CHEMICAL FUME HOODS AND BIOLOGICAL SAFETY CABINETS

Thomas J. Bruno

Engineered safety equipment is preferred over the reliance on personal protective equipment, and the fume hood is one such engineered safety device that is nearly ubiquitous in laboratories. Laboratories concerned with biological specimens (microbes, spores) also commonly are equipped with biological safety cabinets. The following section provides basic information on the function and application of these devices. The purpose here is not to provide design or installation instructions, since most users will find this equipment already installed in their workspaces. Rather, this information is to allow optimal use to be made of the installation that is preexisting.

Types of Chemical Fume Hoods

Most of the chemical fume hoods considered here consist of a cabinet or enclosure set at waist level (above a table or storage cabinet) that is connected to a blower located above the hood or external to the hood through a duct system. The cabinet has an open side (or sides) to allow a user to perform work within. A movable transparent sash separates the user from the work. Most chemical fume hoods have a sill that functions as an airfoil at the work surface below the sash. The connection to the blower might be by use of a v-belt, or it may be direct drive. This allows provision of a smooth flow of air with minimal turbulence. In some installations, axially mounted blowers are used, especially if multiple hoods are ducted into a common blower. Baffles located in the rear of the cabinet provide control of the airflow patterns and can usually be adjusted to provide the best airflow around the experiment or procedure being performed. Many chemical fume hoods are equipped with airflow indicators, low-flow monitors and alarms, and differential pressure sensors to allow the user to operate safely. The major types of chemical fume hoods include the standard/conventional, walk-in, bypass, variable air volume, auxiliary air, or ductless types. Additional types include snorkels and canopies that are portable. Each type must be understood to be operated most efficiently within specifications (see discussion below on Chemical Fume Hood Operations).

Standard or Conventional

The standard chemical fume hood utilizes a constant speed motor, and for this reason the volume of air drawn into the hood will change with movement of the sash position. As the sash is lowered, the velocity of the air drawn into the hood will increase.

Bypass

The bypass chemical fume hood is very similar to the standard/conventional hood except that as the sash is lowered, a vent is opened above the sash to allow additional airflow into the hood. This prevents a large increase in velocity in the working area inside the hood.

Variable Air Volume

The variable volume chemical fume hood controls the volume of air drawn into the hood as a function of sash position, while maintaining the face velocity of the air at a constant rate, within the specifications required. These types of chemical fume hoods are more energy efficient than the standard or bypass hoods because they minimize costs incurred by laboratory heating and cooling.

Auxiliary Air

The auxiliary air chemical fume hood includes an additional blower that injects air into or at the face of the hood, providing additional flow inside the enclosed cabinet. These types of hoods are rarely installed in renovations or new construction but may be encountered in older laboratories. They are less desirable than the standard/conventional, bypass or variable volume types because they require a great deal of energy to operate (although the early designs featured the addition of an auxiliary airstream that was not air conditioned). These devices are mechanically more complex than other types, and consequently more prone to maintenance problems.

Walk-In Hood

The walk-in hood is a chemical fume hood that is mounted directly on the laboratory floor or a slightly raised chemical-resistant platform. It is used for the ventilation of larger pieces of equipment, with the advantage that these pieces of equipment can be wheeled in and out of the walk-in hood. The walk-in hood typically uses two separate sashes.

Ductless Hoods

This type of chemical fume hood does not duct the airflow to outside the laboratory, but rather the airflow is returned to the room or interstitial space after passing through a means to remove contaminants. The contaminants may be removed by HEPA filters, activated carbon cartridges, adsorbents, or catalyst beds. The means of contaminant removal must be inspected and serviced at regular intervals.

Laminar Flow Hood (Clean Bench)

Related at least in principle to ductless hoods are laminar flow hoods, sometimes called clean hoods. These are devices intended to protect the work being performed from particulates in the air, which is accomplished by bathing the work area with HEPA-filtered air either blown at low velocity over the work area or blown from the bottom of the hood as an air curtain. Only approximately 10% of the airflow is through the face of the hood. These are intended to protect the work or samples inside the hood, not primarily the user. These units should not be used in place of chemical fume hoods, rather they are used to protect the work from dust or pollen.

Snorkels and Canopies

Snorkels are flexible ducts routed from a blower duct that can be placed temporarily atop or near an experiment to provide some measure of protection. A canopy is similar, but it incorporates an additional bell-shaped collector that might be suspended above an experiment, but for the same purpose as that for the snorkel.

Chemical Fume Hood Operations

While the operation of chemical fume hoods is straightforward, safe operating practices must be observed. The face velocities should be optimized at between 80 and 120 fpm. Face velocities in excess of 125 fpm can cause turbulent flow and allow outflow of contaminants from the hood and potentially expose the user to hazards. Face velocities are checked periodically by facilities managers. Ideally, chemical fume hoods are located in low traffic areas of the laboratory, away from entry doors. Safe operation of chemical fume hoods requires observance of the following practices, divided into primary (applicable to all installations) and secondary (applicable on a case-by-case basis).

