A Practical Approach to Low Temperature Etyylene Oxide Gas Sterilization
 

By : Mr. Amol Kalyanpur - Pest Control India Ltd.

INTRODUCTION WHAT IS ETHYLENE OXIDE GAS STERILIZATION ? The traditional method of sterilization of surgical instruments has been by autoclaving and dry heating, but these methods are not useful for sterilizing materials like plastic, rubber and electronic equipment as they are heat labile and hence susceptible to high temperatures. E.O gas sterilisation offers effective and trustworthy sterilization at low temperatures and has thus become a standard sterilization method. Another reason for the wide use of E.O gas sterilization is the range of products or materials that can be sterilized using Ethylene Oxide (E.O.) and the capability of a wide range of materials to withstand the treatment without getting damaged. The per cycle cost also makes it an economically viable solution.

It is a proven fact, that ETHYLENE OXIDE ( E.O or EtO ) is an effective sterilizing agent at low temperature with excellent penetration power . It is a chemical sterilant and therefore has to come in contact with the micro-organisms to be de-activated.

ETHYLENE OXIDE vaporizes at 10.7 degrees centigrade and being an effective sterilizing agent, it can destroy micro-organisms even at room temperature. Ethylene Oxide sterilises by alkylation. The inactivation and death of micro-organisms results from alkylating effects on sulfhydryl, carboxyl, hydroxyl and amino groups within the cell. E.O. substitutes for labile Hydrogen atoms on molecules that are needed to sustain life and by attaching to these molecules, E.O. stops these molecules’ life supporting functions. Some of the key molecules that EO disrupts are protein, DNA and RNA. However, like all chemical sterilants, E.O. is a toxic gas and is also flammable and due care has to be taken when operating the sterilizers. The operation and safety instructions have to be clearly understood by the operators before using the equipment.

Sterilization process by ethylene oxide can be practically summed up as the attainment of parameters needed for effective sterilization and the hindrance to this process caused due to the various other factors.

The critical parameters essential for the sterilization process are first described as under and other factors that may cause problems to the sterilization process are considered separately.

Other factors also affect the sterilization process and as such should also be taken up when considering sterilization as a total process and should be thought of, when analysing sterilization cycles; as also sterility failures.

CRITICAL STERILIZATION PARAMETERS

Irrespective of the type of gas ; whether 100% pure E.O gas or a mixture of E.O and inert gases; used for sterilization, the sterilization parameters which are considered critical are as under :

GAS CONCENTRATION :EtO concentration ranging from 300 mg/ltr to 1500 mg/ltr at the highest level has been found effective under varied conditions by various researchers. However for all practical considerations it is recommended that concentration of 500 mg/ltr. to 800 mg/ltr of Ethylene Oxide, be used. The gas concentration is directly related to the inactivation time of the micro-organisms and the higher the gas concentration the lesser is the time required for sterilization .

TEMPERATURE : The EtO sterilization process is a low temperature sterilization process. Gentle heating enhances sporicidal effect of Ethylene Oxide. The rate of inactivation of micro-organisms is approximately halved for every 10 degrees centigrade rise in sterilization temperature. Hence higher the temperature the faster is the sterilization. The temperature range for practical sterilization processes depends on the devices to be sterilized and for hospitals should be kept between 38 to 55 degrees centigrade and could be even validated for a certain load size and content.

HUMIDITY : Humidification of the load is necessary for micro-organisms to be in active phase, which makes them more easily succumb to the sterilisation process and is to be considered as a very important factor which influences the effects of E.O on the micro-organisms. Moisture enhances the penetration of gas into the cell to achieve lethal action by swelling the microbial cells and this aids in the chemical alklylation process that kill the micro organisms. Water acts as a carrier of E.O through the barriers. The water activity in the micro-organism is proportional to the level of humidity of the environment in its surrounding area.

A minimum of 33% relative humidity is considered essential for Ethylene Oxide Sterilisation. Though, the lower level is defined, higher levels are mostly dependent on the effects on the materials and the packing and should not to be detrimental to the device or its packing in any way. Vapours of Steam and the use of distilled water for humidity generation are the recommended practices for humidification in the chamber.

EXPOSURE TIME: The gas exposure time is related to concentration of gas as well as the sterilization temperature as mentioned earlier . The exposure period is also a function of external factors like the packing materials used. Exposure times ranging from one hour to six hours are practically used depending on the sterilization temperature and the concentration of  the sterilant gas.

OTHER FACTORS TO BE CONSIDERED

CHOICE OF Packing Materials: In order to be acceptable, the packing material used for Ethylene oxide sterilization must be permeable to Ethylene oxide gas as well as water vapour and provide a suitable barrier to micro-organisms and permit extended shelf life after sterilization. The sealing of the packs is also a factor to be considered . Improper seals are a sure way of post sterilization contamination. Peel – back bags or tubings, constructed with polyethylene or poly – propylene coated on one side and medical grade paper on the other side, is an excellent packing material to use as well as being economical compared to other suitable materials.

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