Gaseous Decontamination for Critical Environments

By: Jim Polarine and Claire Fritz
October 2009

Hydrogen peroxide vapor systems can address large scale contamination challenges.

Fogging and gaseous decontamination become options when facilities are going through large scale bioburden outbreaks, or are bringing an area up after a shut down, a hurricane, a power failure, or construction events. The majority of the systems used for fogging and gaseous decontamination applications use EPA registered sterilants as an adjunct to cleaning and disinfection. Some facilities pre-clean surfaces with surfactant-based disinfectants and then fog the area with a sporicide such as 3-6% hydrogen peroxide or hydrogen peroxide/peracetic acid blends.

Typical fumigation or gaseous decontamination methods are dry processes that penetrate HEPA filters and can reach areas that are not easily reached by manual disinfection or fogging. These methods can also be validated and challenged with 106 biological indicators to confirm microbial kill. However, the HVAC system to the cleanroom must be shut down before decontamination and the area must be well sealed. Paraformaldehyde is an effective, low-cost fumigation agent, but is a human carcinogen and leaves a residue after the process. Chlorine dioxide gas, another option, is an excellent sterilant, but it has very low allowable human exposure limits (0.1 ppm OSHA TWA; 0.3 ppm STEL); so it is very critical that the space be thoroughly sealed before fumigation. Hydrogen peroxide vapor is rapidly emerging as an excellent alternative for large volume critical environments because it is easy to contain, is non-carcinogenic, leaves no residue, and is environmentally friendly due to its decomposition into water vapor and oxygen.

Sporicidal concentrations of hydrogen peroxide vapor at ambient temperature are approximately 0.1 - 2 mg/L (70-1400 ppm) depending on the volume. The amount of time that is required to kill 90% of a microbial population at a specific concentration is defined as the decimal reduction value (D-value). One D-value is equivalent to one log of spore reduction. Hydrogen peroxide vapor produces shorter D-values for Bacillus subtilis than both paraformaldehyde and ethylene oxide at optimum sterilization conditions.¹ Geobacillus stearothermophilus shows the highest level of resistance and is most frequently used as the challenge organism in validation.²

The shortest D-values are generated when the hydrogen peroxide vapor concentrations are the greatest. Figure 1 demonstrates D-value versus hydrogen peroxide vapor concentration. A 35% hydrogen peroxide liquid is flash-vaporized to generate a gaseous mixture of both water and hydrogen peroxide. While concentration is the primary variable, humidity and total saturation also impact the rate of microbial kill. Figure 2 represents a graph of D-value versus saturation for a constant hydrogen peroxide vapor concentration.

Because the hydrogen peroxide vapor concentration is dependent upon temperature and humidity, the surface temperatures and the relative humidity inside the room will determine the injection rate that will produce the optimal concentration. Vapor condensation should only occur if the injection rate is too high for the temperature and humidity of the room. With the hydrogen peroxide and water vapors below the dew point and noncondensing, exposure conditions will remain uniform and corrosion should not occur on any exposed equipment or surfaces.³ This process is defined as a vapor because hydrogen peroxide’s state at ambient conditions is a liquid; however, this does not imply that hydrogen peroxide cannot remain in a dry, gaseous state

Hydrogen peroxide vapor systems were first commercialized as portable generators in the early 1990s and were utilized specifically in isolator applications for sterility testing and aseptic processing. These systems are completely self-contained and easy to integrate into existing applications but can be limited in capacity. Small cleanroom applications, such as at Pharma Hameln GmbH, began in the mid 1990s. A portable hydrogen peroxide vapor system was installed and validated for the decontamination of an aseptic filling room. The 56-cubicmeter (1970-cubic-foot) room was decontaminated in two hours and then aerated in four hours.⁴

The first integrated, non-portable system became available in the late 1990s. Although these units require an external dry air source and integration into a facility’s HVAC system, they can operate continuously. Figure 3 demonstrates a typical setup for an integrated hydrogen peroxide vapor system for a cleanroom application.

The newest integrated hydrogen peroxide vapor systems now offer almost four times the vaporization capacity of the portable units and much greater air flows, so these systems now have the capability to efficiently decontaminate large volume cleanrooms. Facilities can now plan for the future and install a hydrogen peroxide vapor system as a built-in utility for high-level room decontamination as an adjunct to normal manual disinfection methods, and to drastically reduce bioburden during a shutdown or after new construction and renovation.

Hydrogen peroxide vapor can also be used as a high-level decontamination method for critical items and equipment that need to be brought into a classified environment. These gaseous systems can be built into airlocks or pass-through chambers for an automated and validated equipment decontamination process. However, cycle times can be one to two hours depending on the application, so the use of liquid sporicides may still be the best option for decontamination of items if there are time constraints.

A complete solution for microbial control within a cleanroom should include chemical technologies and processes for varying levels of contamination and all types of surfaces found in the space. This includes formulated chemistries for manual cleaning and disinfection, combined with a high-level gaseous decontamination method for the large-scale contamination challenges that can be expected to occur over the lifetime of a critical environment.

References

1. Klapes, N.A. “New Applications of Chemical Germicides: Hydrogen Peroxide.” Program and abstracts of the ASM International Symposium on Chemical Germicides, American Society for Microbiology, 1990.
2. Kokubo, M., Inoue, T., and Akers, J. “Resistance of Common Environmental Spores of the Genus Bacillus to Vapor Hydrogen Peroxide.” PDA Journal of Pharmaceutical Science and Technology, May, 1998.
3. Hultman, C., Hill, A. and McDonnell, G. “Physical Chemistry of Decontamination with Gaseous Hydrogen Peroxide.” Pharmaceutical Engineering, January/February 2007.
4. Jahnke, M. and Lauth, G. “Biodecontamination of a Large Volume Filling Room with Hydrogen Peroxide.” Pharmaceutical Engineering, July/August 1997.

Jim Polarine is a technical service specialist at STERIS Corporation. He has been with STERIS Corporation for over eight years, where his current technical focus is microbial control in cleanrooms and other critical environments. He is a frequent industry speaker and has worked on several books and article publications related to cleaning and disinfection and contamination control.