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Chemical fume hoods provide a safe, negative-pressure environment by effectively controlling exposure to hazardous materials in laboratories. This allows users to work with volatile, hazardous, toxic, corrosive, and flammable chemicals. There are a variety of chemical fume hoods available that are designed for specific and general chemical use, as well as those that allow for portability and alternative venting. Traditional ducted fume hoods are designed to pull air out of the work space and expel it outdoors through a centralized ducting system. This design comes at an enormous energy cost to laboratories because the conditioned, dehumidified laboratory air is typically pulled through the hood at a high flow rate (100 ft/min). Energy costs associated with the high flow rates and subsequent costs of HVAC systems having to replace the conditioned air are significantly high. Additionally, vented air is directly expelled outside, usually without passing through any filtration systems. Since the 1970s, there has been an increasingly widespread initiative to reduce energy costs and make fume hoods (and laboratories in general) more environmentally friendly.
Ductless chemical fume hoods maintain the same flow rates as standard fume hoods, but they recirculate air from the workstation through high-quality filters and back into the work environment. A fan on top of the hood draws in fumes and particulates from the chemical process occurring in the hood, a prefilter captures many of the particles prior to passing through the main filter, and then the filtered air travels back into the immediate environment. The main filter in the unit may be American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE); high-efficiency particulate air (HEPA); ultralow penetration air (ULPA); activated carbon; or a customized special blended filter. Other customizable options include size dimensions, work surface material, cutouts, airflow rates, and the points of entry (i.e., sashes).
In addition to providing high energy efficiency, eliminating costly duct systems, and capturing pollutants, ductless hoods require minimal maintenance and are uniquely portable. Air monitoring is necessary to check filter saturation levels and ensure safe air quality, but the only maintenance associated with ductless systems is wiping down the hood surfaces to prevent chemical residue buildup, and replacing saturated filters. Installation is simple, and the hoods can be adapted to buildings or field stations without initiating costly construction or retrofits. This design allows the hoods to be transported between field sites, laboratories, and classrooms.
Laboratory fume hood filtration
ASHRAE filters provide protection against particles greater than 0.5 μm, which can be a safety issue since particles that do the most damage to human health are 0.2 μm in diameter. HEPA filters are 99.99% efficient in capturing particulate matter greater than 0.3 μm, while ULPA filters can trap particles as small as 0.12 μm in diameter. The efficiency of activated carbon filtration varies with the chemical. Carbon filtration is appropriate for more than 600 chemicals, but is generally not used with hydrogen, helium, methane, ethane, acetylene, carbon dioxide, nitrogen monoxide, propylene, and others. However, there are some very high-quality superactivated carbon-based molecular filters that provide complete filtration of pollutants and toxic gases. Special blended filters or a combination of activated carbon with an ASHRAE, HEPA, or ULPA filter provides the best protection, depending on the user’s chemicals and methodologies.
Fume hood safety considerations
Ductless fume hoods provide a cost-effective and environmentally friendly alternative to traditional ducted fume hoods, but not all chemicals are appropriate to use in ductless systems. Since ductless hood filters have different affinities and adsorption rates for various chemicals, these hoods are fairly chemical-specific, depending on the filters employed, and are not optimal for applications involving high evaporation, viruses, bacteria, trace metals, perchloric acid, acid digestions, and radioisotopes. It is important to realize that chemicals trapped in the filter and vapors entering the filter may react and alter filter breakthrough and retention.
Perchloric, picric, and nitric acids are particularly dangerous, especially at higher temperatures, and should be used with extra precaution in ducted, stainless steel hoods. Perchloric (an oxidizing inorganic) and picric (an organic) acids require special handling altogether because they are very strong oxidizers at high temperatures, reacting with metals, wood, and other combustibles to form explosive compounds. Picric acid is also potentially dangerous when dry or contaminated because picrate metal salts are potentially explosive compounds. Organic solvents, sulfuric acid, and acetic acid should never be used in a perchloric acid hood. Acid digestion fume hoods provide a safe environment for heavy use of acids (other than perchloric and picric) at high temperatures.
