How filters hold up to biodecontamination

HEPA air filtration systems in applications subject to routine biodecontamination can be affected by the decontamination agents. Camfil looked at its filters to see what effects such agents can have

HEPA filter frames and housings constructed of aluminium or stainless steel are preferred

High Efficiency Particulate Absolute (HEPA) filters are widely used to provide clean air to facilities where micro-organisms cannot be tolerated. HEPA filters are also used to clean the air leaving certain facilities where pathogens may be present. In these situations, the facilities are routinely cleaned using disinfectant solutions and the filtration systems may be decontaminated prior to servicing. In the course of these activities the HEPA filter will be exposed to the decontamination agent with potential detrimental effects.

Commonly used agents can be divided into two broad groups:

1. Gases and vapours used in space decontamination, including

  • a. formaldehyde (gas + water vapour)
  • b. hydrogen peroxide (vapour)
  • c. chlorine dioxide (gas + water vapour)
2. Solutions used for surface decontamination in the facility, including
  • a. bleach (sodium hypochlorite) solution (oxidiser)
  • b. acidic/oxidising solutions including peracetic acid/hydrogen peroxide mixtures
  • c. quaternary ammonium chloride solutions.

This article primarily focuses on group 1, where occasional direct application of the decontamination agent on the installed filter occurs for the purpose of decontamination prior to servicing. Where applicable, information is also provided on the effect of the routine exposure of the filters to ambient levels of vapours from group 2 cleaning solutions used on facility surfaces.

Camfil has considered the exposure of its HEPA filters to decontamination agents and lab testing and field experience indicates that, in general, the materials used by the company for HEPA filters are suitable for these applications. Generally speaking, the compatibility of the filter materials used by Camfil with commonly used decontamination agents is good to excellent.

Due to the hygroscopic and porous nature of wood, filters made with wood frames should be avoided where decontamination of micro-organisms is required. Naturally, there is always interaction between decontamination agents and the materials with which they come into contact. Under most conditions, Camfil HEPA filters withstand these effects without loss of required performance. Since each decontamination agent affects the filters differently, each one is addressed separately.

Formaldehyde

General gas decontamination process summary. Formaldehyde gas is generated on-site at the point of use by heating solid paraformaldehyde in a closed space. The resulting gas is distributed throughout the space along with water vapour to achieve decontamination. The target concentration level of formaldehyde is usually theoretically set to approximately 10.5g/m3 (0.3g/ft3) or approximately 7,800–8,000ppm.

The duration of exposure may range from 2–16hrs or longer and the relative humidity (RH) is usually maintained at 60%–95% during the decontamination cycle. Temperature is ambient to slightly above ambient – approximately 21–35°C (70–95°F).

Camfil has considered the exposure of its HEPA filters to decontamination agents

Following decontamination, the space might be either ventilated to atmosphere or the formaldehyde gas is neutralised using ammonia (by heating ammonium carbonate or a similar compound to generate ammonia gas) and then ventilated.

Compatibility and characteristics of HEPA filters to formaldehyde decontamination. Both lab testing (where performed) and field experience (where known) indicate that the materials used by Camfil to construct HEPA filters show excellent chemical compatibility with formaldehyde under typical decontamination cycles and generally good compatibility with short duration exposure to ammonia consistent with this process. Extended exposure of HEPA filter media and aluminium components to ammonia gas, particularly at high relative humidity, is not recommended.

Fraction negative decontamination studies using 106 B. atrophaeus biological indicators (BIs) demonstrate the effectiveness of formaldehyde in decontamination of the company’s HEPA filters.

Hydrogen peroxide

General vapour decontamination process summary. H2O2 vapour is generated by evaporation or aerosolisation of concentrated (30–35%) aqueous H2O2 solution. Some processes require initial dehumidification prior to introducing the peroxide vapour into the space. While some processes allow micro-condensation to occur, accumulation of condensation should be avoided because the condensate could be very concentrated (60% H2O2) and the concentration of peroxide in the air could drop as a result of condensation.

During the exposure phase, temperature, humidity and dew point in the system should be maintained to avoid widespread condensation of H2O2 on HEPA filters and surfaces. Peak H2O2 concentration may range from approximately 250ppm to approximately 1600ppm. Duration of H2O2 exposure may vary from 30mins to several hours. Aeration follows the decontamination cycle until the residual H2O2 level reaches an acceptably low level to allow access to the decontaminated space.

Compatibility and characteristics of HEPA filters to H2O2 decontamination. Both lab testing and field experience (where performed or known) indicate that the materials used by the company to construct HEPA filters show excellent chemical compatibility with H2O2 under typical decontamination cycles. It is known that over time H2O2 adsorbs onto exposed surfaces and during aeration (or ventilation) desorbs over time. It is also known that different types of surfaces exhibit different levels of adsorption and retention of H2O2.

