The Hitchhiker’s Guide to Hydrogen Peroxide Fumigation, Part 1: Introduction to Hydrogen Peroxide Fumigation

Abstract

Introduction:

When working with pathogens in laboratories, animal or production facilities, and even hospitals, the potential need for room fumigation for decontamination purposes must be taken into consideration. Questions regarding the choice of fumigant, technical aspects of the room, its ventilation, the fumigation system to be used, and other issues will arise and will have to be addressed.

Methods:

This article is based on literature searches and was compiled using the authors’ long-time personal experience in room and filter fumigation using various fumigation systems.

Results:

The article can be used as a guide to establish an effective fumigation system in a laboratory or an animal facility setting and may be adapted for use in hospitals. Different systems for hydrogen peroxide fumigation on the market are presented. Also, technical aspects are discussed.

Discussion:

Hydrogen peroxide is used in various forms for fumigation of rooms, equipment, and filters. Regardless of the individual limitations of these forms, hydrogen peroxide is a versatile fumigation method. However, it is important to consider numerous technical requirements when planning to implement hydrogen peroxide fumigation at an institution.

Conclusions:

Subsequent to the present overview of different fumigation systems based on hydrogen peroxide on the market and their technical requirements, part 2 of this article will focus on validation and verification of hydrogen peroxide fumigation while considering the entire fumigation process. The two parts together will serve users as a guide to establishing hydrogen peroxide fumigations at their facilities.

Hydrogen Peroxide Fumigation: An Overview and Comparison

When working with pathogens in laboratories, animal or production facilities, and even hospitals or health care institutions, room fumigation for decontamination purposes potentially needs to be considered. Especially in laboratories with maximum-containment measures, fumigation is an international requirement. ^1,2 Fumigation is the method of choice to achieve broad-spectrum decontamination of a fumigation zone. In any case, fumigation is often the only delivery approach for decontaminants when surfaces are difficult or impossible to reach using other applications such as wiping and spraying. A laboratory, for example, contains a lot of specialized equipment, such as microscopes, flow cytometers, centrifuges, biosafety cabinets (BSCs), incubators, refrigerators and freezers, and others. Together with all the smaller tools, such as pipettes, rack holders, and so forth, this represents a large surface area that must get in contact with the decontaminant. Furthermore, in high- or maximum-containment laboratories as well as isolation wards at hospitals, the containment zones that require decontamination are not restricted to the room itself but also include air ventilation ductwork, gas-tight dampers, and HEPA filters together with their housings, which makes the overall surface for fumigation even more complex.

Decontamination by fumigation is required in biocontainment facilities in several situations, such as (1) following a biological incident with possible release of aerosols containing (highly) infectious agents, (2) at the end of an animal experiment in which animals could not be housed in primary containment systems such as individually ventilated cages, (3) before maintenance, (4) before repurposing a facility, and (5) for decommissioning a facility and/or its equipment. The decision whether fumigation is required should always be based on a prior risk assessment.

Alternatives to Hydrogen Peroxide

Formaldehyde (FA) has been the method of choice for decades for fumigation of rooms and HEPA filters. ³ It is a broad-spectrum disinfectant, and its mechanism of action is thought to produce intermolecular cross-links between proteins while interacting with ribonucleic acid and deoxyribonucleic acid. The application of FA fumigation is very simple and cheap. A number of facilities worldwide simply use electrical frying pans to boil off FA and release it inside the fumigation zone, keeping equipment costs well below $100. ^{4 -7} However, FA is known to be a human sensitizer and carcinogen and, because of its stability, can leave undesirable residues if not evacuated from the treated area. ^8,9 Alternative fumigants include peracetic acid (PAA), chlorine dioxide (ClO₂), ethylene oxide, and ozone, among others. All these agents are potentially harmful to people, animals, and the environment. PAA is generally applied as a 5% to 15% equilibrium PAA solution containing PAA and hydrogen peroxide and is known to be a very effective biocide. A fogging system is used for room fumigation. ^10,11 Chlorine dioxide gas is a sterilant able to inactivate a very broad spectrum of microorganisms because of its oxidative action but requires well-contained rooms. ¹² Ethylene oxide is a true gas and has many properties that make it a highly effective sterilizing agent with excellent penetration properties. However, the major drawback of using ethylene oxide is its explosiveness when even small amounts come into contact with air. ¹³ For this reason, ethylene oxide must be used only in vacuum sterilizer applications, making large-scale fumigations, such as room decontaminations, impossible. Ethylene oxide is also a known carcinogen. Decontamination by generating ozone is widely used for applications in health care areas and rest homes. ¹⁴

Hydrogen Peroxide Fumigation

Given the aforementioned limitations of the various fumigation systems, a promising alternative, based on hydrogen peroxide, has been developed. Compared with chlorine dioxide or FA, hydrogen peroxide is less hazardous to personnel. It has been shown that hydrogen peroxide is effective against a wide range of pathogens, ^{15 -20} and contrary to, for example, FA, hydrogen peroxide produces nontoxic by-products (water and oxygen) and is thus ecologically safer, is less hazardous to personnel, and requires no postprocess neutralization or cleaning.

