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Short answer
A chemical indicator is a heat- or chemical-sensitive material that changes appearance when a sterilization process reaches the conditions it was built to detect. It shows that an item was exposed to the process. It does not show that the item is sterile. ISO 11140-1 sorts chemical indicators into six types, and the types are descriptions of intended use, not a ranking.
Definition and scope
The CDC’s Guideline for Disinfection and Sterilization in Healthcare Facilities describes chemical indicators as usually heat- or chemical-sensitive inks that change colour when one or more sterilization parameters are present. In the CDC’s words they are convenient, inexpensive, and indicate that the item has been exposed to the sterilization process.
The same word covers products people search for under several names: a sterilization strip, sterilization strips, autoclave tape, an integrator, or simply an indicator of sterilization. They are all chemical indicators; what separates them is which process variables they respond to (see the types below).
This page covers chemical indicators as classified for steam sterilization in ISO 11140-1:2014, as summarised by STERIS Life Sciences. It does not cover how to run a cycle or release a load.
How chemical indicators are used in practice
External and internal placement
The CDC states that chemical indicators are affixed on the outside of each pack to show that the package has been processed through a sterilization cycle, and that external indicators do not prove sterilization has been achieved. It adds that, preferably, a chemical indicator also should be placed on the inside of each pack to verify sterilant penetration. For the packaging side of this, see sterilization pouches.
When an indicator suggests inadequate processing
According to the CDC, if the internal or external indicator suggests inadequate processing, the item should not be used.
The six ISO 11140-1 types
ISO 11140-1:2014, section 4.1, as quoted by STERIS, says the classification structure is used solely to denote the characteristics and intended use of each type of indicator when used as defined by the manufacturer, and that it has no hierarchical significance. Older documents, and the CDC guideline text, speak of numbered classes (the CDC groups chemical indicators into five classes); ISO 11140-1 now uses the word types and lists six.
| Type | What it responds to | Example given by STERIS |
|---|---|---|
| 1 Process | One or more critical variables; shows the unit has been directly exposed to the sterilization process | Autoclave tape or indicator strips, inside or on the outside of packs |
| 2 Specific-use | Defined in the relevant sterilization standards for a specific test procedure | Bowie-Dick test pack, which tests air removal in a prevacuum steam cycle |
| 3 Single-variable | One critical variable, at a stated value | A chemical pellet that melts at a specified temperature |
| 4 Multi-variable | Two or more critical variables, at stated values | Indicator tubes that change colour only at a given temperature held for a specified time |
| 5 Integrating | All critical variables; stated values meet or exceed the performance requirements of the ISO 11138 series for biological indicators | Moving-front indicators |
| 6 Emulating (cycle verification) | All critical variables for specified sterilization cycles; stated values are generated from the critical variables of the specified process | Cycle-specific indicators, typically placed in the pack, pouch or container |
STERIS also notes, quoting the standard, that a chemical indicator should comply with only one type. A Type 5 integrating indicator is therefore not a “better Type 1”: it answers a different question, namely whether the stated values for all critical variables were reached at the indicator’s position.
Chemical indicators compared with biological indicators
This is the comparison that most often gets blurred. The CDC states that biological indicators are the only process indicators that directly monitor the lethality of a given sterilization process, because they use resistant spores. It says chemical indicators should be used in conjunction with biological indicators and, based on current studies, should not replace them. It also reports a study in which chemical indicators were more likely than biological indicators to inaccurately indicate sterilization at marginal sterilization times (for example 2 minutes).
| Chemical indicator | Biological indicator | |
|---|---|---|
| What it is (CDC) | Heat- or chemical-sensitive ink or material that changes colour | Resistant spores |
| What it shows (CDC) | The item was exposed to the sterilization process | Directly monitors the lethality of the process |
| Role (CDC) | Used together with biological indicators; does not replace them | The only process indicator that directly monitors lethality |
The Type 5 definition ties the two together: its stated values are set against the ISO 11138 performance requirements for biological indicators. That is a statement about how the indicator is specified, not a claim that it replaces a spore test. For the spore-test side, see CASRAI’s guide to autoclave validation with biological and chemical indicators.
Where it matters for labs and clinics
- Reading an indicator result. A colour change on tape or a strip documents exposure only. The CDC says external indicators do not prove sterilization has been achieved.
- Reading product literature. Product pages often say “class” where the current standard says “type”. Under ISO 11140-1 the type describes which variables an indicator responds to and at what stated values.
- Specific tests. A Type 2 indicator belongs to a defined test procedure such as the Bowie-Dick air-removal test in prevacuum cycles, so it has a different job from an indicator placed in a routine pack. Cycle design is covered in autoclave cycle types.
- Standards context. Healthcare sterilization practice is also described in ANSI/AAMI ST79, and the wider assurance framework in sterility assurance level.
Common misunderstandings
“A sterilization strip proves the item is sterile”
It does not. A strip or tape shows exposure to the process; the CDC is explicit that external indicators do not prove sterilization has been achieved.
“A higher type number is a better indicator”
ISO 11140-1 states the classification has no hierarchical significance. Each type denotes characteristics and intended use.
“A chemical indicator can replace a biological indicator of sterilization”
The CDC says chemical indicators should be used in conjunction with biological indicators and should not replace them.
“Class 1 to 6 and Type 1 to 6 must be unrelated”
Older documents say classes; ISO 11140-1 now says types and lists six. The CDC guideline text still uses the word classes and groups chemical indicators into five.
“An indicator is the same thing as a validation”
An indicator is one monitoring tool. Validating a sterilization process is a broader exercise; see sterilization validation.
Frequently asked questions
What do chemical indicators show in sterilization?
They are usually heat- or chemical-sensitive inks that change colour, and they indicate that the item has been exposed to the sterilization process (CDC). They do not by themselves prove sterility.
Is a sterilization strip the same as a biological indicator of sterilization?
No. A strip is a chemical indicator. A biological indicator uses resistant spores and, according to the CDC, is the only process indicator that directly monitors the lethality of a given sterilization process.
What is the difference between internal and external chemical indicators?
External indicators are affixed on the outside of each pack to show it has been processed; the CDC says they do not prove sterilization has been achieved. Preferably an indicator is also placed on the inside of each pack to verify sterilant penetration.
How many types of chemical indicator does ISO 11140-1 list?
Six: Process, Specific-use, Single-variable, Multi-variable, Integrating and Emulating, as summarised in the table above.
Does a higher indicator type mean a better indicator?
No. ISO 11140-1, as quoted by STERIS, says the classification has no hierarchical significance.
Sources
- CDC, Guideline for Disinfection and Sterilization in Healthcare Facilities, “Sterilizing Practices”: https://www.cdc.gov/infection-control/hcp/disinfection-sterilization/sterilizing-practices.html (checked 2026-10-29).
- STERIS Life Sciences, “6 ISO Types of Chemical Indicators for Steam Sterilization” (summarises and quotes ISO 11140-1:2014): https://amer.sterislifesciences.com/Resources/Technical-Learning-Library/Technical-Learning-Library-Folder/Articles/6-ISO-Types-of-Chemical-Indicators-for-Steam-Sterilization (checked 2026-10-29).
