Gaseous sterilization - Pharmaceutical Microbiology Third Semester PDF Notes

 Gaseous sterilization

Learning objectives

At the end of this lecture, the student will be able to:

– List the gases used as sterilants

– Explain the mechanism of action, advantages and limitations of the sterilant gases

Gaseous sterilization

• The chemically reactive gases ethylene oxide (CH2)2O and formaldehyde possess broad-spectrum biocidal activity

• Sterilization processes using ethylene oxide sterilization are far more commonly used on an international basis


• In the sterilization of re-usable surgical instruments

• Certain medical, diagnostic and electrical equipment

• Surface sterilization of powders

• An alternative to radiation sterilization in the commercial production of disposable medical devices

Mechanism of action

• Alkylation of sulphydryl, amino, hydroxyl and carboxyl groups on proteins and amino groups of nucleic acids

Gas used



Ethylene oxide




15–100 mg/L




• Even at the higher concentrations and temperatures, the sterilization processes are lengthy

• Unsuitable for the re-sterilization of high-turnover articles

• Delays because of the need to remove toxic residues of the gases before release of the items for use

• Both gases are potentially mutagenic and carcinogenic

• Cause irritation of the skin

Ethylene oxide sterilization

• Ethylene oxide gas is highly explosive in mixtures of >3.6% v/v in air

• Usually supplied as mix containing 10% ethylene oxide + 90% carbon dioxide for sterilization purposes

• Pure ethylene oxide gas can be used below atmospheric pressure in sterilizer chambers from which all air has been removed

• Efficacy of ethylene oxide treatment depends upon achieving a suitable concentration in each article

• The gas diffuses readily into many packaging materials including rubber, plastics, fabric and paper

• Organisms are more resistant to ethylene oxide treatment in a dried state

• Hence the sterilization requires a humidity of 30–70%

Sterilizer design and operation

• Sterilizer consists of a leak-proof and explosion-proof steel chamber

• Capacity: 100 – 300 litre

• Surrounded by a hot-water jacket to provide a uniform chamber temperature

• Vacuum pump for evacuation of chamber

• Inlet for passage of sub-atmospheric pressure steam

• Port for the entry of preheated vaporized ethylene oxide from external pressurized canisters or single-charge cartridges

Operating cycle for ethylene oxide sterilization

• The material is placed in a chamber previously heated to 55⁰C and initial vacuum is applied

• The moisture is introduced to provide a desired relative humidity

• Ethylene oxide is then admitted and required pressure is maintained throughout exposure time

• Gas is the exhausted and slowly filtered air is introduced into chamber until atmospheric pressure is attained

Sterilization using formaldehyde

Low temperature steam formaldehyde (LTSF)

• Formaldehyde gas for use in sterilization is produced by heating formalin (37% w/v aqueous solution of formaldehyde) to a temperature of 70–75°C with steam


• Similar toxicity to ethylene oxide

• Low penetrating power

Sterilizer design and operation

• An LTSF (Low temperature steam formaldehyde) sterilizer is designed to operate with sub atmospheric pressure steam

• Air is removed by evacuation

• Steam is admitted to the chamber to allow heating of the load and to assist in air removal

• Release of formaldehyde by vaporization from formalin (in a vaporizer with a steam-jacket)

• Chamber temperature is maintained by a thermostatically controlled water-jacket

• Steam and condensate are removed via a drain channel and an evacuated condenser

• Two types of cycles: either a simple holding stage or through a series of pulsed evacuations along with steam and formaldehyde admission cycles

• At the end of the treatment period formaldehyde vapour is expelled by steam flushing and the load is dried by alternating stages of evacuation and admission of sterile, filtered air

Other gaseous sterliants

• Propylene oxide – bactericide

• β – propiolactone – virucidal and bactericide

• Glycols – aerial bactericide

• Methyl bromide – insecticide

• Methyl alcohol – fungal decontamination

Factors affecting gaseous sterilization

1. Concentration of the gases

2. Temperature of load

3. Effect of moisture

4. Time of exposure

5. The condition and accessibility of the organisms


• Ethylene oxide and formaldehyde are the commonly used gases used in sterilization

• They act by alkylating the sulphydryl, amino, hydroxyl and carboxyl groups on proteins and amino groups of nucleic acids

• Method requires removal of toxic residues of the gases before release of the items for use

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