The inactivation mechanism of hydrogen peroxide is induced by the generation of free hydroxyl radicals, leading to the oxidation of deoxyribonucleic acid, proteins, and membrane lipids. Hydrogen peroxide inactivation shows a bimodal killing pattern. It is assumed that damage to deoxyribonucleic acid plays a role only at low H₂O₂ concentrations. At high concentrations, damage to other cell components is much more severe. ²¹ Studies suggest that the inactivation capabilities of hydrogen peroxide are greatly enhanced in the vapor phase compared with the liquid phase. ²² This may, however, not be the case for all organisms, as it has been shown that foot-and-mouth disease virus can be inactivated to the same level with aerosolized hydrogen peroxide at a concentration of 7.5% as with vaporized hydrogen peroxide at a concentration of 35%, all while requiring less total volume of fumigant. ²³

Hydrogen Peroxide Fumigation Systems

There are different systems and disinfectants with different concentrations of hydrogen peroxide on the market for fumigation. In principle, hydrogen peroxide can be applied either as a vapor or as an aerosol. Vapors are generated from a 30% to 35% H₂O₂ solution, whereas for aerosols the concentration is often less, ranging from 5% to 12% H₂O₂.

A heat source is required for the generation of vaporized hydrogen peroxide. Two vapor-based processes currently exist on the market. One is the so-called dry system. Here, vapor is generated in a heated evaporator chamber. In addition, supply lines are preheated with dry, heated air to prevent condensation within these lines. The temperature is about 80°C as the air leaves the generator. At the same time, air from the fumigation zone is dehumidified with silica gel. The decontamination cycle consists of 4 phases: (1) dehumidification, (2) conditioning, (3) sterilization, and (4) aeration. Cycle development is crucial, as the parameters for each phase depend on the fumigation zone and need to be determined as exactly as possible to avoid any negative effects during the fumigation process.

The other system, the so-called wet system, forms a layer of microcondensation of hydrogen peroxide vapor over all surfaces. A hot plate with a drop counter generates the H₂O₂ vapor. Preheating of the supply lines is not possible, as no hot air leaves the generator. Air is simply passed over the hot plate, thus dragging the vapor along. Similar to the dry system, the decontamination cycle consists of 4 phases: (1) conditioning, (2) gassing, (3) dwell, and (4) aeration.

There are several nebulizer or fogging systems on the market that supply hydrogen peroxide as an aerosol. All these systems have one or more nozzles through which a fine mist is generated. Most of the resulting aerosols have a diameter of about 4 μm. The hydrogen peroxide solutions used differ in concentration and content. For example, some solutions contain adjuvants, such as silver ions. Added silver ions increase the production of oxygen radicals by a factor of 6 to 8 compared with standard hydrogen peroxide solutions via the catalytically induced reactive oxygen species cascade. Peroxides are thus stabilized, thereby inhibiting their spontaneous self-destruction during storage and allowing them to be transported by air during the fumigation process until they encounter organic matter, which can then be inactivated. Additionally, silver ions are said to enhance the denaturing effect on microorganisms by binding to the sulfur bridges of certain proteins, thereby precipitating or inactivating them. ²⁴

Some generators activate the hydrogen peroxide as it is aerosolized. The terms used to describe this process include “activated,” “boosted,” and “ionized.” The idea is to enhance the formation of highly reactive oxygen radicals, further enhancing the inactivation effect of the fumigant. Plasma activation, for example, generates an antimicrobial H₂O₂ solution that consists of superoxidase water containing various ions, peroxides, superoxide molecules, and oxygen derivatives. The ionic charge of the solution disrupts the cellular membrane permeability, inhibiting enzymatic activities and denaturing cellular proteins. It also opens transport mechanisms across the cellular wall, enabling the rapid permeation of peroxide into the cell. ²⁵

Hydrogen peroxide aerosol systems do not require preconditioning of the room. However, it is desirable to have low relative humidity, as this will enhance the effect of the fumigant and allow more fumigant to be taken up by the atmosphere in the fumigation zone. The aerosol is generated at room temperature. Damage to material is low and not extensively documented. Generally, this has been attributed to the low initial concentration of hydrogen peroxide and the largely decreased occurrence of condensation because the fumigation takes place at room temperature, with no heating of the sterilant.

Depending on the layout of the fumigation zone and equipment present within it, some systems are more adequate than others. Airtightness of fumigation zones is important, though less so for hydrogen peroxide than for proper gases, such as FA and chlorine dioxide. On one hand, it will ensure that no fumigant escapes to adjacent rooms, possibly putting neighboring staff members at risk. On the other hand, fumigant is not lost, and thus inactivation within the fumigation zone is not perturbed. Hydrogen peroxide has a great advantage in being quite well containable and therefore represents less of a danger to personnel outside containment. The reason for this property lies in the physical nature of hydrogen peroxide. It does not form a gas when heated but evaporates and therefore shows reduced motility compared with a gas. Similarly, when aerosolized, the motility of hydrogen peroxide aerosols is much less compared with that of fumigants such as chlorine dioxide and FA. However, being less motile requires active distribution of hydrogen peroxide. This is generally achieved using fans or ventilators, as described below.

