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When Air Remains in the Chamber: Investigating Non-Condensable Gas Problems in Steam Sterilization

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Steam sterilization depends on contact between saturated steam and the exposed surfaces of the load. Air and other non-condensable gases (NCGs) can occupy space that should be filled by steam and interfere with the intended sterilization conditions. For CSSD teams, the practical question is therefore not only whether a cycle reached its displayed temperature and pressure, but whether steam could contact the relevant challenge locations throughout the load. CDC describes sterilization monitoring as a combination of mechanical, chemical, and biological monitoring and emphasizes appropriate loading and steam contact.[1]

What are non-condensable gases?

NCGs are gases that do not condense under the conditions of the steam sterilization process. Air is the most familiar example. An investigation may consider inadequate air removal or a chamber or door-seal problem. It may also review other possible process or equipment contributors, such as steam-quality concerns, gas released from the load or packaging, valves, or associated equipment. These are investigation possibilities rather than conclusions established by the sources for every sterilizer. The specific cause must be assessed using the sterilizer design, service information, facility procedures, and available evidence.

Air can affect the relationship between pressure and temperature at a particular location. A displayed pressure or temperature value should therefore not be treated by itself as proof that steam reached every intended location. Monitoring results must be interpreted in relation to the cycle, load, test method, and challenge device.[1]

Why routine monitoring may not identify every problem

Mechanical records provide information about recorded time, temperature, and pressure, but they do not independently demonstrate uniform steam penetration into every package or device. Chemical indicators respond to specified process conditions, while biological indicators provide a microbiological challenge. Bowie–Dick or other air-removal tests are intended to present an air-removal or steam-penetration challenge and should not be treated as interchangeable with a general chemical indicator used alone.

A published experimental study evaluated chemical, biological, and physical indicators under simulated NCG conditions. Its controlled fault simulations involved chamber leakage and door-seal failure. Results differed among the monitoring approaches, and indicators used without an appropriate process challenge device did not reliably detect small amounts of NCG under the study conditions.[2] The study does not validate the full investigation checklist in this article, and its findings should not be generalized to every sterilizer, load, monitoring product, or process challenge device.

FDA defines a process challenge device (PCD) as an item designed to simulate product being sterilized and provide a defined challenge to the sterilization process. FDA guidance also describes air-removal indicators.[3] This guidance should not be read as establishing broad clinical or operational rules for every facility. A PCD and its indicator should instead be considered as part of a system that includes the sterilizer, cycle, placement, and applicable instructions for use. A passing result from one monitoring method does not independently characterize all other locations or items in a load.

Findings that may warrant investigation

The following findings may justify review under the facility’s approved procedure. They are investigative clues, not validated diagnostic criteria for NCGs:

  • An abnormal, incomplete, streaked, or otherwise unacceptable Bowie–Dick or other air-removal test result.
  • Unexpected variation between repeated tests, sterilizers, test-pack lots, or chamber locations.
  • Evidence suggesting a chamber or door-seal problem, valve or associated-equipment issue, or unusual vacuum behavior.
  • Unexplained pressure–temperature discrepancies, extended air-removal phases, aborted phases, or alarms in the cycle record. A pressure–temperature discrepancy is a reason to review the record, not by itself a validated diagnostic test for NCGs.
  • Wet packages, condensate, or drying results that differ from the facility’s established process history. These observations may have causes other than NCGs and should not be assigned a cause without further review.
  • A recent maintenance intervention, utility or steam-supply change, or software modification. These events may provide context for an investigation but do not, by themselves, establish an NCG problem.

The cited experimental study specifically examined controlled chamber-leakage and door-seal-failure simulations. It does not establish that the operational clues listed above are caused by NCGs or that the checklist detects every such event.[2] Findings should therefore be documented as observations and evaluated through the facility’s approved process.

A practical investigation sequence

1. Preserve the evidence

Retain the cycle record, monitoring results, load contents, load configuration, operator identification, and relevant alarms. Record the sterilizer identification, cycle, date, test-pack or PCD identification, and indicator lot. If permitted by facility procedure, photograph an unusual indicator result before disposal.

2. Review the complete cycle record

Review the air-removal phase, exposure phase, recorded pressure and temperature information, alarms, aborted phases, and drying stage rather than relying only on the final cycle status. Compare the event with previous acceptable cycles performed in the same sterilizer using the same test method. Treat unusual vacuum behavior and pressure–temperature discrepancies as record-review clues, not as stand-alone diagnoses.

