Office of Research

IV. Laboratory Ventilation

1.     Chemical Hoods

Chemical hoods are critical to the safety of laboratory personnel and the advancement of research throughout the University. They are the most important component used to protect laboratory workers from exposure to hazardous chemicals and other agents used in the laboratory. When properly installed, maintained, and used, they can provide protection from these hazards. When this does not occur, the health and safety of laboratory workers and maintenance personnel can be severely compromised, and research can be hindered.       

a.  The purchase, relocation, installation, or modification of all laboratory hoods or other ventilation device requires prior approval by the Office of Environmental Health and Safety and University Facilities.

b.  Airflow into and within the chemical hood should not be excessively turbulent;. Air disturbances at the face of the chemical hood should be avoided. Face velocities for chemical hoods must be a minimum of 60 fpm at a full open sash height. Velocities should typically not exceed 150 fpm due to increased turbulence which can result in deceased capture efficiency, as well as unnecessary energy consumption.

c.  Quality and quantity of ventilation should be evaluated on installation, regularly certified (at least annually), and re-certified whenever corrections or repairs are made or in the event that the hood is relocated. These certifications will be performed by ES or an outside contractor. If your chemical hood certification is not up to date, or needs re-certification because of relocation or repairs, contact ES.

d.  Close the chemical hood sash when the hood is not in use. Work with the sash at the lowest possible position when using the chemical hood. A working sash height of 18" or lower is recommended. Sash should not be raised above 18” except when necessary to place equipment in/out of the hood.

e.  Chemical hoods should be kept clean and uncluttered. Achieving even, laminar airflow across the deck or bench surface of the hood increases the effectiveness of the hood system. The presence of objects in the hood tends to increase turbulence in the hood. For this reason, the number of objects in a hood should be kept to a minimum. In particular, keep chemicals out of the hood unless they are in immediate use. Not only does storage of chemicals in a hood decrease the efficiency of the hood, but it also increases the possibility and seriousness of accidental fires/explosions.

f.  Work within the hood, at least six inches back from the front opening. This greatly improves the capture rate for volatile materials. Paint a line or place a strip of tape 6” inside the hood as a useful reminder. One’s face should never be within the plane of the sash when working with chemicals in the hood.

g.  Run water in hood drains at least once a week if the drains are not normally used.

h.  Conduct all operations inside a chemical hood when working with hazardous chemicals which may generate air contaminants. Work with all hazardous chemicals, regardless of Permissible Exposure Level (PEL) or Threshold Limit Value (TLV), should be handled under a chemical hood whenever possible and always when listed on the SDS or product label.

i.  Take care not to obstruct airflow by the arrangement of equipment and materials in the hood.

j.  Do not use the hood as a waste disposal mechanism.

k.  Do not store chemicals or non-essential apparatus or equipment in the hood. Store hazardous chemicals in an approved safety cabinet.

l.  Hoods should always be "ON" if chemicals are being used in the hood (if chemicals are in the hood, they are considered as being used because they should not be in the hood unless they are in immediate use; or,  if chemicals stored in cabinets under the hood  require ventilation.

m.  Motor-driven electrical equipment used in a chemical hood where volatile flammable materials may be present must be equipped with a non-sparking induction motor.

n.  Keep the slots in the hood baffle free of obstruction by apparatus or containers.

o.  Minimize the floor traffic past the face of the hood and avoid making rapid movements while standing in front of the hood. Turbulence caused by an individual in front of an open hood can greatly enhance exposure.

p.  Work requiring the mixing, stirring, heating, grinding or any other operation that could reasonably be expected to cause aerosolization of hazardous materials should be performed in chemical hood.

q.  Each chemical hood should have a continuous monitoring device (either audible or visible) to allow convenient confirmation of adequate chemical hood performance. If monitoring device indicates that the hood is not working properly, discontinue use of the hood and contact ES. If a hood does not have properly operating alarm installed on the hood, contact ES.

r.  Must be properly ducted to the outside of the building, with proper stack height to ensure that no re-entrainment into the building or any surrounding building occurs.

s.  Sinks incorporated into the hood surface should have a retaining edge surrounding them in order to prevent leaks/spills inside the hood from being released to the sewer system.