Technical Aspects to Be Considered

Contrary to FA fumigation, which is often performed using electrical frying pans, ^{4 -7} hydrogen peroxide fumigations have used complex fumigation systems that require tight controls. It is only recently that less complex, simpler systems have come onto the market, requiring less stringent process controls. Regardless, it is not only the process that needs to be controlled, but one must take the design of the rooms, HEPA filter housings, and equipment, such as BSCs, requiring fumigation into consideration as well. It is therefore important to define the fumigation zone(s) before deciding on a system. Additionally, further decisions with regard to the heating, ventilation, and air conditioning system, airtightness of rooms and installations, and fumigation methods, their limitations, and related processes require consideration.

Fumigation Zone

When defining the fumigation zone, be it for a high- or maximum-containment laboratory, a clean room, an isolation or operating room at a hospital, or a production or animal facility, it is much more complex than simply stating which room needs to be fumigated. The fumigation zone encompasses the entire containment for said room. Figure 1 depicts a simple schematic setup for a maximum-containment laboratory suite that will serve as an example for further discussions and illustrations.

Figure 1.

Schematic ventilation setup for a maximum-containment laboratory suite.

Strictly speaking, the fumigation zone encompasses the entire volume between the supply and the second exhaust HEPA filter. Contamination, should it occur, will not be restricted to the laboratory suite itself. Rather, they will travel by air through the ventilation ducts and the respective HEPA filters on the exhaust side. Thus, one must assume that the exhaust ducts and their HEPA filters are contaminated, with most contaminants trapped in the first HEPA filter. The second HEPA filter, meanwhile, serves as a backup and redundancy filter, should the first HEPA filter fail or be damaged. The supply side of the ventilation system should never be contaminated, as air generally flows from the supply to the exhaust side. Thus, contaminants from the laboratory suite should never arrive in the supply ducts, as they would have to move against the supply airflow into the laboratory suite. However, even the best systems may fail or need to be shut down for various reasons. It is possible that should the laboratory suite be at positive pressure and the HVAC system switched off, airflow is reversed. This is also the reason that a HEPA filter is generally present on the supply side in maximum-containment laboratories. ¹ With all these points considered, the fumigation zone, strictly speaking, does encompass the entire volume between the supply and the second exhaust HEPA filter.

The question now arises as to whether the entire fumigation zone can be fumigated as a whole or whether it must be subdivided into several fumigation zones. On one hand, this depends on the fumigation method used. HEPA filters absorb a large amount of hydrogen peroxide because of their large surface area and the filtration material (generally glass fibers). Additionally, some HEPA filters have wooden frames instead of metal frames, allowing incineration of the filters once they have been removed. However, wood will also adsorb hydrogen peroxide. Taking all of these aspects into account, if one decided to fumigate either the supply HEPA filter together with the laboratory suite or the entire volume depicted in Figure 1, additionally including the exhaust HEPA filters, a large amount of hydrogen peroxide would be absorbed by the supply HEPA filter and thus be lacking later in the laboratory suite or farther down at the exhaust HEPA filters, respectively. This would require larger amounts of fumigant used for fumigation and/or longer fumigation times to achieve the desired effect of fumigation throughout the entire fumigation zone. The challenge remains the same, regardless of whether vaporized or aerosolized hydrogen peroxide is used. Also, one runs the risk of clogging the HEPA filters the more hydrogen peroxide is used in the process because of the increased amount of humidity present, thus potentially blocking the entire ventilation system.

Additionally, it has been shown that it may be possible to fumigate the exhaust HEPA filters together with a large animal room using vaporized hydrogen peroxide when the fumigant was injected directly into the room. ²³ However, using aerosolized hydrogen peroxide in the same fumigation zone with the same setup, biological indicators showed growth following fumigation. The concentration of aerosolized hydrogen peroxide used to successfully fumigate the animal room was insufficient to also decontaminate the exhaust ducts with their HEPA filters, hydrogen peroxide did not reach the exhaust HEPA filters in sufficient amounts, or it was degraded more quickly than vaporized hydrogen peroxide and was no longer available in sufficient amounts once aeration of the room was initiated.

Although the fumigation method used plays an important role, one must, on the other hand, also take into consideration the entire setup of the technical installations, especially the ventilation components. As outlined above, the design of the HEPA filters plays an important role (wood vs metal frames). Additionally, despite the short distances depicted in Figure 1, ductwork may be quite long and HEPA filters quite some distance away from the room air inlets. Although this is not desirable, it cannot always be avoided. Space in any kind of facility is precious and costly. Thus, designers will try to optimize its use, which can lead to long ventilation ducts, increasing the volume to be fumigated. Depending on what system is used, this may lead to additional measures to be implemented, increasing initial investment as well as maintenance costs (see below).

