Laboratory Fume Hoods and Chemical Fume Hoods

Please check out our Laboratory Hoods section for more information or to find manufacturers that sell these products.

A laboratory fume hood is arguably the single most important piece of safety equipment in a lab. A lab worker may never use the fire extinguisher or the eye wash, but the lab fume hood is used on a daily basis. They not only protect workers from exposure to gases during the normal course of experiments, but they provide containment for spills and prevention of explosions and combustion. Fume hoods can also be used as temporary storage for open bottles of chemicals during experiments.

A standard chemical fume hood is basically a box with a sliding door on the front. Air flows in through the opening, mixes with gases being released by the experiment, and is carried up and safely away from the worker. There are other types of laboratory fume hoods that compensate for the shortcomings of a standard fume hood.

A bypass fume hood has an air intake above the movable sash, to minimize turbulent air flow caused by activity in front of the hood. Tests of air flow in fume cabinets show that the face velocity of air coming in can be affected dramatically by something as simple as walking quickly past it.

An auxiliary fume hood is a variation on the bypass hood that draws its air supply from outside air, rather than from heated or air-conditioned inside air. This results in reduced heating and cooling costs.

A variable air volume fume hood includes additional controls to adjust the face velocity of air for different sash positions.

Perchloric Acid Fume Hoods and Radioactive Hoods

In addition to general purpose chemical fume hoods, specialized fume hoods are available for work with biohazards and perchloric acids.

Acid digestion experiments involving the use of perchloric acid require a specialized chemical perchloric acid fume hood, because the build up of perchlorates on the inner surface of hoods can cause explosions. This is accomplished with an interior constructed of an impervious material, such as stainless steel or specially designed man-made materials, and then a washing system is installed so that perchlorates can be rinsed away after use.

Another specialized use for a fume hood is working with radioisotopes. The main requirement for a radioactive hood is that it be made of an impervious material such as steel, and clearly labeled as such on the outside so that workers can take the appropriate precautions. There are some additional design features that are desirable in a fume hood used for radioactive work. Seamless construction and removable baffles make cleanup easier. Reinforced work surfaces and integrated cups are useful to support heavy containers of radioisotopes. Containment needs also differ based on the energy level of the isotope and its half-life.

Ductless Fume Hoods

The ductless fume hood is a relatively new addition to the laboratory fume hood family. Ductless fume hoods filter air and return it to the laboratory environment. Newer models often have the same face velocity as a standard chemical fume hood, and include sensors and alarms for air flow and saturation of the carbon filter.

Ductless fume cabinets have a number of appealing advantages. They are easy to install, as the most difficult part of installation would be the ductwork. They are also easy to move about; many of them even come equipped with wheels. There are numerous circumstances where it would be convenient to bring in a portable fume hood rather than investing in a permanently installed bench top fume cabinet. They capture chemical fumes, rather than venting them into the environment: and, most importantly, they cost much less than a standard fume hood.

However, many institutional environmental health and safety departments discourage the use of ductless fume hoods and will not inspect or certify them. Filters for ductless fume hoods are designed for a specific set of chemicals, and may not capture everything that is used in it. A ductless fume cabinet is also vulnerable to breakthrough of vapors. Chemicals can desorb from the matrix if it is not changed frequently enough. Supplying the equipment with fresh filters is a constant expense, and it requires a lab supervisor to take responsibility for maintenance and correct usage of the hood.

Ductless fume hoods can be used safely if the manufacturer's directions are followed carefully. They are most appropriate for very small volumes of chemicals, such as a few microliters. They should never be pushed beyond these limits or used for high volume or more hazardous chemical work. Biological experiments with small volumes of solvent are an example of a good use of ductless fume hoods.

Low Flow Fume Hoods

A chemical fume hood consumes a great deal of energy. In recent years, a new type of fume hood, generally known as a “low flow” fume hood, has entered the market, driven by an interest in minimizing energy consumption. Air conditioning costs for a building can be increased by several thousand dollars per year for a single standard fume hood. Although there is no consensus on what a low flow hood is, generally the face velocity is less than the standard 100 fpm, with no loss of safety and containment performance. This is achieved by using sophisticated aerodynamic design for the movement of air.

Safe Use of Laboratory Fume Hoods

Lab workers often unintentionally put themselves at risk by misusing laboratory fume hoods. It is important to operate the hood with the sash at the correct level. (This is usually marked by an arrow at the side.) The fume cabinet will not work properly if the face velocity is too low or too high, and materials not in immediate use should not be stored inside the cabinet. Quick movements near or inside the hood can create turbulent flow, and open bottles of chemicals should be placed near the back. A final consideration that is sometimes overlooked is room ventilation. If there is not sufficient air flowing into the room to make up for the air being pulled out by the hood, negative pressure can overcome the fume hood and pull gases into the room. It is important to follow manufacturer instructions and make sure that all laboratory personnel are appropriately trained.

Please check out our Laboratory Hoods section for more information or to find manufacturers that sell these products.