Chemicals that are hazardous, odorous, gaseous or radioactive can be a problem for lab personnel. Fume Hood provides protection from these chemicals. A fume hood is a ventilated enclosure containing gases, vapors and fumes which is used to control exposure of the hood user and lab occupants to hazardous or odorous chemicals and it also prevents their release into the laboratory. (An exhaust fan that is situated on the top of the laboratory building pulls air and airborne contaminants in the hood through a duct work which is connected to the hood and exhaust them to the atmosphere.)
Another purpose is to limit the effects of a spill by partially enclosing the work area and drawing air into the enclosure by means of an exhaust fan. This inward air flow creates a dynamic barrier that minimizes the movement of material out of the hood and into the lab.
Primary parts of the Fume Hood
Face – The face of the hood is the opening where air capture takes place.
Sash – The sash is the glass “window” that travels in the plane of the hood face that opens or closes the hood and protects the user during use.
Baffles – The baffles are located in the back of the hood and direct air in the appropriate direction. The baffles can also be adjusted to account for different vapor densities of chemicals (heavier than air and lighter than air).
Duct – The duct connects the hood to the ventilation system and exhausts to the outside air.
Air foil – The air foil is fixed to the bottom front edge of the hood and is a vent that keeps a minimum gap open at all times but more importantly gives aerodynamic properties that allow better, less turbulent air flow and better capture.
Why do we use a Fume Hood?
Routes of entry of chemicals into our body are Ingestion, Inhalation and the Skin. Inhalation is the major route of entry of chemicals into the body.Chemical fumes and vapours can directly enter our bloodstream and these small particles can lodge in the alveolar region of our lungs. A properly designed and operated fume-hood reduces exposure to hazardous fumes, vapours, gases and dusts. A fume-hood confines hazardous airborne material by diluting it with a large amount of air, drawing it through an exhaust system and then expelling the air in vents locate on the roof of building
When is a Fume Hood Necessary?
The determination that a fume hood is necessary for a particular experiment should depends on a hazard analysis of the planned work. Such an analysis should include:
- Detailed knowledge of the physical characteristics, quantity and toxicity of the materials to be used;
- The Experimental procedure;
- The volatility of the materials that can be present during the experiment;
- The probability of their release;
- The number and sophistication of manipulations; and
- The skill and expertise of the individual performing the work.
Fume Hood in comparison with Laminar Air Flow and Biological Safety Cabinet
Fume Hood
It protects you from fumes you are working with. The fan sucks in the air toward the duct inside of the fume hood towards the outside. This system works only if you bring the hood’s door down at least 2/3 of the way. The narrower the opening, the swifter the air.
Laminar Air Flow
It protects your samples from contamination coming from you and the room. The air is blown at you.
Biological Safety Cabinets
It protects you, your samples and your environmentfrom particulatecontamination. They are NOT designed for harsh or radiolabelledchemicals. To be used for work with low to moderate riskagents NOT with high-riskpathogens. HEPA (High EfficiencyParticulate Air) is the essentialcomponent of these cabinets.
What are the types of Fume Hood?
Major types of fume hood are:
- Constant Air Volume Hood
- Variable Air Volume Hood
- Specialty Lab Exhaust System
Constant Air Volume Hood(CAV) is of following types:
i) Conventional hood : an older, traditionally less elaborate hood design used for general protection of the laboratory worker. Because the amount of exhausted air is constant, the face velocity of a CAV hood is inversely proportional to the sash height. That is, the lower the sash, the higher the face velocity. CAV hoods can be installed with or without a bypass provision which is an additional opening for air supply into the hood.
i.(a) Conventional hood without a bypass
Some conventional hoods do not have a provision for a bypass. Essentially, they consist of an
enclosed cabinet with a connection for an exhaust duct and a movable sash on the front.
i.(b) Conventional bypass fume hood
The bypass fume hood is an improved variation on the conventional fume hood. The bypass is
located above the sash face opening and protected by a grille which helps to direct air flow. The
bypass is intended to address the varying face velocities that create air turbulence leading to air
spillage. The bypass limits the increase in face velocity as the sash nears the fully closed
position, maintaining a relatively constant volume of exhaust air regardless of sash position.
ii) Auxiliary air hoods
This fume hood, sometimes referred to as a makeup air fume hood, was developed as a variation
on the bypass fume hood and reduces the amount of conditioned room air that is consumed. The
auxiliary fume hood is a bypass hood with the addition of direct auxiliary air connection to
provide unconditioned or partially conditioned outside makeup air. Auxiliary air hoods were
designed to save heating and cooling energy costs, but tend to increase the mechanical and
operational costs due to the additional ductwork, fans, and air tempering facilities. In general, installation of this type of hood is discouraged since the disadvantages usually outweigh the benefits.
Variable Air Volume Hood
Variable air volume (VAV) hoods differ from constant air volume (CAV) hoods because of their
ability to vary air volume exhausted through the hood depending on the hood sash position.
VAV hoods reduce the total quantity of supply and exhaust air to a space when not needed,
thereby reducing total operating costs.