To summarize the aforementioned aspects, one would likely have to subdivide the depicted fumigation zone into 3 separate zones: (1) the supply HEPA filter, (2) the laboratory suite, and (3) the exhaust HEPA filters. Therefore, installation of fumigation ports is necessary, with 2 each around the supply HEPA filter, 2 each around the laboratory suite, and 2 each around each exhaust HEPA filter. Because a fumigation zone needs to be isolated from the ventilation system, these ports need to be placed between the airtight dampers and the object to be fumigated, desirably as close to the dampers as possible in order to encompass the fumigation zone in its entirety. With regard to the exhaust HEPA filters, it is possible and has been shown that both filter housings may be fumigated at the same time. ²⁶ Regardless of this, it is still desirable to design the ventilation system in such a way that each filter box may be fumigated individually, thus requiring the fumigation ports as outlined earlier.

Fumigant Supply

An important question to be addressed is how to get the fumigant into the fumigation zone. Using the fumigation ports outlined above, the fumigant can be applied from outside containment. This is desirable and, for certain fumigation zones, such as HEPA filter boxes, the only way. However, depending on the fumigation method used, this may come with further necessary installations. If the fumigation system of choice is vapor based, one must take a close look at ventilation ducts during the design of the facility or, if the facility is already built, when commissioning the fumigation system. Vaporized hydrogen peroxide, as outlined above, reaches temperatures of up to 80°C as it is emitted from the generator. If this vapor falls onto, for example, stainless steel ducts, it will condense because of the temperature difference between the vapor and the duct at ambient temperature and render the process impossible to control or validate. Condensed hydrogen peroxide is not present as a vapor in the fumigation zone and thus will no longer be available for decontamination. Additionally, it is very aggressive and is known to cause damage to a number of materials over time. ²⁷ It is therefore desirable to prevent condensation. This may be achieved in different ways. On one hand, ventilation ducts can be preheated by extending the dehumidification step of the fumigation process, when hot air is blown through the system by the generator. However, this is a lengthy and ultimately costly process. This time span can be shortened while inhibiting condensation by insulating the ventilation ducts. Should this still not suffice, Kümin et al ²⁷ described the use of a gutter heater to preheat the ventilation ducts in order to prevent condensation. These examples are costly, and it is thus desirable to find alternative solutions. Another option is to refrain from using stainless steel ducts and instead use plastic-based ducts. At times, this may, however, not be possible because of the increased combustible load present in the building, possibly not conforming to local fire restrictions.

Aerosolized hydrogen peroxide may also be supplied from outside the fumigation zone. However, especially the newer, more economical systems entering the market are currently not equipped for this. The aerosolizers used in these systems are not designed to propel the fumigant through the duct work into the room to be fumigated. Thus, additional ventilation systems or fans need to be installed to guarantee that the fumigant arrives at its desired location. These fans also help close the loop, enabling the fumigant to be recirculated.

Fumigant Distribution

One next must decide how to distribute the fumigant outside as well as inside the fumigation zone. Distributing the fumigant outside the fumigation zone can basically be achieved in two ways: (1) fumigant is distributed from a centrally located generator, or (2) the generator is moved around from fumigation zone to fumigation zone. Both approaches have their pros and cons. Because generators for vapor-based fumigations are often quite heavy, they are difficult to move around, possibly posing ergonomic difficulties. Also, space may be tight in certain facilities, making it difficult or even impossible to cart the generator around a technical area. On the other hand, a centralized generator requires a further distribution system, ²⁷ which will increase the overall length of the supply lines for the fumigant. Once again, these lines will have to be preheated to avoid hydrogen peroxide condensation, with the same challenges as described earlier for the ventilation ducts.

Hydrogen peroxide, either in its vapor form or as an aerosol, is not a gas such as, for example, FA. Also, it is heavier than air. It thus must be moved around the fumigation zone. An efficient distribution of the fumigant inside the fumigation zone has two desirable effects. It will lead to a more efficient fumigation across the entire fumigation zone, as all surfaces will be more profoundly covered with fumigant. Additionally, it will decrease the likelihood of condensation occurring inside the fumigation zone for vapor-based systems and decrease the likelihood of hydrogen peroxide pools or films forming with aerosolized hydrogen peroxide systems. This will greatly decrease the likelihood of material damage, such as bleaching, rouging, power cuts, and paint cracking, including damage to epoxy coatings, such as shown by Kümin et al. ²⁷