Primary Guidelines

• Before using the chemical fume hood, ensure that it is in working order. A simple airflow test can be done with a laboratory wipe, or one can observe the flow monitor if one is present. Be aware of clattering sounds that might indicate a broken belt, or screeching sounds that might indicate a failed or failing bearing.
• The motor of the chemical fume hood should be running at all times, except for maintenance. If a switch is located inside the laboratory, it should be equipped with a lockout to protect maintenance staff.
• Users should perform all work at least 6 inches inside the plane of the sash.
• The user should avoid placing his/her head inside the chemical fume hood, beyond the plane of the sash.
• Laboratory occupants should avoid traffic in front of the chemical fume hood to minimize turbulence.
• Users should avoid rapid movements in front of the chemical fume hood; this includes rapidly raising and lowering the sash.
• Keep the sash closed down as far as possible.
• The baffles of the chemical fume hood should be free of obstruction.
• If a heating device such as a hot plate is being used in the hood, the interior airflow can be dramatically changed by convection. In those instances, the lower baffles of the hood should be closed or minimized, and the middle baffles fully opened to accommodate the convection.
• Equipment located inside the cabinet that can potentially disrupt the airflow should be raised above the level of the lower baffles by a small shelf or blocks.
• Hoses and power cords that must be run into the chemical fume hood from the outside should be run through the airfoil beneath the sash.

Secondary Guidelines

• Some chemical fume hoods have a small cup sink located inside the cabinet. Water should be run into this sink periodically to maintain it free of obstruction and to keep the P-trap full and thereby prevent sewer gas backup.
• It is imperative to prevent the discharge of chemicals into the cup sink.
• Some chemical fume hoods have compressed air lines that allow the use of air-operated equipment such as vortex tubes (for heating and cooling). When using vortex tubes inside a fume hood, the sash should be fully closed because additional airflow inside the hood is present.
• In a power outage, lower the sash to within an inch of the fully closed position.
• Evaporations and digestions with perchloric acid must only be done in a specifically designed and designated perchloric acid chemical fume hood. The same considerations apply for the use of radionuclides and infectious agents.

Types of Biological Safety Cabinets (BSCs)

Biological safety cabinets, as distinct from chemical fume hoods, are enclosed ventilated cabinets intended to provide both a clean and a safe working environment for aerosols and biological hazards. All exhaust air is HEPA filtered as it exits the biosafety cabinet, removing harmful bacteria and viruses. The Centers for Disease Control (U.S.) lists three classes of biological safety cabinets, Classes I, II, and III.

Class I cabinets are open-front negative-pressure cabinets that provide personnel and environmental protection but do not provide protection to the sample or media (the product) being used in the cabinet. There is airflow into the cabinet (at a face velocity of 75 fpm) that can potentially cause sample contamination. Class I cabinets are often used to enclose specific equipment (centrifuges, harvesting equipment, or fermenters) or ongoing procedures (cultures) that potentially generate aerosols. BSCs of this class are either ducted (connected to the building exhaust system) or unducted (recirculating HEPA-filtered exhaust back into the laboratory, provided there is an interlock with the building exhaust system). Some Class I cabinets are used for animal cage changing and these typically require frequent HEPA filter changes due to odoriferous compounds saturating the filter.

A Class II biological safety cabinet provides protection for both the worker and the sample or product, making it suitable as a sterile compartment for cell culture. This type of cabinet is the most versatile and most common, with face velocities similar to those of Class I. There are four types of Class II cabinets, the main features of which are discussed below. Note that there are additional differences among the types in Class II (Types A1, A2, B1, and B2), primarily concerning the geometry of the airflows and placement of HEPA filters.

Type A1 (formerly Type A) does not have to be duct vented (although it is possible to connect to building ventilation systems by use of canopy exhaust connections), which makes it suitable for use in laboratories inaccessible to ductwork. This cabinet can be used for low to moderate hazard agents that do not include volatile toxic chemicals and volatile radionuclides. The supply air is HEPA filtered to present the sample or media being used with a particulate free airstream with a face velocity of at least 75 fps. This type of BSC cannot be used for volatile and toxic compounds and solvents because small quantities of these materials can quickly load the filter. Type A2 differs from Type A1 in that protection of the operator and the environment is only afforded if the exhaust line is canopy vented to the building exhaust. The face velocities of these units are at least 100 fps.

Type B1 cabinets must be hard vented, with 50% of the air exhausted from the cabinet while 50% can be recirculated back into the room. This cabinet may be used with etiologic agents and treated with volatile and toxic chemicals and radionuclides required as an adjunct to microbiological studies (if the work is done in the directly exhausted portion of the cabinet). The air intake velocity of the B1 type is specified to be 100 fps. Type B2 cabinets must be 100% exhausted through a dedicated duct.

The Class III cabinet is designed for highly infectious microbial agents. It is entirely gas tight, with a non-operating view window (cannot be opened). Access to the interior is through a dunk tank accessible through the floor of the cabinet or through a double-door system. Both the supply and exhaust gas streams pass through HEPA filters. Heavy-duty rubber gloves are used for manipulations in the interior of the cabinet.