Vaporous biodecontamination: A matter of efficiency

The value of routine vaporous biodecontamination is recognised across the industry. What is not clear is choosing the right process for an application. A comparison of vaporised and aerosolised hydrogen peroxide clarifies the choice

In pharmaceutical and life sciences facilities, contamination control is paramount to maintaining product sterility and research integrity. For this reason, both space and equipment biodecontamination have become part of the standard operation procedures (SOPs) for many companies and institutions who have recognised the value of routine vaporous biodecontamination.

Often part of scheduled shutdown maintenance plans, vaporous biodecontamination helps research and production facilities prevent costly contamination events. It also expedites the return to sterile processing or research activities after maintenance work is complete. When used in conjunction with manual cleaning, routine vaporous biodecontamination is a powerful and efficient tool to manage bioburden in critical environments.

Hydrogen peroxide (H2O2) is one of the most widely used sterilants for space decontamination in aseptic and containment environments. This is largely due to its safety, efficacy, material compatibility and lack of residues. Today, almost thirty years since its launch to the market, vaporised H2O2 biodecontamination is used in a variety of applications and is well understood throughout the life sciences industry.

As the popularity of vaporised hydrogen peroxide decontamination has grown, alternative technologies have been introduced offering various price points and performance levels. The most attractive alternative is aerosol spray systems, also known as atomisers. This article compares vaporised and aerosolised hydrogen peroxide by exploring how they are generated, their physical properties, performance differences as well as implications for the successful application of each.

This article compares vaporised and aerosolised hydrogen peroxide by exploring how they are generated, their physical properties, performance differences as well as implications for the successful application of each

Technology background

Vaporised hydrogen peroxide was first introduced by Steris Corporation in the early 1990s under the trademark STERIS VHP. The vaporisation process quickly heats and evaporates precisely measured quantities of an aqueous solution of hydrogen peroxide while controlling airflow, humidity and temperature.

The speed and control of evaporation convert a high percentage of the peroxide into the vapour phase. Once vaporised, individual molecules of invisible hydrogen peroxide enter the atmosphere of the cleanroom/lab and move freely at ambient temperature and humidity levels.

Aerosol systems work by passing an aqueous solution of hydrogen peroxide through a special nozzle at a pressure great enough to create small airborne droplets of hydrogen peroxide and water. Aerosol droplet sizes range from several microns to over 100 microns in diameter. For comparison, a 10-micron aerosol droplet is about 68,000 times larger than a hydrogen peroxide vapour molecule, which is approximately 0.00015 microns. In relative size comparison, visualize the 10-micron atomised droplet as an olympic-sized swimming pool and the vaporised peroxide molecule as a beach ball floating on the surface.

This significant difference in the size of the droplet compared to the vaporised H2O2 molecule is the underlying reason why the two approaches have very different performance characteristics when used in surface decontamination applications. The much smaller vapour molecule can reach all exposed surfaces, enter tiny cavities and readily pass through HEPA filters.

Physical properties of H2O2

To understand the difference between vaporised hydrogen peroxide systems and aerosol fogging systems it’s important to review the forces and principles that impact the performance of these technologies in common decontamination applications.

Because both systems involve injecting an aqueous solution of H2O2 into the air, and most of that solution is water, air saturation and condensation are critical environmental limitation for both applications. Understanding the causes of saturation and condensation is essential to distinguish the differences between the two technologies.

Vapour pressure

Atomised droplets are a liquid composed of water and peroxide molecules. Each type of molecule has a different vapour pressure or tendency to convert from a liquid to a vapour. This is reflected in their boiling points. Water boils readily at 100°C, whereas peroxide boils at 150°C. Thus, water will evaporate from the droplet at a faster rate than peroxide and saturate the atmosphere in the immediate vicinity. Hence, the more rapidly evaporating water component of the solution saturates the atmosphere, and decreases its ability to hold the peroxide component. Because peroxide is more difficult to vaporise, atomisers typically use much lower concentrations of peroxide, often less than 15%.

Vaporised hydrogen peroxide systems overcome issues associated with vapour pressure through the use of a heated vaporiser and a measured supply of dry air to fully evaporate and disperse concentrations of peroxide; typically in the 35-59% range.

Evaporative cooling

As the aerosol droplets evaporate, they remove heat from the immediate surrounding atmosphere. Atomisers using compressed air also create a cooling effect as the air expands. The cooler temperature increases the atmosphere’s per cent saturation of the two substances.