There are several ways to distribute hydrogen peroxide within the fumigation zone. Krishnan et al ²⁸ described the use of tabletop fans to guarantee efficient distribution of vaporized hydrogen peroxide within a laboratory suite. In the described example, they used 6 fans to distribute the fumigant inside the laboratory suite as well as the shower and dirty change room. A similar approach was used to fumigate a BSL-3 laboratory with aerosolized hydrogen peroxide. ⁷ Tabletop fans can be used to guarantee uniform distribution of either vaporized or aerosolized hydrogen peroxide. However, one may wonder at the challenge of attempting to validate a fumigation process when several fans are required to successfully fumigate a room. Will the fans be positioned at exactly the same spot every time? Will the fans be facing the same direction during every fumigation? Will changes in the placement of the fans affect the result of the fumigation? The variables are numerous, and thus the likelihood of human error is increased. To decrease the number of fans used during fumigation, alternative methods have been proposed: the DekoJet ²⁷ and Alpiq 6100. ²³ Although different in appearance, both fans are based on the same principle. Fumigant is either taken up by the fan as it enters the fumigation zone or directly delivered to the fan through a hose. It is then accelerated to approximately 2000 m³/h, creating “a storm in a teacup.” The increased velocity allows a more efficient, uniform distribution, decreasing the likelihood of condensation or the creation of pools or films occurring while shortening fumigation times. Additionally, human errors during setup are less likely to occur, as only one fan is required, even for complex fumigation zones, making placement easier. Also, the high velocity with which the fumigant is distributed lessens the possible negative effect a placement with outlets directed differently than described during cycle development or validation would have compared with tabletop fans.

Electrical Power Supply

An additional aspect, often forgotten, is the need for electrical power sockets that can be turned on and off externally as required. There are two possible reasons these sockets will be useful. First, if the generator, be it a vaporized or an aerosolized hydrogen peroxide unit, is placed inside the fumigation zone, it is desirable to be able to start it from outside and not by an operator inside who would possibly have to rush out of the fumigation zone while the generator is already producing fumigant. Second, it is preferable to start the fans, tabletop or the likes of the DekoJet, externally. If potentially contaminated air is moved around by fans inside a fumigation zone and doors must be opened for operators to exit the room, air may exit with them and thus increase the likelihood of potentially contaminating areas outside the fumigation envelope. This can be avoided by turning the fans on from outside using remotely powered electrical sockets.

Building Management System

It may be possible to integrate a hydrogen peroxide generator into the building management system (BMS). Process-controlled generators such as those that generate vaporized hydrogen peroxide are often factory equipped with ports allowing the generators to be linked to the BMS. One may follow and document a fumigation run in real time. Data exchange is certainly an aspect to strive for. Fully automated processes controlled by the BMS are theoretically possible. All ventilation dampers, the generator, fans, hydrogen peroxide sensor(s), and so on, will require automation. Regardless of all technological possibilities available today, a trained operator is still necessary and fundamental for a successful fumigation. Rooms need to be prepared for fumigation, process controls placed, and checks run on the system. It is thus desirable not only for any institution to aim for an integrated fumigation process but also for trained and experienced personnel to take care of any fumigations.

Limitations of Hydrogen Peroxide Fumigation

As with any other fumigation method, hydrogen peroxide also has its limitations. Operators, users, and designers need to be aware of these, as they represent one aspect that will define what method or system is best suited for the decontamination process to be implemented. Table 1 provides an overview of some aspects related to vaporized and aerosolized hydrogen peroxide as a fumigant compared with others.

Table 1.

Comparison of Fumigant Characteristics Between Vaporized and Aerosolized Hydrogen Peroxide and Other Fumigants.

	Vaporized Hydrogen Peroxide	Aerosolized Hydrogen Peroxide	FA	PAA	ClO2
Effectiveness	Good (increasing amount of data)	Good (increasing amount of data)	Good (in use for decades)	Good (increasing amount of data)	Good (increasing amount of data)
Hazard	Low (nonhazardous end product: water and oxygen)	Low (nonhazardous end product: water and oxygen)	High (suspected carcinogen)	Low (nonhazardous end product: acetic acid, water, and oxygen)	High (toxic)
Distribution	Limited	Fair	Very good (gas)	Fair	Very good (gas)
Containability	Good	Good (probably better than vaporized hydrogen peroxide)	Difficult (gas)	Good	Difficult (gas)
Labor intensity	Low	Low	May additionally require inactivation and cleaning following fumigation	Low	Low
Cycles	Extensive cycle development	Cycle development	In use for decades (cycle parameters known)	Cycle development	Cycle development
Equipment costs	High	Low	Low	Low	High
Operational costs	High	Low	Low	Low	High
Maintenance	Complex system: maintenance required	Simple system: no maintenance	Simple system: no maintenance	Simple system: no maintenance	Complex system: maintenance required
Technical requirements	Often complex technical installations required (temperatures)	Distribution system potentially required (fan)	Sealed rooms essential (gas)	Distribution system potentially required (fan)	Sealed rooms essential (gas)
Material compatibility	Good (temperature, concentration)	Very good (low starting concentration, room temperature)	Good (potentially problems with electrical equipment)	good (at concentrations of 0.5% PAA)	Good

Abbreviations: FA, formaldehyde; PAA, peracetic acid.