t.  Airfoils should be installed at the front edge of the floor of all hoods. This airfoil serves to minimize the effect of turbulence as air enters the hood. Airfoils currently installed on hoods should not be removed.

u.  Keep laboratory doors and windows closed. In closed buildings, ventilation systems are usually designed on the assumption that laboratory doors and windows will be in the closed position. If the doors and windows are left open, unplanned airflow patterns may degrade the efficiency of the hood.

v.  The sash glass offers protection from accidents and, when possible, it is safest to keep the sash between your face and the experiment. However, the sash is not designed to protect against explosions. When an explosion hazard is present, rounded safety shields should be placed between the operator and the experiment and as close as possible to the plane of the hood sash. Full-face protection should also be used in such circumstances.


2.     Hood Failure Procedures

a.  Immediately stop all work in the hood.

b.  If possible, stabilize reactions and turn off equipment (i.e., hot plates) or other electric devices to avoid unplanned re-energizing of this equipment when the hood power is reactivated. Close any opened/exposed containers of chemicals or radioactive materials currently under the hood.

c.  Close the hood sash

d.  If processes/reactions cannot be stopped/contained, the lab(s) should be evacuated until hood(s) are operating.

e.  Do not use the hood until repairs have been made and the hood is re-tested and approved for use by RS (at which time the “DO NOT USE” sign will be removed.

f.  Report the problem to:

      1. The Director of the Lab and/or the Building Manager
      2. University Facilities (656-2186)

Notify others in the area and on additional shifts that the hood is not operating and cannot be used. The hood should be posted with a sign that boldly states that the hood is not functioning and should NOT be used until repairs/corrections have been made and sign is removed.


3.     Types of Chemical Hoods

a.  Conventional Chemical hood

This is a basic enclosure with a moveable front sash and an interior baffle. There can be additional slots in the baffle, perhaps one in the middle and one near the top, through which air can pass to provide more uniform airflow. The remaining air passes through the hood interior and is directed into the exhaust portal over the top of the interior baffle. The performance of the hood is dependent on sash position.

b.     By-Pass Hood

The by-pass air hood is quite similar to the conventional hood, except that it is designed to permit some exhaust air to “by-pass” the sash closure. This has two consequences. The first is that the air velocity near the work surface remains reasonably constant, so that excessive air speeds will not occur. The second is that there is less static pressure and hence less frictional resistance to the flow of air than with the conventional hood, so that the volume of air through the hood remains more nearly the same at different sash heights, permitting better control of the laboratory air balance.

c.     Auxiliary (Make-Up) Air Hood

The auxiliary air hood provides a means of introducing outside air to the hood exhaust and limits the percentage of tempered air removed from the laboratory. Usually, the auxiliary system supplies air downward across the sash and into the hood opening. The installation and configuration of this type of hood is critical and proper operation is difficult to maintain. It is also very difficult to accurately measure face velocities with this type of hood. We do not recommend the use of this type of hood and do not allow new installations of auxiliary air hoods.

d.   Floor Mounted Hoods

Walk-in hoods are hoods which usually rest directly upon the floor or on a pad resting directly upon the floor. They are designed to accommodate tall apparatus which will not fit in a standard hood sitting upon a base unit or bench. Because their height would require an abnormally long sash travel, these hoods are often provided with dual sashes, each of which would cover half the opening or with a single sash which would come down only about halfway, with swinging doors being used to provide access to the lower portion.

4.   Special Purpose Chemical Hoods

a.     Perchloric Acid Hood

Due to the potential explosion hazard of perchloric acid when combined with organic materials, this hood type must be used for perchloric acid digestion. It must be constructed of relatively inert materials such as type 316 stainless steel, ceramic coated material, or PVC. Hoods used for these applications should have integral bottoms, covered interiors, and a drain. Wash down features are required since the hood and duct system must be thoroughly rinsed after each use to prevent the accumulation of reactive residue. Perchloric acid hoods are by their nature of the by-pass type. The hood should be prominently labeled with a sign stipulating that it is for perchloric acid work only. Exhaust systems serving hoods used with heated perchloric acid should not be manifolded into a common exhaust plenum.

b.     Radioactive Hood

Hoods used for radioactive applications should have integral bottoms and covered interiors to facilitate decontamination. These units should also be strong enough to support lead shielding bricks, in case they are required. These hoods are also of the by-pass type.