3. Check the test method and placement

Confirm that the air-removal test or PCD was appropriate for the sterilizer and cycle, within its expiry period, stored correctly, and used according to the manufacturer’s instructions. Verify placement, orientation, assembly, packaging, and handling. An incorrectly prepared or positioned test can produce an invalid result or make interpretation difficult.

4. Examine equipment and process conditions

Review whether the door seal, chamber drain, vacuum system, steam supply, valves, and sensors require inspection under the sterilizer manufacturer’s service procedures and the facility’s maintenance or qualification process. These components and conditions are areas for technical review, not proof of a particular failure. The investigation should distinguish, where possible, among a monitoring-device issue, an operator or setup issue, and an equipment or process-condition issue. The available sources do not provide a universal diagnostic sequence for every sterilizer.

5. Determine product disposition through approved procedures

For a failed Bowie–Dick or other applicable air-removal test, CDC states that the sterilizer should not be used until it has been inspected and passes the test.[1] The facility should also apply its approved hold, quarantine, recall, and reprocessing procedures, as applicable, to potentially affected loads. CDC’s guidance discusses investigation and follow-up in the context of a positive biological indicator; that example should not be treated as an automatic response sequence for every suspected NCG event.[1] Disposition must be determined through facility procedures, sterilizer and monitoring-device instructions, and applicable requirements.

Strengthening routine control

  • Use mechanical, chemical, and biological monitoring as complementary controls, recognizing that air-removal tests address a different challenge from a general chemical indicator.[1][2]
  • Standardize PCD or air-removal-test selection, placement, interpretation, and documentation in accordance with applicable instructions.
  • Trend air-removal results and relevant cycle alarms by sterilizer, operator, test method, and test lot when appropriate.
  • Link monitoring records to maintenance, repairs, utility or steam-supply changes, and changes in load configuration.
  • Train staff that a passing result provides evidence about the conditions challenged by that method; it is not, by itself, a blanket guarantee about every item or location in the load.
  • Involve appropriate CSSD, quality, infection-prevention, and equipment-service personnel when results are inconsistent or recurrent.

Practical implications and limitations

NCG control is a system issue involving the sterilizer, steam, air-removal process, load, packaging, monitoring method, and interpretation of results. A single indicator cannot independently characterize all of those elements. Conversely, a normal-looking printout should not automatically dismiss an abnormal air-removal result or a credible equipment concern.

The available experimental evidence used controlled chamber-leakage and door-seal-failure simulations and should not be generalized to every sterilizer, load, indicator, or PCD.[2] Facilities should follow the sterilizer manufacturer’s instructions, monitoring-device instructions, applicable requirements, and validated quality procedures.

Conclusion

Effective steam sterilization requires more than achieving a displayed target value. NCGs can interfere with steam contact, and different monitoring approaches may respond differently to a defined challenge. A defensible investigation combines an appropriate air-removal test or PCD, complete cycle-record review, careful documentation, equipment and process review, and controlled product disposition. Unusual results should be treated as prompts for structured investigation rather than as isolated pass/fail events.

Educational disclaimer: This article is for educational purposes and does not replace applicable requirements, validated facility procedures, sterilizer instructions, or monitoring-device instructions for use.

Sources

  1. Centers for Disease Control and Prevention. Steam Sterilization. https://www.cdc.gov/infection-control/hcp/disinfection-sterilization/steam-sterilization.html
  2. Performance evaluation of chemical, biological and physical indicators in the process of sterilization under the effect of non-condensable gases. PubMed / International Journal of Environmental Research and Public Health. https://pubmed.ncbi.nlm.nih.gov/33188866
  3. U.S. Food and Drug Administration. Premarket Notification (510(k)) Submissions for Chemical Indicators: Guidance for Industry and FDA Staff. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/premarket-notification-510k-submissions-chemical-indicators-guidance-industry-and-fda-staff

References

  1. Steam Sterilization. Centers for Disease Control and Prevention. 2024-02-05.
  2. Performance evaluation of chemical, biological and physical indicators in the process of sterilization under the effect of non-condensable gases. PubMed / International Journal of Environmental Research and Public Health. 2020-11-19.
  3. Premarket Notification (510(k)) Submissions for Chemical Indicators: Guidance for Industry and FDA Staff. U.S. Food and Drug Administration. 2002-09-27.

Educational overview. Follow the applicable product instructions, validated procedures, local regulations and your facility’s approved policies.