Respiratory Protection

"Keeping Radioactive Material Out of You"

There are many forms of respiratory protection, some better than others.  For our work here at the research center, we use fume hoods to ensure volatile materials or radioactive gases are not released into the work area or researcher's breathing zone.  This section covers the safe use of not only radioactive material use fume hoods, but chemical and biological protective fume hoods as well.

Basics

A chemical fume hood is a partially enclosed work space that is exhausted to the outside. When used properly, hazardous gases and vapors generated inside the hood are captured before they enter the breathing zone. This serves to minimize your exposure to airborne contaminants. The common parts of a fume hood and their major functions are:

Hood Body -- The visible part of the fume hood that serves to contain hazardous gases and vapors.

Baffles -- Moveable partitions used to create slotted openings along the back of the hood body. Baffles keep the airflow uniform across the hood opening, thus eliminating dead spots and optimizing capture efficiency.

Sash -- By using the sash to adjust the front opening, air flow across the hood can be adjusted to the point where capture of contaminants is maximized. Each hood is marked with the optimum sash configuration. The sash should be held in this position when work involving the fume hood is being performed and closed completely when the hood is not in use.

Airfoil -- Found along the bottom and side edges airfoils streamline air flow into the hood, preventing the creation of turbulent eddies that can carry vapors out of the hood. The space below the bottom airfoil provides source of room air for the hood to exhaust when the sash is fully closed.

Work surface -- Generally a laboratory bench top, but also the floor of a walk-in hood, this is the area under the hood where apparatus is placed for use.

Exhaust plenum -- An important engineering feature, the exhaust plenum helps to distribute air flow evenly across the hood face. Materials such as paper towels drawn into the plenum can create turbulence in this part of the hood, resulting in areas of poor air flow and uneven performance.

Face -- The imaginary plane running between the bottom of the sash to the work surface. Hood face velocity is measured across this plane.

Hood Types and Uses:

Chemical fume hoods are approved for two general types of uses: General Purpose and Radioisotope.  Hoods approved for each of these uses will appear alike but require different functional and operating parameters.

General purpose hoods, the most common use type, are used to prevent exposure to toxic, irritating, or noxious chemical vapors and gases. A face velocity of 100 - 150 feet per minute (fpm) provides efficient vapor capture while reducing hood turbulence.

Radioisotope hood systems are ideally made from welded stainless steel to ensure against absorption of radioactive materials. In order to comply with licensing requirements, radioisotope hoods require a face velocity of 100 - 150 fpm. The exhaust systems of these hoods are not shared.  The exhaust system must be labeled and must exhaust at least 7 feet above any occupied surface.  The Radiation Safety Officer approves all such hoods for use and may require additional controls of the researcher dependant on experiment, isotope, and quantities to be exhausted.

The best way to identify the use to which a particular hood is approved is to look at the yellow sticker attached to the sash frame.

Chemical hoods are checked annually by the Industrial Hygiene group and labeled for approved use.

Radioisotope hoods are checked quarterly by the Health Physics group and labeled for approved use.

Conventional Hoods -- Conventional hoods represent the original and most simple of the hood design styles. With a conventional hood the volume of air exhausted is constant, regardless of sash height. As the sash is lowered the opening area decreases, resulting in an increase in face velocity. Since face velocity changes dramatically with sash position it is particularly important when working with conventional hoods to maintain the sash at its optimal height as indicated by the yellow sticker attached to the hood frame. Optimal sash height represents the point where face velocity equals 100 - 150 fpm.

Bypass Hoods -- Bypass hoods have an added engineering feature and are considered a step up from conventional hoods. An air bypass incorporated above the sash provides an additional source of room air when the sash is closed. As the sash is lowered the bypass area becomes exposed, effectively increasing the face opening and dampening face velocity fluctuations. Because variations in face velocity still occur, it remains important to utilize the optimum sash height as indicated on the yellow sticker attached to the hood frame.

Auxiliary Air Hoods — With this type of hood a dedicated duct supplies outside air to the face of a bypass hood. The main advantage of an auxiliary air hood is the energy savings realized by reducing the amount of heated or air conditioned room air exhausted by the hood. While energy savings can be substantial, the unconditioned air flow can cause discomfort for those working near the hood. It is important to keep in mind, however, that the auxiliary air supply is necessary for proper functioning of the hood. Any alteration of the air supply system such as sealing off the auxiliary air duct will adversely affect hood operation and may result in hazardous chemical exposures. If the sash of an auxiliary air hood is kept closed most of the unconditioned air will bypass through the hood, reducing its effect on room temperature and humidity. Remember to check the optimum sash height since it will affect face velocity in a manner similar to that for bypass hoods.

