Thermal Disinfection and A0 Values: What a Washer-Disinfector Cycle Has to Prove
A four-chair dental practice in Ontario upgraded from a benchtop ultrasonic bath to a washer-disinfector because a consultant told them the machine would "do the cleaning and the disinfection in one go". Six months later the practice could show a stack of cycle printouts and could not answer the one question a visiting auditor asked: what does A0 600 mean for the instruments in that load, and how do you know the load got it?
That question is the whole subject. A thermal disinfection claim is not a temperature on a display; it is an accumulated exposure, and the number that expresses it exists so that a clinic, a machine and an inspector can talk about the same thing.
What is thermal disinfection and where does it fit?
Thermal disinfection uses hot water at a defined temperature for a defined time to reduce microbial load on instruments that have already been cleaned. It sits between cleaning and sterilization: the instruments come out of the washer visibly clean, safe to handle with bare hands for inspection and packaging, and ready for the autoclave.
Its advantage over chemical disinfection is residue. What comes out of the washer is also what goes into the pack: a Class 5 chemical integrator placed inside the tray is only meaningful if the instruments reached the washer's thermal stage in the right condition. Hot water leaves nothing behind if the water itself is suitable, whereas a chemical disinfectant has to be rinsed to a validated level. Its disadvantage is heat: anything that cannot tolerate 80 to 90 °C cannot go through the process, which is why heat-sensitive items follow a different route.
What does the A0 value actually calculate?
A0 integrates the lethality of the exposure across the whole cycle, not just the hold phase. Each second at temperature T contributes 10 raised to the power of (T − 80) divided by 10, and the contributions are summed. A second at 80 °C contributes 1. A second at 90 °C contributes 10. A second at 70 °C contributes 0.1.
Three consequences follow. First, a short spell at a higher temperature can be worth more than a long spell at a lower one. Second, the ramp-up and cool-down phases contribute, which is why the arithmetic has to use the measured curve rather than the setpoint. Third, a machine that fails to reach temperature for part of the hold loses A0 quickly, because the loss is exponential rather than linear.
The concept and its calculation are described in ISO 15883-1, whose 2024 edition replaced the 2006 text that many clinic files still quote.
| Specification | Equivalent at 80 °C | Equivalent at 90 °C | Typical use in a clinic |
|---|---|---|---|
| --- | --- | --- | --- |
| A0 60 | 1 minute | 6 seconds | Thermal step carries a low-level claim |
| A0 600 | 10 minutes | 1 minute | Instruments that will be sterilized afterwards |
| A0 3000 | 50 minutes | 5 minutes | Larger margin specified by the facility |
Which A0 value should a clinic specify?
A0 600 is the figure that appears most often in Canadian dental and clinic specifications, because it corresponds to 80 °C for ten minutes or 90 °C for one minute — both achievable in a benchtop washer-disinfector without stressing instruments. A0 60 appears where the thermal step is not carrying the main claim. A0 3000 appears where a facility wants a larger margin.
The specification should come from the instrument manufacturer's processing instructions, which ISO 17664-1:2021 requires to be provided, and from provincial infection control guidance. A clinic that writes "A0 600" into its procedure because a supplier's brochure said so, without checking the instrument instructions, has documented a number rather than a decision.
How do you know the load, not just the machine, reached temperature?
Four layers, and a clinic generally has three of them without calling it verification.
The first is the machine's own cycle record: the temperature curve, the hold time, the calculated A0 and often the conductivity of the final rinse. The second is periodic validation with independent temperature measurement placed in the load, at the points where heating is hardest — inside a tray, in a lumen, near the door. The third is the maintenance and calibration history, because a sensor that drifts produces a convincing curve that is wrong. The fourth is the visible condition of the load: no residual soil, no water pooling, no detergent film.
Chemical indicators do not belong in this list. Indicators described in ISO 11140-1:2014 are designed for sterilization processes, and a Class 1 tape on a tray proves nothing about a washer cycle. Confusing the two is a common error when a clinic tries to apply autoclave habits to the washer.
What goes wrong in practice?
