Sunday, August 5, 2012

Confusion About Killing Bacteria

Cleaning is not sterilization or disinfection. These activities or processes produce different outcomes.
Cleaning is removal of materials from surfaces; materials which may be inert, such as polymers or minerals, or alive, such as bacteria. Sterilization and disinfection kill live bacteria. But they are unlikely to remove the non-live bacteria husks.
There is some confusion about the difference between sterilization and disinfection, and specifically about use of hydrogen peroxide to perform disinfection or sterilization.
Bacteria are alive, they reproduce, create waste, have DNA, and consume matter to produce energy. Killing of bacteria is either disinfection or sterilization.
The difference between sterilization and disinfection is one of degree:
  • Disinfection is the process of eliminating or reducing harmful microorganisms from inanimate objects, air, and surfaces. To the extent elimination is removal from surfaces, disinfection provides a cleaning function.
  • Sterilization is the process of killing (destroying) all the various organisms and their spores present on surfaces, in liquids, in medication, or in compounds such as biological culture media. Sterilization implies no cleaning function.
Image 1
Decomposition of molecules can produce free radicals, which are atoms or collections of atoms having unpaired electrons in their outer shells. Hydrogen peroxide (H2O2) produces both hydroxyl (HO*) and hydroperoxy (HOO*) radicals.
Both attack cell walls, and often destroy cells by causing them to collapse. H2O2 is an equal-opportunity killer of cells. For humans the Personal Exposure Limit (PEL) is 1.0 ppm, and the Immediately Dangerous to Life or Health (IDLH) limit is 75 ppm.
In air, hydrogen peroxide has crucial applications as a low temperature antimicrobial gas used to decontaminate enclosed and sealed volumes such as laboratory workstations, isolation and pass-through rooms in hospitals, aircraft interiors, anthrax-ridden buildings, structures contaminated with nerve gas,1 and especially volumes to which access is difficult.
Vaporized hydrogen peroxide is registered by EPA as a sterilant2 that inactivates bacterial spores on environmental surfaces in an enclosed area.
There is more than one method by which hydrogen peroxide may be applied to surfaces and volumes, and the methods are often confused.3 The methods of application are:
  • Aerosols - a commercial system produces a fine mist (particle sizes between 8 and 10 microns) of 5% H2O2 in air, with <50 acid="acid" aerosols="aerosols" arabica="arabica" as="as" catalysts.="catalysts." collapse="collapse" gum="gum" h="h" ions="ions" over="over" phosphoric="phosphoric" ppm="ppm" silver="silver" sub="sub" the="the" time="time">2
O2 reacts, and then degrades to safe conditions. There is little published validation of this method in hospital situations.
  • Non-Condensing Vapor - in a four-step sequence, an enclosed volume is first dehumidified. Then 35% H2O2 is vaporized under controlled conditions of temperature, humidity, and pressure so that there is no condensation. This state is maintained in the enclosure for a period of hours during which super lethal concentrations of H2O2 of several hundred ppm are maintained in air for disinfection. Finally, the enclosure is purged with air (catalytic aeration) so that the concentration of H2O2 is below the PEL.
  • Common enclosures where this method has been tested are aircraft cabins and buildings. No toxic residues remain as the H2O2 is converted to CO2 and water. There is considerable published validation of this method, but not in hospital situations.
    1. Condensing Vapor - the same 35% H2O2 vapor is produced in an enclosure using a dual-axis vapor distribution system, which ensures that the H2O2 is introduced to the room evenly until the air is saturated. The condensing oxidant forms a liquid film about one micron thick onto exposed surfaces where micro-organisms may reside. After minutes (probably) of treatment the environment is similarly purged with air, leaving behind no toxic residues.
    There is considerable published validation of this method in hospital situations. It's believed4 to be superior to applied aerosols, because of concern about distribution of the aerosol particles. Six-log reductions with various strains of bacteria have been reported,5 as well as have failures.6
    Image 2
    Hydrogen peroxide is becoming a preferred disinfectant and sterilant primarily because it is straight forward to avoid toxic residues (vs formaldehyde). But, it is clear that additional development is needed and the successes noted above are likely to drive that to completion, and the confusion about it away.
    1. HPV is not effective against all nerve gases. Long-available nerve gases such as GD (soman), VX (the most toxic ever), and HD (sulfurmustard) are not treatable by HPV. However, HPV when combined with low levels of ammonia gas is reactive toward these death threats. See Wagner, et. Al., “Vaporized Hydrogen Peroxide (VHP) Decontamination of VX, GD, AND HD,” 2003. Available for download at
    2. Yet there is considerable recent concern that HPV is a not sterilant against all bacterial spores. Among other recent publications, see Anderson, B.,M. “Does ‘Airborne’ Hydrogen Peroxide Kill mycobacterium tuberculosis?,” Journal of Hospital Infection, Vol. 77, No. 1, 2011, pages 81 to 83. The answer appears to be no, which raises the question about the certainty of EPA’s endorsement.
    3. Anderson, B.M., “A Call for Clarity When Discussing Hydrogen Peroxide Vapor and Aerosol Systems,” Letters to the Editor / Journal of Hospital Infection, Vol. 77, 2011, pages 76 to 92.
    4. Fu, T.Y., Gent, P., and Kumar, V., “Efficacy, Efficiency and Safety Aspects of Hydrogen Peroxide Vapor and Aerosolized Hydrogen Peroxide Room Disinfection Systems,” Journal of Hospital Infection, Vol. 80, No. 3, 2012, pages 199 to 205.
    5. Falagas, M.E., Thomaidis, P.C., Kossantis, I.K., Sgouros, K., and Samonis, G., “Airborne Hydrogen Peroxide for Disinfection of the Hospital Environment and Infection Control: a Systematic Review,” Journal of Hospital Infection, Vol. 78, No. 3, 2011, pages 171 to 177.
    6. Pottage, T., Macken, S., Walker, J.T., and Bennett, A.M., “Meticillin-resistant ataphylococcus aureus Is More Resistant to Vaporized Hydrogen Peroxide than Commercial geobacillus stearothermophilus Biological Indicators,” Journal of Hospital Infection, Vol. 80, No. 1, 2012, pages 41 to 45.

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