Two aspects, both related to the fact that hydrogen peroxide is not a gas when used for fumigation, require special attention. As outlined above, hydrogen peroxide must be actively distributed during the fumigation process to achieve uniform distribution within the fumigation zone. Furthermore, differences between vaporized and aerosolized hydrogen peroxide fumigation have been observed with regard to their capability to decontaminate the exhaust ventilation ducts and HEPA filter(s) together with the room (see above and Kümin et al ²³ ). Although this may affect investment and operational costs and is more labor intensive, it has the positive effect of allowing the fumigant to be more easily contained. It has been shown that rooms not designed for fumigation and not providing the standard level of leakiness for either high- or maximum-containment laboratories can successfully be fumigated with aerosolized hydrogen peroxide without any negative effects on work going on next to the fumigation zone. ⁷ Hydrogen peroxide could never be detected outside the fumigation envelope. This particular characteristic will have a profound effect on design and building costs, as the lower airtightness of the fumigation zone required can be factored into the design of a facility.

Material compatibility is always an issue with disinfectants. On one hand, hydrogen peroxide has been known to affect wall paints, including epoxy coatings. ²⁷ It is therefore important to ascertain not only that materials used in a zone to be fumigated are compatible with the fumigant to be used but also that fumigation cycles are properly developed and validated. There is a distinct difference between using vaporized and aerosolized hydrogen peroxide. Whereas vaporized hydrogen peroxide is evaporated in a generator and exits the generator at temperatures above ambient (up to 80°C), aerosolized hydrogen peroxide is aerosolized at ambient temperature. Thus, there are no temperature differences between the fumigant and any surfaces it comes into contact with. There will therefore never be any condensation when using aerosolized hydrogen peroxide. Additionally, condensed hydrogen peroxide may have much higher concentrations of peroxide than initially used for evaporation, possibly causing more damage quicker. ^29,30 Furthermore, hydrogen peroxide solutions used for aerosolization have generally lower starting concentrations than those used for vaporization (eg, 7.5% vs 35% ²³ ). Using aerosolized hydrogen peroxide, it has been possible to fumigate rooms that had been coated with dispersion paint without any damage. ⁷ Dispersion paint does not conform to international standards for laboratory wall coatings (eg, World Health Organization ¹ ). Dispersion paint does not provide a smooth and easy-to-clean surface, is not impermeable to liquids, and is said to be nonresistant to hydrogen peroxide. Nevertheless, these rooms could be fumigated without any negative effects on the paint. Overall, it may be said that aerosolized hydrogen peroxide has slightly better material compatibility than vaporized hydrogen peroxide.

It should be noted that although elevated levels of relative humidity have been observed during fumigation using either vaporized or aerosolized hydrogen peroxide, no electrical equipment has ever been harmed in any fumigation performed by the authors or others. ^{7,11,23,27,31} This includes equipment such as BSCs, laptop computers, small rodent biocontainment and individually ventilated cages with their ventilation units, (fluorescence) microscopes, freezers, incubators, and other equipment. Some of this equipment was left running intentionally, such as BSCs, as they aided in the distribution of the hydrogen peroxide and were thus internally fumigated at the same time. Thus, electrical equipment may well get fumigated without fear of damage.

Besides possible damage to any material or equipment present within the fumigation zone, there is also material that may negatively affect the fumigation cycle. For example, porous material, such as paper, cardboard, and fabrics, may adsorb hydrogen peroxide during the fumigation process. On one hand, this will lower the amount of available fumigant and thus endanger the success of a fumigation. On the other hand, these materials will release hydrogen peroxide again over time following the fumigation, possibly leading to longer aeration times and thus unnecessarily prolonging the fumigation cycle. This is particularly cumbersome for facilities that need to be released back into operation as quickly as possible, such as operating rooms, patient rooms, and production facilities.

Other materials act as catalyzers and promote the breakdown of hydrogen peroxide into water and oxygen, leading to decreased amounts of fumigant available for decontamination and thus negatively affecting the process. Copper is a well-known example of such a material. ³²

Finally, another limitation of hydrogen peroxide as a fumigant, which is often disregarded, may be found in the organisms to be inactivated. Some organisms produce catalases, an enzyme commonly found in nearly all living organisms exposed to oxygen. The enzyme catalyzes the decomposition of hydrogen peroxide to water and oxygen. There is evidence to suggest that in certain circumstances, organisms that produce catalases may be more resistant, and this should be considered in the risk assessment. Interestingly, a number of hospital-associated organisms are known producers of catalases. Staphylococci ³³ and Micrococci are catalase positive. Other catalase-positive organisms include Listeria, Corynebacterium diphtheriae, Burkholderia cepacia, Nocardia, the family Enterobacteriaceae (Citrobacter, E coli, Enterobacter, Klebsiella, Shigella, Yersinia, Proteus, Salmonella, Serratia), Pseudomonas, Mycobacterium tuberculosis, Aspergillus, Cryptococcus, and Rhodococcus equi. Despite their intrinsic resistance to hydrogen peroxide, it has been demonstrated that a complete inactivation could be achieved for a number of these microorganisms. ^34,35

