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DECONTAMINATION


Bob Jobbins – Authorising Engineer (Decontamination) at Brighton & Sussex University Hospitals NHS Trust.


Gas plasma sterilisation: science fact or fiction?


This article is concerned with finding a suitable on-site solution for the sterilisation of re-usable medical devices that are made of heat sensitive materials.


Re-usable medical devices are becoming ever more complex and are being produced in materials that are more suitable for particular devices and/or offer cost reductions in the manufacturing process. However, some of these materials are heat sensitive and the majority of this type of device is destroyed when heated to over 60˚C. The temperature bands of 121˚C and 134˚C are the most commonly used for steam sterilisation between clinical uses, meaning that for some devices steam cannot be used as a sterilising medium. There are a number of low temperature


sterilisation processes available, of which H2


sterilisation, with or without plasma, is possibly the most recent. The others include:


O2


• Low temperature steam with formaldehyde and ethylene oxide


• Radiation – This is used for large-scale commercial use only.


• Magnetic – As above.


Gas plasma has been researched since the 1960’s. A process which is simple in principle, but to produce a robust reliable sterilisation


– Both of these processes were historically used within hospitals for low temperature sterilisation but have declined in recent years due to environmental issues. Large ethylene oxide (EtO) plants are used commercially where the process and environment can be more effectively monitored.


process, needs technology and computers. This is mainly because the parameters are critical to achieve repeatable sterilisation conditions. Plasma is considered to be the fourth


state of matter – solid, liquid, gas and plasma. It occurs when a high energy electromagnetic or radio charge breaks the molecules apart, creating a cloud of free radicals, electrons and ultra violet light. An example of plasma occurring naturally is during a lightning strike and the residual smell of brimstone. The process is sub-atmospheric and the


plasma can be generated either directly in the chamber itself where the load is in direct contact with the plasma, or externally where the load is subjected to the afterglow.


The sterilisation action Many people argue that the plasma sterilisation stage of a cycle does not have any effect on the sterilisation of the load. Others maintain that the physical effects of producing the plasma gives rise to the generation of free radicals and ultra violet light, both of which have a profound effect on bacteria.


The UV light will destroy the genetic DNA of the cell and also erode the microorganisms virtually piece by piece, and the free radicals combine with the cellular chemical bonds of the microrganisms to form gaseous compounds destroying these bonds. Plasma itself is limited in effectiveness as


it is reliant on the penetrative properties of UV light. The sterilisation action is now usually completed by a cocktail of chemical agents, primarily hydrogen peroxide (H2


with, depending on the supplier of the steriliser/chemicals, a percentage of other


O2 )


Figure 1: In comparison to ethylene oxide the time-weighted average (TWA) exposure limit of hydrogen peroxide is five times greater, although it is not considered carcinogenic.


8 hr TWA ppm mg.m-3


Ethylene oxide Hydrogen peroxide


75-21-8 5 9.2 – EH40/2005 Workplace Exposure Limits. 32 STEL


ppm mg.m-3 –


7722-841 1 2.4 2 2.8 Carc


‘Many people argue that the plasma sterilisation stage of a cycle does not have any effect on the sterilisation of the load.’


chemicals, such as paracetic acid, all of which combine to kill spores and bacteria using an oxidising chemical reaction. This is, to some extent, collaborated by the by Steris in its VPro


use of vapour H2O2


steriliser, which does not utilise plasma and also its use by various manufacturers, for example Bioquell, for environmental disinfection.


It is obvious that further research is


required to fully understand the process, but it can be shown that the combination of low vacuum, hydrogen peroxide and plasma does, in fact, produce a sterilising environment.


Health and safety risks The main health and safety risk is in the transport, storage and use of the cartridges containing the chemical. Concentrated hydrogen peroxide is a hazard that can cause serious medical problems. As a liquid,


Bob Jobbins


Bob Jobbins is an authorising engineer (Decontamination) employed by Brighton & Sussex University Hospitals NHS Trust (BSUH). After initially training as a marine engineer on steam ships, he joined the South East Thames Regional Health Authority in 1986 and was soon seconded to the sterilizer section. On the dissolution of Regions he worked for a short period with Capita – still testing and advising on decontamination before being employed by BSUH as a decontamination specialist.


He obtained a degree with the Open University and after completion of the Advanced Course in Sterilizer Technology became an IHEEM registered AE in 2008.


IFHE DIGEST 2014


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