In laboratory environments, at levels above a toxic threshold, gaseous contaminants and volatile organic compounds can cause chronic and acute health effects. Chronic health effects may include eye, nose and throat irritation, headaches, fatigue, dizziness, nausea, and allergic skin reaction; acute health effects may include systemic damage to the liver, kidneys, and central nervous system, and various types of cancer.
With ductless hoods, workstations, and enclosures that are not vented to the outside, gas phase filtration is critical in order to provide operator safety and protection from airborne particulates.
Gas filter design
Ductless hoods, workstations, and enclosures are designed to incorporate different types of gas phase filters. These filters may include a particulate prefilter followed by either an ultra-low particulate air (ULPA) filter or a high-efficiency particulate air (HEPA) filter. ULPA and HEPA filters trap suspended particles to remove gaseous impurities and volatile organic contaminants. The gas then passes through activated carbon filters.
Particulate prefilters
A particulate prefilter is an electrostatically charged fiber filter designed to remove submicron particulates from the air before it passes through the adsorption filters and thus protect them from contaminants such as smoke, acid mists, and other submicron particulates.
HEPA and ULPA filters
HEPA and ULPA filters are disposable air filters designed to trap a vast majority of very small particulate contaminants from an air stream. The HEPA filter has a 99.997% efficiency rating for collecting 0.3-micron particles, and the ULPA filter has a 99.999% efficiency rating for collecting 0.125-micron particles.
The filter media is made up of a thin sheet of boron silicate microfibers that form a complex three-dimensional matrix that traps particulate matter but allows gases to pass through. The media captures and contains contaminant particles throughout the filter’s depth. Both HEPA and ULPA filters are used to remove chemical powders and particulates. However, they are not designed for the removal of gaseous pollutants.
Activated carbon filters
Activated carbon filters are designed to remove gaseous pollutants. They are made from a variety of high-carbon-content substances including coal, wood, coconut shells, and bamboo, and are available either as granular activated filters or bonded activated carbon filters.
Granular carbon filters
Granular activated carbon filters have a relatively larger particle size and a smaller external surface. They are suitable for the adsorption of gases and vapors. One drawback, however, is that they release carbon dust.
Bonded carbon filters
In bonded carbon filters, the carbon is held in a predictable matrix, thus preventing a carbon shift that can lead to “dead spots” common in traditional granular filters. One of the advantages of bonded carbon filters is that they minimize the release of carbon dust found in traditional granular carbon filters.
Bonded carbon filters are either standard carbon filters or carbon filters impregnated with a chemical additive that helps the chemical adsorption process of the gaseous contaminant by chemically reacting it with the chemical additive.
Impregnated activated carbon filters chemically adsorb organic vapors. Filters designed to absorb a specific chemical or group of chemicals are shown in the Activated Bonded Carbon Chemisorptive Filters table here. The filters in this table are generally characterized by a filter absorption index that is a guide to approximate how much of the toxic contaminant the filter may adsorb. The filter absorption index does not indicate the filtration efficiency, but the approximate weight of the contaminant that may be adsorbed within the filter. In general, an index of “1” or “2” is a good indication of the filter’s suitability for use with that specific chemical (see Filter Absorption Index table here.
Silica and activated carbon filters
These filters are made from a chemically and thermally enhanced filtration technology called Silconazyne (AirClean). Characteristics of Silconazyne filters include silica and high-capacity activated carbon, along with a structure layered inside a bonded matrix. These features can result in a dust-free filter that can be used in a cleanroom without an exhaust HEPA filter and can be used with 99% of the chemicals shown in the Filter Absorption Index table shown here.
Benefits
A key advantage of activated carbon gas filters is that an operator can choose which type of filter to use based on their chemical specificity to the gaseous contaminant or volatile organic compounds. Another benefit is that two or more HEPA and activated carbon filters can be stacked to enhance the gas phase filtration efficiency. For single chemical applications, the two filters may be the same; for mixed chemical applications, two different filters can be used. In other cases, when the air contains both gaseous and particulate contaminants, an activated carbon filter and HEPA filter may be used.
However, the ability of gas phase filters to remove gaseous contaminants and volatile organic carbons depends on maintaining the filtration efficiency. To maintain gas filtration efficiency, both HEPA and ULPA filters must be monitored for leaks. If any leaks develop, the filter must be replaced.
With activated carbon filters, filtration efficiency depends on the affinity of the chemicals in the filter for the gaseous contaminants or the volatile organic carbon. The level of this affinity can be evaluated by monitoring for leaks of the gaseous contaminants or volatile organic carbons in the exhaust, and any odor or smell in the laboratory air. As with HEPA and ULPA filters, if any leaks develop, the activated carbon filter must be replaced to maintain filtration efficiency.
More information on gas phase filtration can be found at https://www.aircleansystems.com/.
Lina Genovesi, Ph.D., JD, is a technical, regulatory, and business writer based in Princeton, NJ, U.S.A.; e-mail: [email protected]; www.linagenovesi.com