Introduction
Gas is used to sterilise large areas in industrial and medical environments examples of where they are used includes using gas to sterilise a production area after sterility has been broken this may be due to maintenance work being carried out in the area. It is also used to sterilise equipment this can include equipment that is used for surgical procedures and equipment that is used in pharmaceutical sterile environments. Chemicals such as formaldehyde and ethylene oxide are used the later being the most successfully used gas used on a large scale due to the ability to sterilise without causing damage to equipment and having the ability to defuse out of the equipment when given the correct airing during the sterilisation cycle.
Ethylene oxide is effective in killing all microorganisms and during the validation process of ethylene oxide sterilising cycle’s spores of bacillus subtilis var.niger are used as this is the most resistant organism to act against ethylene oxide. To ensure effective sterilising certain conditions must be met this includes-:
· The temperature of sterilisation
· The concentration of ethylene oxide
· The exposure time
· The penetration of the load
· The humidity of the area
Health and safety
When using or coming into contact with ethylene oxide health and safety precautions must be considered, equipment must be worn to protect the eyes, skin and the respiratory passage.
· gloves must be worn to protect skin on hands
· protective clothing must be worn to protect skin
· a full face mask with breathing equipment must be worn to protect eyes, face and for protect from gas inhalation
The gas vapour is irritating to the eyes and the skin and in liquid form can cause burns. If low doses are inhaled it can cause delayed nausea and high concentrations can cause narcotic and neurotoxic effect followed by vomiting, coughing and irritation to the respiratory passage. This can lead to emphysema and pulmonary edema.
Relative humidity of sterilising atmosphere
The relative humidity must be increased to between 40 and 50% to produce a suitable force that will provide moisture across the load to create optimum conditions at the surface of the microorganisms and increase the rate of diffusion through to the wrapped equipment. Moisture in the sterilisation cycle is required during the pre-vacuum stage to prevent the dry spores being dehydrated and becoming resistant to the action of ethylene oxide.
Temperature of the sterilisation cycle
The temperature of a sterilisation cycle can affect the time it takes for the process to complete where a cycle at room temperature can be achieved over a longer period of time and a high temperature of 60oC will decrease the cycle time.
Concentration of ethylene oxide and time of exposure
Manufactures of ethylene oxide sterilises recommend exposure to 850-900mg/litre for 3 hours or 450mg/litre for 5 hours at 64oC
Ethylene oxide is able to penetrate through paper, fabrics, different types of plastics and rubber. This allows materials to be sterilised suitably packed in appropriate containers, it is still important to make sure articles of gaseous sterilisation are clean. The efficiency of the process is reduced by organic matter however it does not prevent it. Diffusion of moisture is prevented by organisms within crystals. Spores protected in this way are capable of resisting sterilisation due to contact with steam, ethylene oxide, or dry heat these conditions effect sterilisation. Therefore it is important that care is taken to prevent physical protection of microorganisms in gas impermeable deposits. Sterilised articles should not be used until the absorbed gas has escaped because ethylene oxide is absorbed by many substances.
Ethylene oxide sterilisers
Design
The features of suitable equipment include:
· An exposure chamber that is gas tight and able to withstand high pressure and vacuum.
· A means of heating the chamber e.g. steam, or hot water jacket
· A baffled inlet of the gas mixture
· A method of completely vaporising the gas mixture and warming it to the sterilising temperature.
· A means of extracting air before, and the gas mixture after, sterilisation.
· A system for adding water to provide the right humidity.
· Provision for the admission of sterile air at end of the process.
· Safety valve and suitable indicators and recorders of temperature and pressure
Method of use
· The chamber is loaded
· Water is introduced to prevent vacuum dehydration of microorganisms
· Door is closed and temperature raised to sterilisation level.
· The heat exchanger is raised to high temperature (about 100oC)
· A high efficiency vacuum pump is used to reduce the air pressure in the chamber to about 1.5kPa.
· More water is added if necessary
· Warm gas mixture is added until correct pressure is reached
· Exposure time is allowed
· Gas is deabsorbed.
Control of the process
Similar method to steam sterilisation have been used – physical, chemical and biological monitoring of the process. The temperature and pressure should be recorded throughout the sterilisation cycle. Conventional physical and chemical monitoring is not able to provide assurance that each item of the load has been subjected to the predetermined conditions and gas concentration. Biological monitoring is the most important, either paper or aluminium foil strips coated with known concentrations of Bacillus subtilis var niger spores after exposure they are tested for sterility.
Application of ethylene oxide sterilisation
Ethylene oxide is suitable for sterilising where it is known that the microorganisms are on the surface of the particles and not embedded inside them. Ethylene oxide can be used to sterilise equipment, instruments and articles that are made from plastic, rubber, metal and other materials without causing damage. Catheters and syringes can also be sterilised. Other articles such as intravenous sets, prostheses, blood oxygenators, bottles and vials are also suitable. Some plastics may become damaged however the damage is not caused with pure ethylene oxide or mixtures with carbon dioxide. Contact with liquid ethylene oxide must always be avoided. Fragile rubber articles survive more treatments with ethylene oxide than steam. Equipment such as cystoscopes, bronchoscopes, opthalmoscopes and Geiger-Müller counters can also be sterilised with ethylene oxide.
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