Evaporative cooling also occurs inside the vaporizer of the vapour H2O2 system. As the vaporiser cools a thermostat activates a heater to compensate and maintain a constant and controllable evaporation rate.

Condensation

Just as water will boil and evaporate more readily than hydrogen peroxide, the opposite is true when condensation occurs. Should a mixture of the vapours become too concentrated or cooled to the dew point, peroxide will be the first to condense. This reduces its concentration in the vapour phase and creates tiny droplets of concentrated peroxide on surfaces. These can act as magnets attracting other airborne droplets and vapour. This is known as coalescence.

Two steps are essential to avoid condensation: the complete vaporisation of the liquid peroxide solution and adequate dispersion of the vapour in the air to prevent saturation of the atmosphere.

Saturation percentage

Injecting too much or too little peroxide can lead to ineffective cycles. At a given temperature, air can only hold a certain amount of water and hydrogen peroxide vapour. This is the dewpoint. The maximum hydrogen peroxide concentration that can be maintained as a vapour at a given temperature and humidity level is referred to as 100% saturation; the proportions may vary.

Controlling saturation is the most critical component of any space biodecontamination process. Over-saturating the atmosphere in an enclosure leads to surface condensation and accumulation of concentrated liquid hydrogen peroxide. This can damage sensitive materials. Moreover, the pooling of liquid hydrogen peroxide on surfaces can be a safety hazard, deter the decontamination process and prolong aeration times.

The effective use of both vaporised and aerosolised hydrogen peroxide requires distribution of the vapour/aerosol throughout the room or enclosure

Both vaporised hydrogen peroxide and aerosol systems can be optimised to achieve comparable levels of kill against microorganisms, viruses and fungi.

Coalescence and gravity

Coalescence is the process in which two aerosolised droplets make contact and become one. Larger droplets have less surface area and will evaporate more slowly. Thus, larger droplets have a higher likelihood of attaching to other droplets and are less buoyant in still air, causing them to fall. This is only relevant to the atomisation process.

The small molecule size of vaporised hydrogen peroxide allows it to act as a gas with gravity not impeding its buoyancy and distribution. In contrast, the thousand-fold higher density of aerosolised droplets requires that they be rapidly dispersed by an air stream or they will accumulate on a substrate near the point of dispersion, creating a wet spot.

Performance differences

However, a vaporisation system is far more efficient than an aerosol system because it can attain a higher, more evenly dispersed H2O2 concentration level in the environment without exceeding the dew point. By reaching higher concentrations faster, vaporisation systems can achieve higher levels of kill (six or even twelve log bioburden reduction) compared to the aerosol system over the same period.

Aerosol systems can theoretically achieve comparable kill levels to vaporised systems by employing a longer cycle and injecting greater amounts of H2O2 In practice, however, it is difficult for aerosol systems to achieve high, evenly dispersed H2O2 concentrations before condensation occurs. Therefore, in most applications/environments, aerosol systems are only able to obtain a three or four log reduction before condensation. Wet surfaces and liquid pools form (prompting the termination of the exposure phase).

The use of an aerosol may, nonetheless, be effective for applications that require the wetting of the surfaces. Such treatments typically need an even layer of product and certain contact time to achieve a required disinfection level. The results from a wet process may have a lower level of log reduction or uneven distribution versus a vaporised process.

Distribution

The effective use of both vaporised and aerosolised hydrogen peroxide requires distribution of the vapour/aerosol throughout the room or enclosure. Adding airflow at the point of injection can facilitate sufficient dispersion. For large spaces, the HVAC systems (if recirculating and well-sealed) may be employed to enhance dispersion.

The distribution potential of aerosols is limited by the large droplet size and the effect gravity has on them compared to the smaller and lighter vaporised H2O2 molecule, allowing vaporised H2O2 to be distributed much further than aerosolized droplets. In addition, the smaller size vapour molecules can easily pass through HEPA filters whereas atomised droplets will likely coalesce on the surface of the filter media. This difference means that a vaporisation system can decontaminate a far larger area than an aerosol system using the same amount of H202 solution.

Output and capacity

The capacity of a single vaporised hydrogen peroxide generator to decontaminate at a given log-reduction level is significantly higher than a single aerosol generator due to its efficacy and the saturation and distribution limitations previously described.

Consequently, for a given size enclosure, the number of aerosol systems required would be several times greater than the number of mobile vaporised systems needed. A single large integrated vaporisation unit can decontaminate an entire building in sections of tens of thousands of cubic feet. Several portable vaporised hydrogen peroxide systems would be needed to achieve the same task, and even more portable or stationary aerosol systems would be required.