Specialty Lab Exhaust Systems:
They are of following types
i) Walk-in hood : A walk-in hood is a hood which sits directly on the floor and is characterized by a very tall and deep chamber that can accommodate large pieces of equipment. Walk-in hoods may be designed as conventional, bypass, auxiliary air, or VAV. If you have a walk-in hood, contact ORS for operating protocol and inspection procedures.
ii) Fume exhaust connections: “snorkels”
Fume exhaust duct connections, commonly called snorkels, elephant trunks or flex ducts, are
designed to be somewhat mobile allowing the user to place it over a small area needing
ventilation. However for optimal efficiency, these connections must be placed within six
inches of an experiment, process, or equipment. These funnel-shaped exhausts aid in the removal
of contaminated or irritating air from a point source to the outside.
iii) Canopy hoods : Canopy hoods are horizontal enclosures having an open central duct suspended above a work bench or other area. Canopy hoods are most often used to exhaust areas that are too large to be enclosed within a fume hood. The major disadvantage with the canopy hood is that the contaminants are drawn directly past the user’s breathing zone.
iv) Glove boxes : Glove boxes are used when the toxicity, radioactivity level, or oxygen reactivity of the substances under study pose too great a hazard for use within a fume hood. The major advantage of the glove box is protection for the laboratory worker and the product.
v) Perchloric acid and radioisotope fume hoods: Perchloric acid hoods have wash-down capabilities to prevent the buildup of explosive perchlorate salts within the exhaust systems.
Operating Performance of a Fume Hood
- Location :The location of the hood affects its efficiency. Ideally, fume hoods should be located in an area of minimal traffic. When a person walks by a fume hood, turbulence can be created causing contaminants to be drawn outside the hood. Also, if the air diffuser is located directly above the fume hood, air turbulence may be created causing contaminants to escape into the room. The air flow into the room has an effect on the fume hood. All doors and windows should be kept closed to maintain the negative pressure of the lab with respect to the outside corridor. This ensures that any contaminants in the lab will be exhausted through the fume hood and not escape into the hallway.
- Face velocity : Face velocity is a measurement of the average velocity at which air is drawn through the face of the fume hood. Face velocities too high or to low can be detrimental to the performance of the fume hood. The acceptable range of the average face velocity may vary between 60-100 feet per minute (fpm) depending on hood type and hazard. If non-carcinogenic materials are being used the acceptable face velocity for minimally hazardous materials is 50 fpm. Currently, all fume hoods are certified for work with hazardous chemicals if the air velocity is between 80 and 120 fpm. At velocities greater than 125 fpm face velocity, studies have demonstrated that the creation of turbulence causes contaminants to flow out of the hood and into the user’s breathing zone.
- Air flow indicators : Small pieces of tinsel are taped to the bottom corner of the sash. Inward movement of the tinsel indicates air is being drawn into the hood. Air flow indicators do not determine face velocity. They only indicate that air is being exhausted through the fume hood.
Safety Guidelines for Fume Hoods
- Keep the Sash as low as possible to minimize the risk of exposure.Sash acts a Safety Shield and protects the individual’s face, so one should be looking through the sash to perform work. Height of Sash should be adjusted depending on the height of the person using the hood.
- An Airflow Indicator should always be there. This is a small piece of crepe paper (or similar) attached to the bottom of the sash that blows with the air current. This is the only way to know for certain that air is flowing through the hood in the proper direction. The indicator should be blowing into the hood (sometime the flow is reversed by accident during maintenance). NOTE: An airflow indicator only indicates the direction of airflow and does not indicate whether the fume hood has the proper face velocity.
- Lab doors and windows should be closed. These extra sources of inlet air can affect the
Performance of the hood, cause turbulent air currents in the room or cause the room to loose its negative pressure.
- There should be limited traffic near hoods when in use. Pedestrian traffic or fast movement in front of hoods can cause turbulence and can negatively affect the capture ability of the fume hood.
- Clutter should be less as much as possible and do not store large amounts of chemicals in the hood. Excess clutter and chemicals can impede airflow especially to the lower openings.
Risks involved with the improper use of Fume Hoods
A fume hood is a piece of safety equipment that can be misused to the extent that they
can be less effective than expected. Injury from misuse can arise from two causes:
- From the fume hood not providing adequate flow rates for the work required (i.e. sash being left open or from excess clutter that reduces the containment of noxious substances).
- From the hood itself (i.e. if the fan belts are slipping, the exhaust duct has blockage due to paper towels being sucked into the duct, the duct damper is restricted). Always
realize the most likely person to be injured is the hood user. Escaping noxious material
into the laboratory can also affect all laboratory occupants.
A. Power Outages
In case of a power outage, the hood sash should be lowered within an inch to maintain
a chimney effect to keep some air flowing into the hood.
B. Exhaust
Care should be taken with use of paper products, aluminum foil, and other lightweight
materials within a hood. For example, a single paper towel or chemical wipe can potentially
decrease the airflow into the hood if it restricts exhaust flow.
Misconceptions Associated with Fume Hoods
- When working with highly hazardous materials, the higher the face velocity
the better: It is important to have a face velocity between 0.3 m/s (60 fpm) and 0.5 m/s (100 fpm), but velocities higher than this are actually harmful. When face velocity exceeds 0.6 m/s (125 fpm) eddy currents are created which allow contaminants to be drawn out of the hood, increasing worker exposures.
- A chemical hood can be used for storage of volatile, flammable, or odiferous
materials when an appropriate storage cabinet is not available: It is appropriate to keep chemicals that are being used during a particular experiment inside the chemical hood, but hoods are not designed for permanent chemical storage. Each item placed on the work surface interferes with the directional airflow, 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 not add significantly to worker exposures. Unlike a chemical hood, flammable materials storage cabinets provide additional protection in the event of a fire.
- 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 chemical 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.