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SAFE SCIENCE: BE PROTECTED
Safety in the Hood!

By Dr. Ken Roy

(See the rest of the Safe Science Series)

I. Laboratory-type Hoods – Gets A Boost – Legally!
On 15 June 2006, California’s Occupational Health and Safety Standards Board adopted safety regulations titled “Ventilation Requirements for Laboratory-Type Hood Operations.” By definition, a Laboratory-Type Hood is a device enclosed except for necessary exhaust purposes on three sides and top and bottom, designed to draw air inward by means of mechanical ventilation, operated with insertion of only the hands and arms of the user, and used to control exposure to hazardous substances. This and other standards are helping to lead the way to improve safety in school laboratories by requiring well defined engineering control devices. It also is leading the way in innovative technology. For example, the standard requires hoods to be equipped with a quantitative airflow monitor that continuously indicates whether air is flowing into the exhaust system during operation by 1 January 2008.

Why is this important? It is important because it helps set the professional standard for teachers in school laboratories throughout the country. These kinds of advances help science teachers in having legal muscle to update and maintain safe working conditions in their laboratories for all occupants. It helps teachers like the one from the Midwest who was told she had to have a canopy hood in her chemistry laboratory for experiments. It was determined to be cheaper and certain to meet the need for her chemistry class according to the building architect. But did it really meet the need? Not according to NFPA-45 which sets the standard for laboratory-type hoods. A canopy type hood is inappropriate for the kinds of experiments done in school science laboratories. The California standard, along with other safety standards, again serves as a model for science teachers in working with architects, builders, administrators and other.

II. Anatomy of A Hood!
Fume hoods come in a variety of sizes, but the conventional school lab hood has a width of about 4 to 6 feet. Remember the purpose of the hood is to isolate the contaminated air and remove it from the laboratory. In this way, occupants are not exposed to hazardous chemicals. Having said that, always make sure the hood is operating within its set parameters. For example, at one school, three out of seven newly installed fume hoods had the exhaust motors reversed – bringing the hood contents into the laboratory instead of out of the building!

The University of Louisville has listed the following Basics of Laboratory Chemical Hoods:

The typical components of a chemical hood and their major functions are:

Hood Body - The visible part of the chemical 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 – The sliding “door” to the hood. By using the sash to adjust the front opening, airflow 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 working in the hood and closed completely when the hood is not in use. The sash may be temporarily raised above this position to set up equipment, but must be returned to the optimum sash height setting prior to generating contaminants inside the hood.

Airfoil – Located along the bottom and side edges the airfoil streamlines airflow 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. Removing the airfoil can cause turbulence and loss of containment.

Work surface - Generally a laboratory bench top, but also the floor of a floor-mounted 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 airflow 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 airflow 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.

III. Going To College For Fume Hood Usage!
Proper usage of the chemical hood is critical for worker protection. The following prudent practices are supplied by Cornell in its Laboratory Inspection Manual to encourage the safe use of fume hoods:

1. Work at least six inches inside hood.
2. Keep the hood clear of clutter. Keep the amount of material in the hood to a minimum.
3. Lower sash to lowest possible position. Keep the sash between your face and the experiment.
4. Avoid rapid movement in and traffic in front of the hood.
5. Elevate large equipment off the work surface of the hood to improve air flow.
6. Use an airflow indicator (tell tale) such as an eight-inch strip of light material dangling from the sash.
7. Run water in hood drains often to keep drain traps full.
8. Use a safety shield in addition to the hood sash if a danger of explosion is present.
9. Insure adequate illumination inside hood.
10. For most effective fume hood operation, keep laboratory doors closed.
11. If you think that your hood is not functioning properly, call your building's facility coordinator and report the problem so that the repair process can be initiated.

1. Again, make sure there is at least an annual inspection of the hoods for proper preventative maintenance. This should include re-calibration to ensure proper function.
2. Filters: Filters should also be part of the inspection process. Included in this should be air flow patterns and measurement of face velocities. A smoke bomb can be used to test mechanical ventilation. Just make sure that the test has been coordinated with the fire department to ensure disabling of area smoke detectors prior to this activity.
3. Chamber: Before using the fume hood, the chamber should be inspected. Make sure nothing is blocking the baffle, air foils or bypass. The hood should be drawing air. Something as simple as a broken belt off of a pulley could have disastrous ramifications! For most operations, a face velocity of 100 fpm is sufficient.
4. Utilities: Make sure any utilities (gas, water, electricity) are functioning properly if used near or in the hood.

IV. Reducing the Hood Habit!
Remember that the microscale concept to curriculum; this is, using approximately 1/10th the amounts called for in laboratory, helps to reduce the need for using a fume hood. This approach seriously brings down the cost of doing science and also the need for using certain engineering controls on a regular basis. However, if the local laboratory ventilation can not handle the products of an experiment, make sure the hood is all you want it to be – ready for use and effective in evacuation of unwanted chemical vapors.

Resources:

Cornell Lab Inspection manual site: http://www.ehs.cornell.edu/lrs/ inspections_old/LabInsMan99/lim.intro.htm#hoods

NFPA 45: http://www.nfpa.org

OSHA Regulations: http://www.osha.gov

University of Louisville – Department of Environmental Health & Safety: http://louisville.edu/admin/dehs/lsfume.htm#Basic

Dr. Ken Roy is the K12 Environmental Health & Safety for the Glastonbury Public Schools in Glastonbury, Connecticut, an authorized OSHA Instructor and NSELA Safety Consultant. He can be reached by fax at 860-652-7275 or email:Royk@glastonburyus.org

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