Chlorine Dioxide Gas vs. Vapor Phase Hydrogen Peroxide
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Chlorine Dioxide (CD) and formaldehyde are the only effective decontaminating agents for the decontamination of buildings, rooms, isolators, and bio-safety cabinets that are "true" gases.  Hydrogen peroxide is a liquid at room temperature and requires boiling or vaporization to generate the vapor.  The boiling point for 35% hydrogen peroxide is 109 C while room temperature is only 21-22 C. A vapor is basically a superheated fluid that when introduced into a room wants to return to its original form (a liquid) as condensation. Chlorine dioxide on the other hand does not require condensation or lack of condensation for an efficacious process.  Gaseous CD does not require tight control of Dew Point temperatures (ie. difficult to obtain uniform condensation or lack of condensation in a real world application).  Gaseous CD has Quicker Aeration due to minimal absorption and lack of condensation on surfaces.   Additionally, ClorDiSys' CD systems can include integrated, accurate, and repeatable CD concentration measurement and control.  During VPHP cycles, many factors including the injection rate and dwell time must be calculated and validated for each chamber/room selected.  The addition or subtraction of equipment and other items in the room (as well as their positioning) also affects these calculations, meaning cycle development is essential prior to any changes within the space.  Cycle development for CD cycles only consists of roughly calculating the chamber/room size.  The amount and position or equipment and items does not play a role in CD cycle development at all.  This greatly simplifies validation efforts and assures efficacy and repeatability.
Efficacy:
Being a true gas, chlorine dioxide observes natural gas laws.  This means that chlorine dioxide, by nature, uniformly fills the spaces where it is held.  Hydrogen peroxide vapor starts to condense back into a liquid state at temperatures below 228 F.  This leads to many of the differences between the two methods in terms of effectiveness.
	Chlorine Dioxide Gas	Vapor Phase Hydrogen Peroxide	Description
Distribution	Follows natural gas laws to achieve complete and uniform distribution throughout space.	Hydrogen peroxide vapor is poor at passive diffusion because of hydrogen bonding characteristics.1	Contact is essential in decontamination, poor distribution leads to poor decontamination
Penetration	Able to penetrate into cracks, crevices and into some organic materials.	Unable to penetrate well due to tendency to condense on surfaces.  Unable to penetrate gaps of 5mm (0.196")2	Cracks, crevices, and gaps are commonly found in rooms and chambers.  Poor penetration leads to poor decontamination.
Relative Humidity	Optimal range between 60-75%	Initial levels vary but finals levels can reach 85%.	Increased humidity levels are essential in all spore reduction as the high Rh causes spores to swell and crack, allowing the agent to penetrate.
Concentration Monitoring	Integrated, validated photometric sensor which measures concentration accurately in real-time.	Chemical sensor which may be integrated at extra cost. Inaccurate concentration monitoring due to non-uniform distribution within space.	Chemical sensors can become saturated and read inaccurately.  Chlorine dioxide gas is able to be photometricaly measured due to its yellow-green color to provide precise measurement and control.
EPA Registration	Yes	Yes	Both methods are registered with the US EPA as sterilants.  Product labels must be read for approved applications.
NSF Approval	Yes	No	Only chlorine dioxide gas and formaldehyde gas are approved by NSF International for BSC decontamination.
Safety:
Both VPHP and CD are sterilants, which makes them dangerous by nature.  Due to differences in their chemical properties and the processes used during decontamination, chlorine dioxide gas is a much safer method.
	Chlorine Dioxide Gas	Vapor Phase Hydrogen Peroxide	Description
8-hr TWA (time weighted average)	0.1 ppm	1 ppm	Both methods use concentrations much higher than 8-hr TWA level.
Odor Detection	Yes, at 0.1ppm	No	The ability to smell CD at the 8-hr safety level allows the user to be aware of exposure while still at safe levels.  VPHP cannot be smelled so users are only aware of exposure at higher concentrations when coughing and choking occurs.
Carcinogenicity	IARC - NO ACGIH - NO	IARC - NO ACGIH - YES (confirmed animal carcinogen)	The American Conference of Governmental Industrial Hygienists (ACGIH) classifies hydrogen peroxide as a confirmed animal carcinogen.
Cycle Times (2500 ft3 room)	3-4 hours	6-12 hours	Cycle times are shorter with CD gas due to its faster aeration time to safe levels.  VPHP can take hours to aerate down to safe levels after a decontamination cycle.
Distributive Properties	Great (true gas)	Poor (vapors condense)	Gas laws state that gasses, such as CD gas, naturally distribute to uniformly fill their container. Hydrogen peroxide is a liquid at room temperature and will start to condense at temperatures below 228 C.
Ability to Penetrate Water	Yes	No	CD gas can penetrate water, decontaminating the water itself as well as the surface beneath.  Hydrogen peroxide dilutes and breaks down in water.
Equipment Location	Outside Room	Location depends on manufacturer	Keeping generation equipment outside of the room is the safest option, as equipment can be handled and shut down in case of emergency.
