Why are TPP values changing?

For nearly four decades, thermal protective performance (TPP) has been one of the most recognizable numbers associated with firefighter protective clothing. It appears in certification data, procurement specifications and product literature, often interpreted as a shorthand indicator of thermal insulation against extreme heat exposure. Yet recently, many in the fire service and protective clothing community have noticed something unexpected: TPP values for otherwise familiar material composites appear to be declining.

This trend is beginning to create understandable confusion. Departments may wonder whether their gear has become less protective. Manufacturers may struggle to explain certification outcomes that differ from past results. Individuals with organizations that specify gear may question whether long-standing purchasing criteria remain valid.

The short answer is that the observed changes appear to be largely rooted in measurement methodology, though the effect of individual materials or material technologies has not yet been thoroughly researched. Understanding why requires stepping back to examine what TPP represents, how it is measured and how recent standards evolution has probably influenced reported values.

What TPP measures — and what it doesn’t

TPP is fundamentally a laboratory-derived metric that quantifies the thermal insulation provided by a protective clothing composite when exposed to intense convective and radiant heat. In the standard test, a specimen of layered material — outer shell, moisture barrier and thermal barrier — is placed above a heat source delivering a combined exposure designed to simulate flashover-level thermal energy. Heat transfer through the specimen is measured using a copper calorimeter, and the time required to reach a second-degree burn prediction threshold is multiplied by the incident heat flux to yield the TPP rating.

Higher numbers indicate greater resistance to heat transfer, while lower numbers reflect faster heat penetration. The test exposure is intentionally severe and is not meant to represent normal working conditions but rather the limited time available for escape from extreme events. The results of the test are further not intended to infer a level of “safe” time for firefighters to be exposed to emergency fireground conditions.

Since its introduction in NFPA 1971 in the 1980s, the minimum required value for turnout garment composites has remained essentially unchanged. Yet this apparent stability has masked an important reality: The number itself depends on how the measurement is conducted. TPP is not a fundamental material constant; it is an outcome shaped by instrumentation, calibration and procedural assumptions.

The emerging pattern of lower values

Following the transition from NFPA 1971 (2018 edition) to the lead up of products being certified to the new 2025 edition of NFPA 1970, testing across hundreds of composite submissions showed average reductions in reported TPP values on the order of 2–3 cal/cm².

This data presented by a certification organization similarly demonstrated a shift, with average measured values decreasing from about 40.4 to 37.4 across a large set of composites — a difference of roughly 3 points when comparing results produced using different editions of the referenced ISO method.

This decline has practical consequences. Material systems previously positioned just above the minimum compliance threshold may no longer qualify under current testing conditions, even though no physical change has occurred in the composite itself. Approximately 15–20% of composites previously certified were found not to meet the minimum requirement under updated evaluation conditions.

Importantly, other performance attributes — such as total heat loss (THL) that is also conducted on the same three-layer garment composites — did not show corresponding degradation, reinforcing the likelihood that measurement evolution rather than material deterioration explains much of the observed shift.

Photo/International Personnel Protection, Inc.

Methodological evolution: The primary driver

The most significant factor behind changing TPP values lies in updates to the referenced test method, specifically the transition from two edition of the same International Standards Organization (ISO) method: ISO 17492:2003 to ISO 17492:2019. This test method establishes the equipment design, the setup of the testing and how samples are placed, and provided detailed procedures for calibrating the test apparatus and running the test. NFPA has used this standard since 2007 (4 editions ago).

Equipment configuration

Several differences in burner orientation, burner surface design, infrared radiation source type, and thermocouple configuration have been identified between the earlier and updated methodologies. These differences alter how heat is delivered and measured at the specimen surface. It was further learned that even seemingly small variations — such as burner angle ranges or mesh geometry — can influence convective heat distribution. A key part of the equipment, the sensor used for measuring heat transfer through the material, changed. For TPP testing, the sensor is a copper calorimeter; it moved from using averaged readings from multiple thermocouples to single-thermocouple measurement. This change has been shown to introduce differences in response characteristics and affect signal interpretation.

