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
Applications
• 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 | Concentration | Temperature |
Ethylene oxide | 800–1200mg/L | 45–63°C |
Formaldehyde | 15–100 mg/L | 70–75°C |
Limitations
• 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
Disadvantages
• 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
Summary
• 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