Networking and control

Using a single, high capacity vaporised hydrogen peroxide generator as a utility with pipes connecting it to rooms throughout a facility allows users a great deal of control and consolidation over the biodecontamination process in terms of automation, integration, validation, maintenance and data capture.

However, if users want the versatility and economy of small mobile units, both foggers and small vaporised H2O2 units can meet this need. Both systems offer the ability to network multiple generators together so that group of rooms can be decontaminated simultaneously.

Most vaporised hydrogen peroxide systems are precisely controlled using industrial PLCs to monitor and/or control temperature, humidity, airflow, and H202 injection rates. Depending on the type of system, the process uses heaters, dehumidifiers, flow meter pressure transducers and remote sensors to carefully monitor ambient conditions. Some systems can dynamically adjust injection rates to maintain target saturation levels during the cycle.

Financial considerations

Although limited in their efficacy and capacity, aerosol systems offer a decontamination solution at a significantly lower cost compared to vaporisation methods. An aerosol system is relatively simple in design with fewer features and require less maintenance.

Operation of aerosol systems is straightforward, with start-stop type operation and few settings, if any.

Vaporisation systems vary, with some operating automatically and others requiring some degree of operator training. These systems typically feature touch screen controls.

Biodecontamination chemistries

The concentration of liquid hydrogen peroxide used for aerosol biodecontamination usually ranges from 8-12%. Aerosol chemistries sometimes include additives offering alternative performance characteristics.

One variation adds surfactant and sequestering agents to the hydrogen peroxide formulation to facilitate uniform wetting and drying. Peracetic acid and hydrogen peroxide mixtures are also used for aerosol biodecontamination to improve lethality.

Material compatibility should be considered before using peracetic acid/hydrogen peroxide mixtures as they may be corrosive to ordinary steel, aluminium, copper and brass.

Some chemicals used for fogging are only EPA-registered as disinfectants, which are less effective than a sterilant in reducing a bioburden load of bacterial spores.

Vaporised hydrogen peroxide systems typically use a hydrogen peroxide solution in a concentration of 35%, which is highly purified so that it leaves no residue on surfaces and will foster better equipment reliability. Users should choose an EPA-registered sterilant with published label claims showing broad efficacy against bacterial spores, viruses, moulds and other types of fungi.

Final word

There are appropriate applications for both vapour and aerosol biodecontamination processes. Vaporisation has many advantages: outstanding efficacy and material compatibility, HEPA filter penetration, and the capacity to treat large areas. Aerosol systems, in contrast, are very economical, low maintenance and simple to operate. Nevertheless, vaporisation is recommended when high-level biodecontamination or sterilisation of an area is required, and a six-log or higher kill of spores, viruses, and moulds is expected.

Aerosols may be appropriate for smaller spaces where a three or four-log bioburden reduction is acceptable. Aerosols may also be suitable in a workspace that cannot be well-sealed or where the limited distribution range of the system is sufficient.

References

Spiegelman, J. and Alvarez, D. ,Cheating Raoult’s Law to Enable Delivery of Hydrogen Peroxide as a Stable Vapor Gases and Instrumentation Magazine January/February 2015 pg. 14-19.

Hultman, C. Hill, A. and Mcdonnell, G. The Physical Chemistry of Decontamination with Vaporization Hydrogen Peroxide. Pharmaceutical Engineering January February 2007, Vol. 27 No.1

Schumb, W.C. Hydrogen Peroxide, monograph series, American Chemical Society, Reinhold Pub. Corp., 1955

Finnegan, M., Linley, E., Denyer, S., McDonnell, G., 2, Simons, C., Maillard, J.Y.,  Mode of action of hydrogen peroxide and other oxidizing agents: differences between liquid and gas forms J Antimicrob Chemother 2010; 65: 2108 –2115

Authors

This article is co-authored by a team at Steris, including:

Steven Feinstein, Director, Sales Environmental Services
John Klostermyer, VHP Application Engineer, Vaprox Hydrogen Peroxide (VHP)
William Warren, Vice President, VHP Equipment & Consumables, VHP Equipment and Consumables
Derek Newbould, Senior Product Manager, VHP Equipment and Consumables

N.B. This article is featured in the February 2019 issue of Cleanroom Technology. The digital edition is available online.

Featured Companies

STERIS (more information, website)