Not only is it important to account for the effect of catalases when developing fumigation cycles with hydrogen peroxide, one should also consider testing cycles against the targeted organisms themselves. Additional considerations regarding the use of biological indicators as fumigation controls will be presented in the second part of this series. ³⁶

Fumigant Containment

Containing a fumigant is important. On one hand, if one can be certain that the fumigant does not escape from the fumigation zone during fumigation, the entire fumigant quantity supplied to the fumigation zone will be available for the decontamination process. Therefore, the fumigation will be much more efficient and more likely to succeed, as has been suggested by Coppens et al. ³⁷ On the other hand, fumigant leaking from the fumigation envelope poses a substantial health risk to people (coworkers, patients, etc) in adjacent rooms. In certain circumstances, it may be required to evacuate rooms adjacent to the fumigation zone during fumigation. This can have severe side effects on the operation of a facility and result in reduced availability of facilities and higher fumigation costs due to reduced productivity.

When designing a new facility, the choice of fumigant and its associated fumigation system should be considered. Depending on what fumigant is to be used, this will affect the level of airtightness required for the entire facility or at least for those parts to be fumigated in the future. A gaseous fumigant such as FA will require a higher level of airtightness than, for example, a vapor- or aerosol-based fumigant, as outlined above. However, how does one define airtightness, and what is required for fumigation? Internationally, there are currently two national standards available that define numerical values for airtightness of high- and maximum-containment laboratories. The Australian and New Zealand standard ³⁸ recommends a maximum air leakage rate of 10⁻⁵m³/Pa · s, measured at either a positive or a negative pressure of 200 Pa. The Canadian Biosafety Standards and Guidelines ³⁹ prescribe a pressure decay test as part of the commissioning tests, with a maximum 250-Pa loss of pressure over a 20-min period starting at 500 Pa positive or negative pressure, respectively. Both values appear to define sufficient containment requirements, as fumigations using different methods are in use in these countries. ^4,5,11,28 Also, facilities in other countries that comply with said standards have been fumigated using various methods. ^23,27 However, it is important to note that the Canadian standard is much more stringent, as it is run at a higher starting pressure and does not take the overall room surface into account. We have personally seen a room pass the Australian and New Zealand standard while failing the Canadian standard (data not published). Smaller rooms are especially prone to this, as even a small leak has a much more profound effect on the pressure decay than the same leak in a larger room. The German Association of Engineers has attempted to address this discrepancy by defining leakage rates for clean rooms, high- and maximum-containment laboratories, and isolators, taking the respective room surfaces into account. ^40,41 Thus, room leakage rates are normalized by their total surface area, which allows a comparison among different rooms. Coppens et al ³⁷ proposed an alternative approach, a so-called blower door test. However, the test was developed to quantify leakage rates in passive houses to optimize energy loss and describes values well below those of the standards and guidelines mentioned above.

In our opinion, regardless of the fumigation method to be used, one should strive to comply with the German guidelines when designing a new facility that requires fumigation. We have found that facilities complying with said guidelines can be safely fumigated using either FA or hydrogen peroxide in its various forms. ^23,27 However, what if a facility is already built and in operation and does not comply with any of the aforementioned standards or guidelines? Although it would be difficult to contain FA within a fumigation zone not built to these standards, hydrogen peroxide may well still be suited for fumigation of such rooms. ⁷ Others have also shown that fumigation of leaky rooms with aerosolized hydrogen peroxide is feasible. ³⁷ Herein lies one of the advantages of using aerosolized hydrogen peroxide, making it an economical, easy-to-use, and versatile fumigation method.

Fumigation System Selection Process

Clearly, there are many technical aspects to be taken into consideration when choosing a fumigation system. So how does one go about deciding on the fumigation method to be used? We would like to propose the following points to be considered prior to defining what method is to be used at a facility.

New or Existing Facility

A facility that is about to be designed and built may house any fumigation system desired and can be made to fit the requirements of the fumigation system of choice. All options are therefore available. However, if an existing facility suddenly requires fumigation, options may be much reduced. Questions such as those outlined below will need to be answered prior to making a choice.

Fumigation Zone

It is important to clearly define what needs to be fumigated. Depending on whether the target of fumigation is a laboratory, a production or an animal facility, a patient room, a HEPA filter box, or equipment such as a BSC, different fumigation systems can be used. Also consider whether it is desirable to fumigate several fumigation zones together or separately. Not all systems on the market allow the simultaneous fumigation of, for example, a laboratory and its exhaust HEPA filter. ²³ On the other hand, it may well be possible to fumigate BSCs or small rodent cage systems together with the room in which they are placed. ^7,11,27

Application of Fumigant

It will be necessary to define how fumigant enters the fumigation envelope. Some systems are already designed to provide the fumigant from either within or outside the fumigation envelope. ^23,26,27 Others are better suited to expel the fumigant from within the fumigation envelope. ^7,23 However, these systems may well be adapted to also provide the fumigant from outside. This may well require additional technical adjustments, though. Regardless, the decision has an impact on the design of the facility, and it will be necessary to consider what is feasible.