Data acquisition and sensitivity

Measurement frequency and sensor resolution have also improved. The newer approach incorporates more frequent sampling and tighter temperature accuracy tolerance, generally considered an improvement. These changes can affect the recorded rate of temperature rise at the calorimeter, which directly determines calculated burn prediction time. In practice, increased precision may yield more conservative results, lowering the resulting TPP value without any change in protective capability.

Calibration and flux determination

Calibration procedures have likewise evolved, including different methods for achieving the prescribed combined heat flux and confirming exposure conditions. Compounding this issue, an identified error within ISO 17492:2019 may lead to incorrect flux calculations depending on interpretation of temperature-rise parameters.

Additionally, laboratories discovered that certain equipment components required modification or adjustment to meet the updated specifications, including sample holders, thermocouple wiring and burner alignment.

These findings underscore a critical point: When measurement infrastructure changes, the resulting performance values cannot be assumed to align numerically with legacy data sets.

Are materials also changing?

While method-based factors dominate the explanation, the possibility of material evolution contributing to lower values cannot be dismissed. Material suppliers have made changes in some of their materials for a variety of reasons, including new requirements in the NFPA 1970 standard. Moreover, protective clothing systems continue to evolve in response to heat stress concerns, contamination issues, durability demands and regulatory pressures.

However, as of January 2025, no systematic study has demonstrated a clear correlation between specific material changes — such as outer shell finishes — and the observed downward trend in composite TPP results. It is important to point out that in some cases, new materials have entered the market for which there are no precedent products. Nevertheless, the current lack of evidence suggests caution in attributing the change to material degradation or design shifts without further data.

Broader implications for the fire service

There are several broader implications for the fire service here.

• Procurement specifications: Many departments historically incorporated minimum TPP thresholds into purchasing requirements. Because current measurement conditions yield lower numbers, these specifications may unintentionally exclude otherwise acceptable gear material options. This could be significant, particularly if the dropped composite had better breathability (remember no drops in total heat loss) that now available compliant composites. Departments satisfied with current performance may need to revisit language to ensure requirements reflect contemporary measurement realities rather than legacy numeric expectations.
• Interpretation of protection: A reduced TPP number does not automatically mean reduced protection. It may simply reflect a different measurement baseline. Without understanding test context, comparisons between historical and current values risk misinterpretation, particularly when communicating with line personnel or municipal decision-makers.

• Standards communication: One challenge highlighted by this recent experience is the difficulty of communicating method-based impacts following major standards revisions. For this reason, there is activity within the committee responsible for the NFPA standards that use TPP testing to thoroughly investigate the observed trend. There is also a proposed amendment to the NFPA 1970 standard that will alert departments for the possibility of changed TPP values.

  This issue illustrates a broader lesson: When test references change, proactive outreach is essential to ensure consistent understanding across the fire service community.

The continuing role of TPP

Despite its limitations, TPP remains an important comparative tool when properly interpreted. It provides a standardized means of evaluating thermal insulation within a defined exposure framework and has contributed substantially to improvements in firefighter protective clothing design and performance.

However, the current experience reinforces a fundamental truth familiar to anyone working with protective equipment evaluation: Performance metrics are only as stable as the measurement systems behind them. Values should always be considered within the context of test configuration, calibration and procedural assumptions. Numerical shifts may reflect measurement evolution rather than changes in protective capability.

Moving forward

The changing TPP landscape offers an opportunity for reflection and education. For researchers, it highlights the importance of measurement harmonization and validation when standards evolve. For manufacturers, it underscores the need to communicate clearly about certification transitions. For departments and individuals that specify gear, it reinforces the value of understanding what performance numbers actually represent.

Ultimately, the question is not whether TPP values are changing — they are. The more meaningful question is why.

The answer may lie largely in how we measure thermal protection, not in what firefighters wear, even though new information could show impact from changes in some material technologies as a further contributing factor.

Recognizing this distinction ensures that the fire service continues to make informed decisions grounded in technical reality rather than numeric perception — a perspective essential to maintaining both trust and safety as protective clothing standards continue to evolve.

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