The failures are mechanical and human in roughly equal measure. Spray arms blocked by a stray instrument or by scale. Racks overloaded so that water cannot reach the underside of a tray. Instruments loaded with hinges closed, so the joint never sees the water. Detergent dosed by guesswork, leaving residue that later masks soil or interferes with the sterilization chemistry. Final rinse water that is hard enough to leave mineral film on stainless steel.
Each of these produces a valid-looking printout, because the machine did reach temperature. The measurement was correct and the process was wrong, which is exactly the gap that validation and load inspection exist to close.
What should the written procedure contain?
Six elements, in the order they occur in the working day: pre-cleaning and the hold time before it starts, ultrasonic or manual cleaning with the solution strength and temperature, loading rules for trays and lumens, the cycle to select and the A0 it must reach, the acceptance criteria at the end of the cycle, and what to do when a criterion fails.
Acceptance should be written as a check a new employee can perform: instruments visually clean, no water in lumens, hinges move freely, no film or spotting, cycle record filed. Failures should name the action — reprocess, quarantine, report — rather than leaving it to judgement at seven in the morning.
Which records does a clinic actually keep?
Three records carry the weight. The cycle record, printed or exported, with date, machine identifier, cycle name, maximum temperature, hold time and A0. The daily check that the machine is functioning as expected, which many clinics fold into the first cycle of the day plus a log entry. And the periodic validation report with its temperature measurements and the identity of the person who performed and reviewed it.
Retention is a provincial and organisational question rather than a technical one, but the practical rule is that the record must survive longer than the instrument's service life in your building. A record destroyed after a year cannot answer a question about a patient treated eighteen months ago.
How do consumables follow the cycle?
Everything downstream of the washer depends on what came out of it. Drying matters because a wet instrument in a self-sealing sterilization pouch creates a moist environment inside the package, and moisture is the enemy of the sterile barrier. Packaging choice follows the load: pouches for single instruments and small sets, wrap for cassettes and dense trays.
Monitoring follows the packaging. A Class 5 chemical integrator inside a tray tells the reader what the load experienced during sterilization, and a strip of indicator tape on the outside distinguishes processed from unprocessed packs during handling. Both are sterilization records, not washer records, and the two sets of documentation should stay separate so that neither is mistaken for the other.
A practice that is building this system from scratch can start with the sterilization equipment collection, where washers, sealers and monitoring consumables sit together, and write the A0 specification into the procedure at the same time as the machine is installed rather than a year later.
References
- ISO 15883-1:2024 — Washer-disinfectors, Part 1: general requirements, terms and definitions (checked 25 October 2026)
- ISO 15883-1:2006 — Washer-disinfectors, Part 1 (previous edition) (checked 25 October 2026)
- ISO 15883-4:2018 — Washer-disinfectors, Part 4: requirements for washer-disinfectors for thermolabile endoscopes (checked 25 October 2026)
- ISO 17664-1:2021 — Information to be provided by the medical device manufacturer for the processing of medical devices (checked 25 October 2026)
- ISO 17665-1:2006 — Sterilization of health care products, moist heat, Part 1 (checked 25 October 2026)
- ISO 11140-1:2014 — Chemical indicators, Part 1: general requirements (checked 25 October 2026)
- ISO 11138-1:2017 — Biological indicators, Part 1: general requirements (checked 25 October 2026)
- ISO 11737-1:2018 — Microbiological methods, Part 1: bioburden (checked 25 October 2026)
- ISO 11737-2:2019 — Microbiological methods, Part 2: sterility testing (checked 25 October 2026)
- ISO 15223-1:2021 — Symbols to be used with medical device labels (checked 25 October 2026)
- ISO 13485:2016 — Medical devices, quality management systems (checked 25 October 2026)
- ISO 14971:2019 — Application of risk management to medical devices (checked 25 October 2026)
- ISO 7153-1:2016 — Surgical instruments, materials, Part 1: metals (checked 25 October 2026)
- ISO 15190:2020 — Medical laboratories, requirements for safety (checked 25 October 2026)
- Infection Prevention and Control — Public Health Ontario (checked 25 October 2026)
- Infection Prevention and Control — Royal College of Dental Surgeons of Ontario (checked 25 October 2026)
Printable companion: the compliance log centre carries cycle-record sheets with the temperature, hold time and A0 columns an inspector will look for.