Spontaneously decaying radioactive materials produce ionizing radiation (alpha and beta particles), which can strip electrons from atoms, break chemical bonds, cause contamination, build up on fume hood surfaces and in crevices, and have carcinogenic effects on living biological tissue. Alpha emitters (americium-241, plutonium-236, uranium-238, thorium-232, radium-226, radon-222, and polonium-210) eject alpha particles, leaving an atom that has lost two protons along with two neutrons. Losing an alpha particle changes the atom to a different element. Beta emitters (tritium, carbon-14, potassium-40, cobalt-60, strontium-90, technetium-99, iodine-129, iodine-131, and cesium-137) eject beta particles, leaving loose electrons that are picked up by a positive ion. Acute and chronic effects of alpha and beta radiation result in tissue damage and cancers. Seamless, stainless steel, ducted hoods have been specially designed to protect users from ionizing particles.
Lab fume hood storage and work surfaces
Fume hoods should provide a safe and functional workstation for laboratory workers, and are often purchased with vented, flammable, or corrosive storage base cabinetry. Flammable (flash point < 100 °F) and combustible chemicals (flash point >100 °F) impose a fire hazard when stored/used improperly. The most common fume hood safety hazards that arise in laboratories stem from cluttered workstations, improper chemical storage, chemical fume hood incompatibility, and failure to check the hood flow indicator to make sure that it is working properly before starting work. When choosing a ducted or ductless system, it is important to get all of the necessary components to maintain good lab practices. Cabinets are available for regular, solvent (flammable and combustible), and corrosive (acids or bases) storage. Some cabinets are designed without a bottom for easy access to equipment that can be rolled into and out of the cabinet. If storage is not necessary, simple base stands or carts (for portable hoods) are available to elevate and support the hood and work surface.
Work surfaces vary in composition, abrasion, chemical, and heat resistance. Solid epoxy resin work surfaces provide the highest-level resistance, allowing users to perform tasks with high concentrations of solvents, acids, and bases. Solid phenolic resin stations are intended for high chemical resistance, but are not as hard and durable as epoxy stations. Stations made of composite resin provide the least amount of chemical resistance and are intended for general use only. Stainless steel work surfaces are also available for perchloric acid and radioisotope fume hoods, both of which require ducted exhaust. All work surfaces can be purchased with or without spill containment for extra protection, and with cutouts for cup or trough sinks. However, the more plumbing that a hood requires, the less feasible portability becomes. Sinks would likely be plumbed to acid waste systems, and if gas, compressed air, N2, or vacuum turrets are necessary, then disconnecting and reconnecting the unit to move it becomes more of a bother.
Fume hood design and other considerations
In the past, lab designers have avoided ductless fume hoods because they feel that there is a compromise between safety and efficiency. With advancing filter and monitoring technologies, ductless systems are becoming more favorable as they provide a “greener” laboratory product. However, monitoring is key, because flow rates must be kept at appropriate levels, and it is difficult to tell when filters become saturated. Ductless hoods can be very heavy, and additional plumbed options can eliminate their portability. Nevertheless, ductless hoods are desirable over ducted hoods because the filters capture potentially harmful pollutants, allow for proper waste removal, and are incredibly energy efficient. Criticism arises with the potential exhausting of fumes from chemical spills and forced evaporation because the concentration of vapors versus the flow rate and filterability becomes compromised.
Table 1 – Ductless fume hood manufacturers and distributors
Ducted hoods have been deemed “safer” by many ductless critics because a wider range of chemicals can be used, chemical breakthrough of the filter is not an issue, and they ensure good air flow. There are some products available that can be adapted to ducted systems to trap toxins and pollutants in exhaust prior to atmospheric release, which eliminates pollution coming from the ducts. Regardless, ducted hoods still face enormous energy consumption and costs.
All of these options may seem overwhelming, but the first thing to consider is the desired chemical use and laboratory infrastructure. Ductless hoods are cost-effective, do not pollute the atmosphere, and provide a great deal of flexibility through portability and lack of construction requirements. However, filters of ductless hoods can be limited by chemical incompatibility and absorbability.
Features of ductless fume hoods vary between manufacturers (see Table 1). Listed in the box above are important points to consider when choosing a ductless fume hood.
Emily S. Tozzi, M.Sc., is a freelance science writer with a Master’s degree in Soils and Biogeochemistry from the University of California, Davis; e-mail: [email protected].
Please check out our Ductless Fume Hood / Filtering Fume Cabinet section for more information or to find manufacturers that sell these products