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