Perchloric Acid Hoods — When heated above ambient temperature, perchloric acid will vaporize and condense on hood, duct and fan components. In addition to being highly corrosive, condensed vapors can react with hood gaskets, greases and other collected materials to form explosive perchloric salts and esters. A perchloric acid hood is built with welded stainless steel hood surfaces, duct work, and fan to minimize the corrosive and reactive effects. More importantly, perchloric acid hoods have a wash-down system of water fog nozzles dispersed throughout the hood and exhaust system. By washing down the hood following each use of heated perchloric acid, any materials deposited within the system are removed, preventing the buildup of hazardous perchlorates.

Variable Air Volume Hoods — Variable air volume (VAV) hoods are the most sophisticated of the hood types, requiring technically proficient design, installation and maintenance. The primary characteristic of VAV hoods is their ability to maintain a constant face velocity as sash height changes. Sash height is continuously monitored and total fan exhaust volume adjusted so that the average face velocity is maintained within acceptable parameters.

Biosafety Cabinets and Clean Benches — Although fume hoods, biosafety cabinets and clean benches can look similar, they have very different uses. A chemical fume hood is designed to contain hazardous vapors and gases and exhaust them outside the building. A clean bench is designed to protect biological specimens by bathing the work area with air free of particulate contamination. Because a clean bench forces air out from the back of the hood, across the work surface and toward the worker it protects only the specimen, not the user. A biosafety cabinet provides biological protection for both specimen and user. Particulate-free air is passed down from the top of the hood and across the work surface, and is captured before entering a worker’s breathing zone. The air is then re-filtered before being exhausted, usually back into the laboratory. Because all clean benches and most biological safety cabinets exhaust air back into the work area, they cannot safely be used with hazardous gases and vapors. Even when a biosafety cabinet is equipped with 100% external exhaust, the HEPA filter can impede the venting of volatile materials. While small volumes of low toxicity volatiles may be used in such a cabinet, highly flammable or highly toxic volatiles must always be used in a chemical fume hood.

Monitoring Hood Function

The ultimate goal of fume hood use is to contain the contaminants generated within the hood. While direct measurement of containment is difficult and costly, there exists a surrogate measurement for containment that is simple and straight-forward -- hood face velocity. A face velocity of 100 fpm - 150 fpm with a minimum of turbulence is generally considered a good compromise between competing air forces and the creation of turbulent eddies. With the exception of VAV hoods, face velocity is strongly dependent on sash height. It is therefore important when using the hood to position the sash at the height indicated by the arrow on the yellow fume hood approval sticker. When not in use the sash should be closed completely. This will reduce noise levels and ease the load on the heating or air conditioning system.

The yellow sticker affixed to the side of the hood indicates the most recently determined hood face velocity and the date by which the hood should be rechecked. This face velocity information represents air flow results at the time of the test. Hood function can change from one moment to the next due to system irregularities such as hood storage and use, broken belts, electrical malfunctions, or maintenance activities. For this reason it is important that you always verify air flow prior to conducting procedures inside the hood. This can be done by checking the air flow gauge, if so equipped. In the absence of a gauge, you can tape an inch wide strip of tissue to the lower corner of the sash. Air flow can be visually assessed by noting that the tissue is pulled gently into the hood.

Contact the following to report an inoperative fume hood:

Contact Physical Plant directly to report problems with the physical structure of the hood such as an inoperable sash.

Standard Operating Procedures

Common Fume Hood Myths

Myth - With an auxiliary air hood lab temperature problems can be remedied by covering the supply air duct.

While this might provide marginal temperature control, it will cause a stream of air to be forced down the face of the hood that will actually draw contaminants out of the hood. For hoods with unconditioned make up air the best solution is to keep the sash closed whenever the hood is not being used. With the sash closed all of the unconditioned air is exhausted by the fume hood. With the sash open some unconditioned air will escape into the room.

Myth - When working with highly dangerous materials, the higher the face velocity the better.

While it is important to have a face velocity between 100 and 150 feet per minute, velocities higher than this are actually harmful. When face velocity exceeds 125 feet per minute eddy currents are created which may allow contaminants to be drawn out of the hood, increasing worker exposures.

Myth - A fume hood can be used for storage of volatile, flammable, or odiferous materials when a appropriate storage cabinet is not available.

While it is appropriate to keep chemicals that are being used during a particular experiment inside the fume hood, hoods are not designed for permanent chemical storage. Each item placed on the work surface interferes with the directional air flow, causing turbulence and eddy currents that allow contaminants to be drawn out of the hood. Even with highly volatile materials, as long as a container is properly capped evaporation will not add significantly to worker exposures. Unlike a fume hood, flammable materials storage cabinets provide additional protection in the event of a fire.

Myth - The airfoil on the front of a hood is of minor importance. It can safely be removed if it interferes with my experimental apparatus.

Airfoils are critical to efficient operation of a fume hood. With the sash open an airfoil smoothes flow over the hood edges. Without an airfoil eddy currents form, causing contaminates to be drawn out of the hood. With the sash closed, the opening beneath the bottom airfoil provides for a source of exhaust air.