By Jeff and Grace Stull
The ability of firefighter protective clothing to shield the body from extreme heat is one of the most critical aspects of fire service occupational safety. Despite decades of advances in materials, garment design and testing standards, burn injuries remain a risk during structural firefighting.
One concept that emerged decades ago from laboratory evaluations is “alarm time” — the window between the onset of pain (time-to-pain) and the threshold for a second-degree burn (time-to-burn). Alarm time is not just a laboratory metric; it has profound implications for how much reaction time firefighters have when exposed to heat.
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Here we’ll explore alarm time in detail, drawing on prior industry research and observations related to burn injuries arising from the investigation of firefighter burn injuries in both ordinary fires and those considered “near misses.” It also encompassed a review of how alarm time plays a role when affected by fireground conditions such as wet gear and the impact of compression for reinforced garment areas. And we’ll conclude with some implications for firefighters on the fireground.
Defining alarm time
In heat transfer testing of protective clothing, three threshold points are typically measured:
- Time-to-pain: The moment when heat transfer through the clothing raises simulated skin temperature to approximately 44 degrees C, corresponding to the sensation of pain.
- Time-to-burn: The moment when simulated skin temperature reaches approximately 55 degrees C, associated with onset of blistering or a second-degree burn.
- Alarm time: The interval between these two thresholds. It represents the critical “escape window” — how long a firefighter has after feeling pain before sustaining irreversible injury.
Alarm time is valuable because it provides a measure of reaction margin. Clothing that produces longer alarm times allows firefighters to adjust position, withdraw from high-heat areas, or call for assistance before injury occurs.
Historical context and testing methods
Since the late 1970s, thermal protective performance (TPP) testing has served as the benchmark for evaluating firefighter gear. TPP exposes a fabric composite to a combined radiant and convective heat flux of 2 cal/cm²·s, representing flashover or backdraft conditions. The test records time-to-second degree burn and sets NFPA 1971’s minimum TPP rating of 35. However, this approach does not explicitly measure alarm time. It was not until after its formal introduction in the mid-1980s that later research began emphasizing the difference between tolerance time and blister time.
Other test methods have since complemented TPP:
- ASTM F1060 (conductive heat resistance) simulates contact with hot surfaces under compression. This test method requires the measurement of both the time to pain and time to a second-degree burn. The test is most commonly applied to gloves, but a version of the test is also used in conductive and compressive heat resistance (CCHR) testing minimally performed on the reinforced areas of shoulders and knees of firefighter garments. In that application, only the time to second-degree burn injury is reported.
- ASTM F1939 (radiant protective performance, RPP) simulates radiant-only exposures from flame or hot surfaces. The test uses TPP-like metrics (i.e., the report of time-to-second degree burn and provides an overall RPP rating). It is not used for structural gear but is instead used for aluminized proximity protective clothing and wildland garments, though with very different exposure heat fluxes. (Note: A much lower radiant heat load is applied for wildland firefighting garments.)
- ASTM F2371 (transmitted and stored heat energy) uses an extended, but relatively low radiant exposure to measure heat transfer through a clothing composite, which is then subjected to a rapid compression for assessing how energy stored from the initial transmitted heat can then pass through to the sensor. As with the above tests, the principal measurement of this test is the time to second-degree burn injury.
These tests capture performance differences not revealed by TPP alone. While alarm time is generally not measured, when calculated, it offers insight into how usable the protective margin is under realistic exposures.
Moisture and alarm time
Moisture plays a complex role in thermal insulation. Firefighters’ gear routinely becomes wet, whether through sweat, hose spray or steam. Traditional assumptions suggested that moisture always decreases insulation by bridging fabric layers and facilitating heat transfer. Yet decades of research show that water can sometimes extend protection by acting as a heat sink, depending on exposure type. For example:
- In radiant exposures, modest moisture often reduced alarm times because absorbed water rapidly transferred heat to underlying layers.
- In some conductive or convective exposures, moisture increased alarm times because added water mass delayed heat rise.
One study cited in the moisture review showed glove materials sometimes exhibited longer alarm times when wet while at other times performance decreased. The deciding factors included the amount of water, whether the shell or liner was saturated, and whether compression was applied. In many instances, moisture in the lining next the skin has been shown to be the worst case when it comes to comparing the time and rapidity for the onset of burn injury, often because of a reduced alarm time.
These variations mean that firefighters cannot assume wet gear is always better or worse; it depends heavily on exposure conditions. On the other hand, there is value in the clothing staying dry because its performance becomes predictably more consistent. Hence, manufacturers often select materials that lessen the amount of moisture that can be absorbed.
Reinforced knees and localized protection
Our research review focused on reinforced knee areas, an often-overlooked aspect of turnout clothing. Knees face both conductive heat transfer (from kneeling on hot or water-soaked surfaces) and radiant heat (from facing flame fronts).
