop.
- Provide a method of switching the extract fan on or off.
- Provide a method of turning the internal light on or off.
Specific extra functions can be added, for example, a switch to turn a waterwash system on or off.

## A warning
Even though a properly functioning fume hood provides adequate ventilation, users should not directly smell any chemicals even when using one. Doing this could cause severe damage to the trachea and lungs. A safer method is to wave the palm above the test tube or beaker to waft reduced amounts of the substance towards the user's nose although there is still a danger if toxic fumes are present.