Size the consumables that follow the washer, not just the machine: the sterilization usage calculator turns weekly instrument sets into pouch and indicator counts, the bulk quote form prices them by case, and a wholesale account covers multi-site ordering on purchase orders. Reselling? become a distributor.
Not sure how this applies to your own setup? Ask a sterilization compliance specialist — you’ll get a written answer specific to your equipment and province, with the regulation or standard cited.
Related Reading
- ISO 15883-1:2024 washer-disinfectors: validation and routine testing for Canadian facilities
- Benchtop ultrasonic cleaner vs a central washer-disinfector: when to upgrade
- Case review: a washer-disinfector validation run that failed
- Detergent residue and sterilization failure: the rinse step explained
Frequently Asked Questions
What does an A0 value actually mean?
A0 is a way of expressing a thermal disinfection exposure as one number: the equivalent time in seconds at 80 °C, calculated with a z value of 10 °C. A cycle that reaches 90 °C for one minute and a cycle that holds 80 °C for ten minutes both come out at A0 600, because the arithmetic credits the hotter cycle for its temperature. The point of the number is comparability, so two machines running different programmes can be judged against the same specification.
Which A0 value should a Canadian clinic ask for?
The value follows the device and its use, not the brand of machine. Instruments that will be sterilized afterwards are usually run at A0 600, which corresponds to 80 °C for ten minutes or 90 °C for one minute. Higher figures such as A0 3000 exist for situations where a greater margin is wanted, and lower figures such as A0 60 appear in specifications where the thermal step is not the main claim. The clinic's own device instructions and provincial guidance settle the question.
Does thermal disinfection replace sterilization?
No. Thermal disinfection reduces microbial load and is the step that makes instruments safe to handle during inspection, assembly and packaging. Sterilization, described for moist heat in ISO 17665-1:2006, is the terminal process that makes the pack sterile. A washer-disinfector cycle that reaches A0 600 is not a substitute for an autoclave cycle, and a clinic that treats it as one has removed the only step that carries a sterility assurance claim.
Is a printed cycle report enough proof?
A report shows what the machine's sensors measured, which is not the same as what the load experienced. Verification therefore combines the record with periodic validation using independent temperature measurement, and with the machine's calibration and maintenance history. When a clinic is asked to prove that a cycle worked, the answer is a record plus the evidence that the sensors still tell the truth.
Why do instruments come out wet if the machine is working?
Wet loads after thermal disinfection usually trace to load configuration rather than to the machine: instruments nested inside a bowl, tray mesh covered by a solid item, or a lumen that holds water because it was not positioned to drain. Water that stays on a hinged instrument during cooling finds its way into the joint. Loading for drainage is a technique, and it is the step most often learned by accident.
What water quality does the final rinse need?
The final rinse is the water that dries onto the instrument, so what is dissolved in it stays there. ISO 15883-1:2024 addresses the quality of the final rinse water because hardness, chloride content and conductivity all affect instruments and the disinfection claim. A clinic on hard municipal water that sees spotting or early corrosion should be looking at the rinse water before it blames the detergent.
How often should the washer be validated?
Validation is not a one-time event. The machine is validated when installed, revalidated after significant repair or relocation, and checked routinely through the records that each cycle produces. The routine record is the daily proof; validation is the periodic proof that the routine record means something. A clinic should be able to show both without searching a filing cabinet.
Where does ultrasonic cleaning fit if there is a washer?
Ultrasonic cleaning is the step that removes soil from surfaces that the washer's spray cannot reach reliably, which is why it usually precedes the washer rather than replacing it. Solution strength, temperature and bath condition all influence what arrives at the thermal stage, and a multi-enzyme ultrasonic concentrate is dosed against the bath volume rather than by eye.
Last updated: October 2026. CliniEco Medical is a licensed medical device establishment (MDEL #35334).
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