Ethylene Oxide

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Chapter: Pharmaceutical Engineering: Sterilization

Ethylene oxide is a gaseous alkylating agent. It alkylates proteins, ribonucleic acid and DNA in microorganisms.


Ethylene oxide is a gaseous alkylating agent. It alkylates proteins, ribonucleic acid and DNA in microorganisms. It replaces labile oxygen with ethylene hydroxide. Ethylene oxide is utilized as a surface sterilant. Bulk crystalline materials can occlude vegetative bacterial cells or spores, with crystals. Conse-quently, ethylene oxide will not reach them. The final step prior to sterilization is an aseptic recrystallization step.

Ethylene oxide is a colorless gas with an aromatic odor. The threshold limit for the odor is 700 ppm. The OSHA specification for worker exposure is 10 ppm. The toxicity of ethylene oxide is similar to that of ammonia. It causes conjunctival and respiratory irritation, dizziness, headaches, and vomiting. It is known to be mutagenic and may be carcinogenic. There are some byproducts of ethylene oxide (boiling point 283.8 K) including ethylene glycol (boiling point 471.9 K) and ethylene chlorohydrin (boiling point 401.4 K). Pure ethylene oxide is flammable and explosive. It is generally mixed with propellant (88:12) or carbon dioxide (90:10). Ethylene oxide polymerizes in the liquid state. In this form, it may plug lines or spray polymerized sludge on the product. The product expires in 90 to 120 days because of the polymerization.

Ethylene oxide inactivates all microorganisms. The cidal rate depends on the concentration of gas, the temperature of sterilization, the duration of expo-sure, and the water content of the microorganism. Inactivation follows classical first-order kinetics. Inactivation is irreversible. Relative humidity is synergistic with ethylene oxide. At 30% to 60% relative humidity, the microorganism hydrates. The water acts as a vehicle to transport the gas through polyethylene and polypropylene. Polystyrene traps ethylene oxide and dissipates it over years and is thus not appropriate for ethylene oxide sterilization. Temperatures of 313 to 333 K are suitable for heat-labile items. Cycle times are longer if temperatures, relative humidities, or ethylene oxide concentrations are lower. Generally, concentrations of 350 to 700 mg/mL are employed. Cycle times vary from 4 to 12 hours.

Following sterilization, the load is degassed. This is a dynamic process wherein filtered air is passed over the product. The time for degassing is 12 to 72 hours. This is usually performed in the treatment chamber but may be moved to a sterile facility. The process is monitored using Bacillus subtilis var. niger as a biological indicator. Spore strips (106 spores/strip) can be purchased for this purpose. During validation, the load is probed with thermocouples in addition to B. subtilis spore strips. Gaseous mixture is sampled from different points in the sterilizer for gas chromatographic analysis.

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