Accordingly, and due to the physics of adsorption, lab testing and field monitoring indicate that the presence of a HEPA filter in a system may delay the attainment of peak concentration levels downstream of the HEPA filter (due to the enormous surface area of the filtration media). The HEPA filter will also capture droplets of aerosol in the air stream if they exist. H2O2 in the vapour phase will pass through the HEPA filter and downstream concentration levels will rise accordingly, approaching levels similar to upstream concentration level once adsorption has occurred.

After exposure, during the aeration phase, the opposite effect is observed. Downstream H2O2 levels will momentarily peak at the start of aeration due to rapid desorption from the filter media. Reduction in downstream concentration levels will initially lag behind that of the upstream level. The overall aeration time may or may not be extended due to the presence of the HEPA filter depending upon the type and area of other surfaces present in the system. Fraction negative decontamination studies using 106 G. stearothermophilus BIs demonstrate the effectiveness of H2O2 in decontamination of Camfil HEPA filters.

Chlorine dioxide

General chlorine dioxide (ClO2) Gas decontamination process summary. ClO2 gas is generated on-site at the point of use by either gas-solid phase or by solid-liquid phase methods. Regardless of the generation method, the decontamination cycles are similar. The resulting ClO2 gas is distributed throughout the space along with water vapour to achieve decontamination. The target concentration level of ClO2 gas is typically in the range of 1–5 mg/l (347–1,735 ppm). The duration of exposure may range from 30mins to 2hrs or longer and the RH is usually maintained at 60–75% during the decontamination cycle.

The space being decontaminated is kept dark because light accelerates the breakdown of ClO2. Temperature is ambient to slightly above ambient, approximately 21°C–30°C (70°F–86°F). Following decontamination the space is ventilated through a scrubber to capture and neutralise the ClO2 gas.

Compatibility and characteristics of HEPA filters to chlorine dioxide decontamination. Both lab testing (where performed) and field experience (where known) indicate that the materials used by the company to construct HEPA filters show good to excellent chemical compatibility with ClO2 under typical decontamination cycles. HEPA filter frames and housings constructed of aluminium or stainless steel are preferred to those constructed of coated carbon steel.

Rust staining

Repeated exposure of welded or fabricated stainless steel to high levels of ClO2, especially in high humidity environments, will result in surface rust staining if iron impurities are present on the exposed metal surface. The rust stains are superficial and do not negatively affect the integrity of the filter or housing. However, if the desire is to have a very smooth clean surface and to avoid rust stains, it is recommended that the housing be treated either by pickling or passivation or other means to remove residual iron contamination on surfaces after fabrication and prior to exposure to ClO2.

Rust stains present on previously exposed surfaces may be removed in the field by application of a pickling solution or other cleaning agent that removes iron oxide. Once properly cleaned, the stains will not return upon subsequent exposure to ClO2. Careful studies indicate that while low carbon stainless steel is advantageous, the use of 316/316L alloy stainless steel offers no additional advantages in this regard compared with 304/304L stainless steel.

Direct application of cleaning agents onto any part of the filters should be avoided

HEPA filter potting compound made of polyurethane will show a characteristic colour shift to yellow following exposure to ClO2; however, tests indicate no measurable change to the bulk properties and no loss in performance of the polyurethane. Fraction negative decontamination studies using 106 B. atrophaeus BIs demonstrate the effectiveness of ClO2 in decontamination of Camfil HEPA filters.

Long-term exposure to decontamination agents used in facility cleaning. Certain component materials and filter assemblies have been exposed to the cleaning agents listed in Group 2 in lab tests. These tests were intentionally designed to be harsh since practical concerns limited exposure time.

Because environmental exposure and experience concerning the installed base of filters takes many years of observation, the full facts about long-term exposure to environmental contamination are not currently known. Generally, in Camfil’s lab tests and field experience there were no severe interactions between materials of construction and exposure to these agents with the following notable observations:

  • Direct application of liquid or aerosolised cleaning agents onto any part of the filters should be avoided
  • If a gel is required to be used in an application (e.g. pharmaceutical, biological or food) where oxidative decontamination or oxidative cleaning agents will be used, the preferred gel is silicone gel.

The use of polyurethane gel will shorten the effective lifetime of the filter because it may oxidise on the surface and exhibit a ‘skinning effect’ after many years of service. Prolonged and repeated exposure of silicone gel to oxidising agents such as sodium hypochlorite will result in fading of the colour pigment.

This colour shift to clear does not of itself indicate a problem with the gel, but may indicate that the gel is being subjected to very harsh or unusual conditions. Exposure of silicone gel to chlorine and to strong acids and bases over many years may result in damage to the gel beyond colour change that may compromise the performance of the gel system.

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