Aeration Time (2500 ft3 room)	30-60 minutes	Typically overnight	In case of emergency, aeration is initiated for both methods.  The longer the aeration time is, the longer an area is exposed to dangerous levels and cannot be accessed. VPHP aeration times are lengthy because of absorption into materials and condensation on surfaces.
Residues	None	Upon evaporation hydrogen peroxide can concentrate	Residual hydrogen peroxide that is allowed to dry on organic materials such as paper, fabrics, cotton, leather, wood or other combustibles can cause the material to ignite and result in a fire.
Equipment Needed:
Depending on the size of your application, multiple generators may need to be purchased.  Chlorine dioxide gas diffuses naturally, behaving according to gas laws, which allows a single CD Generators to decontaminate very large volumes.  Vapor Phase Hydrogen Peroxide generators have lower volume capacities due to the vapor's poor diffusion rates which necessitates a line-of-sight injection and nearly one generator per room.
Room Size CD Generators Required VPHP Generators Required Square Feet Cubic Feet Cubic Meters 50 ft2 500 ft3 14.16 m3 1 1 100 ft2 1000 ft3 28.32 m3 1 1 150 ft2 1500 ft3 42.48 m3 1 1 200 ft2 2000 ft3 56.63 m3 1 1-2* 500 ft2 5000 ft3 141.58 m3 1 2-3* 1000 ft2 10000 ft3 283.17 m3 1 6-7* 1500 ft2 15000 ft3 424.75 m3 1 10 2000 ft2 20000 ft3 566.34 m3 1 13-14* 3000 ft2 30000 ft3 849.51 m3 1 20 * Lower number of generators used if room consists of simple geometry (rectangular) and is empty of equipment.
Cycle Times:
Cycle times vary among both chlorine dioxide gas and vapor phase hydrogen peroxide generators mainly because of the difference in aeration times.  Chlorine dioxide gas can aerate from a chamber in 12-15 air exchanges, normally around 30 minutes for rooms, and under 5 minutes for isolators, Biological Safety Cabinets, and HEPA housings when direct venting is possible.  VPHP aeration times are lengthy because of absorption into materials and condensation on surfaces. Below are examples of published cycle times for both isolator decontamination and room decontamination.
Isolator Decontamination Generator Volume Cycle Time Steris VHP ~25 ft3 3-6 hours3 Bioquell Clarus ~25 ft3 3-3.5 hours3 Clordisys Minidox-M    31 ft3 1.3 hours4
Room Decontamination Generator Volume Cycle Time Steris VHP 300 ft3 7.5 hours5 Steris VHP 530 ft3 10+ hours9 Steris VHP 760 ft3 4.5 hours + overnight aeration6 Bioquell Clarus 2500 ft3 10-11 hours7 Clordisys Minidox-M 2700 ft3 3.5 hours8
Material Compatibility:
Both chlorine dioxide and hydrogen peroxide are oxidizers.  To examine a quantitative measure of how corrosive a chemical is, one can examine the chemical's oxidation / reduction potential.  The higher a chemical's oxidation / reduction potential, the more corrosive it is.  As can be seen in the table below, chlorine dioxide is less corrosive than both bleach and hydrogen peroxide.  More info on material compatibility can be found here.
Biocidal Agent Oxidation / Reduction Potential (V) Ozone 2.07 Peracetic Acid 1.81 Hydrogen Peroxide 1.78 Sodium Hypochlorite 1.49 Chlorine Dioxide 0.95
References:
1. Orlowski, Martin. Redifining Decontamination Safety. ALN Magazine, March 2011.
2. Steris Case Study M1941, Industry Review: Room Decontamination with Hydrogen Peroxide Vapor. Publication ID #M1941EN.2002-09 Rev. C, Steris, 2000.
3. Caputo, Ross A. and Fisher, Jim.  Comparing and Contrasting Barrier Isolator Decontamination Systems. Pharmaceutical Technology, November 2004.
4. Czarneski, Mark A. and Lorcheim, Paul. Isolator Decontamination Using Chlorine Dioxide Gas.  Pharmaceutical Technology, Volume 29, No 4, April, 2005
5. Steris Case Study M1456, VHP Case Study #1 Hydrogen Peroxide Gas Decontamination of A Material Pass-Through (MPT) Room, Publication ID #M1456(8/99), Steris, August, 1999.
6. Steris Case Study M1455, Case Study #3 - VHP 1000 Decontamination of a 760 ft3 room Containing Blood and Urine Analyzers, Publicaiton ID#M1455/990810 (8/99), Steris August 1999.
7. Room Decontamination Presentation to Council on Private Sector Initiatives, Washington, DC, by Henry Vance PE of Alpha Engineering, February 11, 2002
8. Lorcheim, Paul. Decontamination using Gaseous Chlorine Dioxide, A case study of automatic decontamination of an animal room explores the effectiveness of this sterilization system.  Animal Lab News, Vol. 3, No. 4 , p25-28, July/August 2004.
9. Rogers James., Choi Young W., and Richter, William R., "Effects of Drying and Exposure to Vaporous Hydrogen Peroxide on the Inactivation of Highly Pathogenic Avian Influenza (H%N!) on non-porous Surfaces", Applied Biosafety Vol. 16 No. 1, pp4-8, 2011.