Sometimes, especially in research laboratory settings, our planned reactions go awry. There are far too many documented incidents involving chemical reactions that resulted in serious damage and injuries.1 Perhaps some were avoidable, but there are certainly some that were not. If we are trying our best to conduct these experiments or protocols safely, we are most likely working in a chemical fume hood. We trust that the hood and its accompanying exhaust system are performing optimally and up to the task, especially if things get off track. This month’s article describes the different safety systems available and provides some guidance on the proper installation and operation of exhaust systems.
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Laboratory exhaust systems fall into three main classes: chemical fume hoods for working with corrosive acids and bases, volatile solvents, and other hazardous chemicals; biological safety hoods, which can be designed to protect the work (clean air bench) or the worker (true biosafety cabinet); and standard exhaust hoods typically used in mechanical or machine shops or production areas. This discussion will focus on the first category, the chemical fume hood, since this is a critical element of most laboratories and research, production, or other facilities. Laboratory fume hoods are designed to protect the worker by containing and exhausting harmful or toxic fumes, gases, or vapors from chemicals used in the hood. In simple terms, a typical fume hood is a box or enclosure with an exhaust duct and a blower mounted so that room air from the laboratory is pulled into and through the hood. The hood entry is designed to create directional airflow into the hood. The “pull” at the hood opening is termed the “face velocity” and is usually measured in feet per minute (fpm). Proper face velocity of the hood is critical in maintaining protection for the worker. Too little flow allows room air currents or disturbances to overpower the hood and draw contaminants into the room. Too much flow can result in turbulence and eddies that also lead to contaminants escaping the hood. Baffles and other aerodynamically designed components determine how air moves into and through the hood. Contaminants inside the hood are diluted with room air and exhausted by the hood’s blower and duct system to the outside, where they are dispersed. Let’s look at some of the different basic exhaust design principles of chemical fume hoods.
Lab design basics for chemical fume hoods
Layout of the laboratory and location of the chemical fume hood are both very important for optimum performance and minimal interference. Avoid placing fume hoods near doorways or exits. Ten feet from any door or exit is recommended by the National Fire Protection Association.2 This is to prevent a fire or chemical release from blocking the exit.
Also, to the extent possible, locate fume hoods away from high-traffic areas, air supply diffusers, doors, and windows. Any area that produces air currents or potential turbulence could affect the ability of the hood to capture and exhaust contaminants as designed.
Do not locate chemical fume hoods opposite workstations, desks, microscope benches, or other areas where personnel spend significant time. As above, the reason should be obvious, as any incident in the hood could involve or injure anyone seated in front of the hood.
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Ensure that there is an emergency eyewash and safety shower within ten seconds travel time from every fume hood. This requirement is triggered whenever a worker could be exposed to corrosive, flammable, toxic, or severely irritating substances.
Very often we inspect labs and find things stored on top of cabinets or in front of hoods or vents, completely disrupting the airflow. Therefore, make sure cabinets and equipment do not block or interfere with the fume hood opening or the laboratory’s supply or exhaust vents.
Optimum fume hood performance
Chemical fume hood performance is based primarily on constant and consistent face velocity. Although the OSHA Lab Standard, the OSHA Standard for Occupational Exposure to Hazardous Chemicals in Laboratories, 29CFR1910.1450, does not specify procedures for safe hood operation, exhaust volumes, or face velocities, it does contain this statement: “airflow into and within the hood should not be excessively turbulent; hood face velocity should be adequate, typically 60-100 feet per minute”; albeit, this appears in the non-mandatory Appendix A.3
The generally accepted consensus for face velocity is around 100 fpm, on average. We feel it is more important to provide an airflow indicator and routine performance certification or testing. The flow indicator should be located so that it is easily viewed from the front of the hood. It could be a magnehelic gauge that shows inches of water and is marked for a corresponding 100 fpm; a digital fpm readout, both with audio and/or visual alarms; or a simple strip of yarn or Kim wipe. The key is a visual indication that the hood is drawing in air and exhausting it through the duct system.
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The most critical part to maintaining hood performance is a regular check or testing of fume hood flows. Routinely checking the hood for adequate flow and velocity must be incorporated into your lab safety program. We recommend that you post flow test results or performance checks directly on the hood and request a recheck if you suspect a problem.
Hood exhaust design criteria
Obviously, the fume hood must be connected to an exhaust system. First and foremost, the system must be dedicated to the hood only. Other exhausts or equipment should not connect to the hood’s exhaust duct, unless these are designed and approved by a competent mechanical/ ventilation engineer.
Next, all fume hood exhaust ducts must be constructed entirely of noncombustible material. Any gaskets must be resistant to chemical degradation and fire.
It is also important to keep air velocities at optimum levels for the duct size. Air velocities should be enough to prevent any condensation of liquid or condensable solids on the duct walls. The American Conference of Governmental Industrial Hygienists recommends velocities from 1,000 to 2,000 fpm.4
Avoid or minimize horizontal duct runs and turns or bends. All horizontal duct runs must slope at least one inch per ten feet downward to a suitable drain in the direction of airflow.
Mount blowers externally, if possible, and as near the exhaust stack as possible. This will keep the duct runs at a negative pressure and ensure that any leaks are drawn into the exhaust stream. If blowers are mounted on top of the hood, all ducts downstream will be pressurized and any leaks will push contaminated exhaust into surrounding spaces.
Exhaust stacks are a critical end component. They should extend at least eight to 12 feet above the highest point of the roof and be secured against wind damage. Ensure that the stack is located at least 50 feet from any air intake. Finally, all exhaust stacks should be fitted with a rain cap that does not impede the exhaust flow.
1. Lab Safety Explosion Incidents—Chemistry, American Industrial Hygiene Association, Falls Church, VA. 2016. https://www.aiha.org/get-involved/VolunteerGroups/LabHSCommittee/Incident%20Pages/Lab-Safety-Explosions-Incidents---Chemistry.aspx
2. NFPA 45: Standard on Fire Protection for Laboratories Using Chemicals, National Fire Protection Association, Quincy, MA. 2015. http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards?mode=code&code=45
3. Occupational Exposure to Hazardous Chemicals in Laboratories, Occupational Safety and Health Administration, U.S. Department of Labor, Washington, DC. 2006. http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10106
4. Industrial Ventilation, American Conference of Governmental Industrial Hygienists, Cincinnati, OH. 2015. http://www.acgih.org/forms/store/ProductFormPublic/industrial-ventilation-a-manual-of-recommended-practice-for-design-28th-edition