Chemical hoods used for radioactive materials should be marked “RADIOIOSTOPE HOOD” and in addition should be labeled with a “CAUTION-RADIOACTIVE MATERIALS” sign bearing the standard radiation symbol. The isotopes being used should be identified on the label.

c.     Ductless Hoods

These hoods are designed to remove hazardous vapors from the work area as the exhausted air passes through an absorbent, such as activated charcoal. These hoods require constant attention and often do not provide adequate face velocity. The filters are designed for specific chemicals and will not protect against the variety of chemicals used in a typical university lab. The user is also faced with expenses to replace filters and dispose of the expended filters. Routine monitoring is also required to ensure that “breakthrough” is not occurring and users are not being overexposed. This type of device would be used only for special situations and is not normally recommended.


Other Ventilation Devices

a.     Ventilated storage cabinets, canopy hoods and snorkels should be provided as needed, provided they are installed properly and with the approval of RS. Each canopy hood or snorkel should have a separate exhaust duct. Canopy hoods have their uses, where it is desired to capture and exhaust hot fumes carried upward by convention currents until they come close enough to the canopy so that the fumes become entrained within the hood. The speed of the air movement in the vicinity of the hood face, due to the air flowing through the canopy, falls off very rapidly to about 7.5% at a distance equal to the effective size of the canopy opening. If the canopy is at a reasonable distance away from the bench top, the airflow at the work surface due to the hood will be on the order of the average air movement speed within the room, or less. A further disadvantage would be that the fumes, if drawn upward, would pass through a worker’s breathing zone. For these reasons, canopy hoods are not recommended as general usage laboratory chemical hoods.

b.     Any ventilation used to control the release of hazardous substances must be exhausted to a once-through system; not recirculated into the building’s general atmosphere.

5.     Biological Safety Cabinets

The biological safety cabinet is the principal device used to provide containment of infectious splashes or aerosols. Biological safety cabinets are divided into three classes based upon the type of protection provided. Class I and II cabinets use an air curtain and Class III uses as physical barrier to protect personnel. Class II and III cabinets filter the air before it is blown onto the work surface, and all three have filtered exhaust. HEPA (high efficiency particulate air) filters are used since they are efficient in removing at least 99.97% of particles 0.3 microns in diameter or greater. HEPA filters do not remove gaseous contaminants. As the filter becomes loaded, the resistance to air movement through the filter increases, with the result that the rate of air flow will decrease. Therefore, airflows must be adjusted periodically to assure proper performance. Also, these cabinets are subject to the same requirements with regard to location as chemical hoods.

Biological safety cabinets must be tested and certified after installation and before use, any time they are moved, and at least annually. Certification of biological safety cabinets will be performed by a certified agent. Costs for certification and necessary replacement of HEPA filters will be billed back to individual departments. If your biological safety cabinet needs to be certified/recertified, contact the University Biological Safety Officer.

For specific information on the three classes of biological safety cabinets, refer to Clemson University’s Biological Safety Manual or contact the University Biological Safety Officer.


6.     Horizontal or Vertical Laminar Flow Cabinets

Horizontal or vertical laminar flow cabinets (clean benches or blow out hoods) are not biological safety cabinets or chemical hoods. They lack a front window and provide protection for only the work surface, not the worker. Clean filtered airflow is forced across the work area and either directly or indirectly blown at the worker, and therefore these cabinets should not be used for work with potentially hazardous materials, including antibiotics used during media preparation.


7.     Special Ventilation Areas

a.     Glove boxes are usually small units that have multiple ports in which arm-length rubber gloves are mounted, and the operator works through these. Glove boxes generally operate under negative pressure, so that any air leakage is into the box. Exhaust air from glove boxes and radioactive iodine chemical hoods should be passed through scrubbers or other treatment before its release.

b.     Isolation rooms use the same principles as glove boxes, except that the protected worker is within the unit. The unit itself operates under negative pressure, and the exhaust air requires special treatment before release.