Distribution

Is it necessary to distribute the fumigant within the fumigation zone? Is access to the fumigation zone available to enter with a distribution system? Is there space for a distribution system within the fumigation envelope? Can the facility afford a distribution system (cost)? Is a distribution system necessary outside the fumigation zone (eg, in the technical area)? Once again, the answers to these questions will have an impact on the choice of fumigation system and the design of the facility.

Airtightness

It will be necessary to know the leakage of the fumigation zone. In the case of a fumigation zone with low leakage rates, a gaseous or hydrogen peroxide–based system is an option. Leaky fumigation envelopes, on the other hand, may well require the use of a hydrogen peroxide–based fumigation system because of its good containability.

Organisms

It is important to define what organisms must be inactivated. Not all organisms show the same susceptibility to different fumigation systems. Ideally, one selects a fumigation system with broad applicability to numerous organisms. Unfortunately, this is not always possible. Also, knowing what organisms need to be inactivated will aid in the development of fumigation cycles. It has been shown that, for example, foot-and-mouth disease virus is less susceptible to vaporous hydrogen peroxide than bacterial indicator spores. ²³

Turnover Rates

How quickly does the fumigation zone have to be back online? Can the facility afford to have the fumigation zone out of operation over several days, or must it be back in operation within a short period of time? The decontamination time is similar for most fumigation systems and in the range of several hours, depending on the size of the fumigation zone. However, the aeration time may vary depending on the fumigation system. FA needs to be purged from the room for several days in some circumstances. ²³ Hydrogen peroxide can generally be purged overnight and in some cases even within a few hours, allowing the fumigation zone to be back online much quicker.

Availability

Not all fumigation systems are available globally. A lot depends on the local market and what is available there. Certainly, with the world becoming one global market, it is now much easier to buy anything from anywhere. However, it may also be taken into consideration that even though it is possible to buy a piece of equipment from anywhere, it may not necessarily be possible to find technicians to install or maintain the equipment. Also, can spare parts or the products to run the system be easily obtained? It is worthwhile to check what is available in a given local market and base the choice of fumigation system on that.

National Regulations, Standards, and Guidelines

National regulations, standards, and guidelines may not permit the use of certain fumigation systems. FA, for example, has been banned for certain applications in the European Union under the 1998 Biocidal Products Directive (Directive 98/8/EC). It is therefore paramount to check with local authorities with respect to which fumigation systems are and are not allowed in a given jurisdiction.

Costs

As with most aspects of our lives, costs play an important role. On one hand, a fumigation system comes with initial investment costs. Once obtained, though, there are also costs to run the system and to maintain it. Both need to be considered. It has long been claimed that FA is the cheapest fumigation system on the market. Recently, there have been new products based on hydrogen peroxide coming onto the market that rival the investment and operational costs of FA, though. ^3,7,25

Should one answer all of these questions, it should be possible to obtain a suitable and affordable fumigation system for any application or facility. Although all fumigation systems have their pros and cons, their limitations, and their supporters, we have had great success with hydrogen peroxide in either its vaporized or aerosolized form and recommend it wholeheartedly.

Conclusions

This article was written as a guide for those involved in planning and performing fumigations. Its focus is on hydrogen peroxide as a fumigant. Hydrogen peroxide, although it has its limitations, has obvious advantages over other fumigants. It is environmentally friendly and offers increased personnel safety, as no toxic end products are formed. Short cycle times lead to short turnaround times and, thus, to increased availability of fumigation zones to their users and intended purpose. Hydrogen peroxide shows good material compatibility. Hydrogen peroxide fumigations can be validated (further details on validation and verification of hydrogen peroxide fumigation will be published in the second part ³⁶ ) and have been shown to work successfully in numerous situations and with various agents.

It is important to consider hydrogen peroxide’s limitations as a fumigant, as these will affect the necessary technical installations of a facility. We have attempted to outline a decision process for future users of fumigation systems to determine what system best suits their purpose.

Subsequent to the present overview of different fumigation systems based on hydrogen peroxide on the market and their technical requirements, in part 2, ³⁶ to be published shortly, we focus on the validation and verification of hydrogen peroxide fumigation while considering the entire fumigation process. The two parts together will serve users as a guide to establishing hydrogen peroxide fumigation at their facilities.

Although hydrogen peroxide has been used as a fumigant for several years, and more and more data are available, supporting the notion that it is a fumigant with a broad spectrum of activity and a wide variety of applications, it is only recently that it could be shown to be a low-cost, versatile, and robust fumigant.

These recent developments may well pave the way for further applications and wider use of hydrogen peroxide fumigations. With this article, we hope to have supported these developments and look forward to future successes.

Ethical Approval Statement

Not applicable to this study.

Statement of Human and Animal Rights

Not applicable to this study.

Statement of Informed Consent

Not applicable to this study.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Kathrin Summermatter https://orcid.org/0000-0001-5519-5966