In work from the late 1990s, significant studies of various reinforcement configurations evaluated using modified ASTM F1060 and F1939 protocols revealed findings that helped influence the development of the current CCHR test, required for reinforced layers of the knee and shoulders of firefighter protective clothing. These findings highlighted several key points:
- Material composition matters: Leather reinforcements performed better under wet conditions, while Hypalon-coated reinforcements performed better when dry. This underscored that alarm times are material-dependent.
- Wet vs. dry effects: At high radiant flux (84 kW/m², the same heat level used in TPP testing), wet conditions often extended alarm times due to water’s heat-sink effect. At lower flux (8.4 kW/m²), dry conditions usually produced longer alarm times.
- Repeated use and laundering: With repeated laundering and exposures, alarm times tended to shrink, sometimes to as little as 12 seconds. This reduction reflects permanent material degradation from heat and wear.
The knee reinforcement study underscores that alarm time is not static. It changes as gear ages, absorbs water or faces different heat exposure intensities.
Alarm time as a safety metric
From both areas of study, several conclusions about alarm time emerged:
- Alarm time is dynamic: It varies with exposure intensity, moisture levels, compression and garment condition.
- Longer is not always better: While a long alarm time provides more reaction margin, in some conditions, moisture extended alarm time by lowering initial pain onset but still led to faster ultimate heat transfer. Firefighters may not recognize this hidden danger.
- Localized reinforcements complicate performance: Added layers may change alarm times unpredictably. Some reinforcements degraded under repeated laundering, which reduced alarm time despite greater thickness.
- Testing gaps remain: NFPA 1971 does not mandate reporting alarm time, even though it arguably reflects real-world safety more directly than TPP values alone.
Practical implications for firefighters
For the firefighter in the field, alarm time translates into reaction time under thermal heat exposure stress. Key lessons include:
- Listen to the body: The sensation of heat or pain is a real warning. Firefighters should be trained to interpret these sensations as cues to reposition or retreat.
- Moisture management: Wet gear may feel cooler initially but can suddenly transfer stored heat. Firefighters should avoid assuming wet gear is always protective. It is believed that this sentiment is better appreciated as many manufacturers promote gear that is capable of distributing moisture more easily and thus stay drier.
- Gear maintenance matters: Repeated washing and wear degrade insulation, shortening alarm time. Departments should inspect reinforced areas (knees, shoulders, elbows) closely over the gear’s service life. The replacement of knee reinforcements should be considered as clothing ages with use.
- Design tradeoffs: Lightweight gear may meet TPP minimums but offer shorter alarm times, while heavier reinforcements may add mass but not always extend alarm margins. Balance is critical.
Toward better standards and testing
The above information suggests that future protective clothing standards should integrate alarm time as a performance measure. Alarm time reflects not only material insulation but also realistic firefighter survivability. Including it would encourage manufacturers to design clothing that maximizes reaction margins under multiple exposure conditions.
Further protective clothing testing should better account for the following:
- Wet versus dry performance
- Compression and kneeling
- Repeated laundering and heat cycling
New sensor technologies may allow more precise simulation of skin responses over longer exposures, addressing current limitations with copper calorimeters on which most existing test methods are based.
Final thoughts
Alarm time represents one of the most meaningful measures of firefighter protection. The research shows it is influenced by material choices, reinforcement designs, exposure intensity, moisture levels and garment wear. While NFPA standards have focused on TPP ratings and minimum burn thresholds, alarm time deserves equal consideration because it aligns more closely with real-world firefighter decision-making.
By understanding and prioritizing alarm time, the fire service can move closer to protective clothing that not only withstands heat but also gives firefighters the crucial seconds they need to save themselves and others. As one study demonstrated, alarm time in reinforced knees shrank to as little as 12 seconds under repeated use. Those 12 seconds may be the difference between a near miss and a career-ending injury.
Note: The views of the author do not necessarily reflect those of the sponsor.
REFERENCES:
- Stull, J. O. (2000). “The effect of moisture on firefighter protective clothing thermal insulation: a review of industry research.” ASTM Special Technical Publication, 1386, 557-576.
- Stull, J. O. (2000). “Comparative thermal insulative performance of reinforced knee areas of firefighter protective clothing.” ASTM Special Technical Publication, 1386, 312-328.
ABOUT THE AUTHOR
Jeffrey and Grace Stull are president and vice president, respectively, of International Personnel Protection, Inc. They are members of several NFPA committees on PPE as well as the ASTM International committee on protective clothing. Mr. Stull was formerly the convener for international work groups on heat/thermal protection and hazardous materials PPE as well as the lead U.S. delegate for International Standards Organization Technical Committee 94/Subcommittees on Protective Clothing and Firefighter PPE. They participate in the Interagency Board for Equipment Standardization and Interoperability and have authored the book, “PPE Made Easy.” Send questions or feedback to the Stulls via email.