Author - fireservice@admin

During the past decade or so, the health and safety impact of firefighting equipment has been a significant driving force for fire related standards – including those developed by such noteworthy organizations as… the National Fire Protection Association (NFPA), the Canadian General Standards Board (CGSB) and many others. Consequently, there have also been substantial developments in the in the science, especially the research, associated with such impact.

There is, perhaps, no finer example of the profound and positive effects such research can have on our daily routines than in the changes we have seen in the firefighter turnout ensemble – commonly called our ‘bunker gear’.

This article is intended as a summary of one such research experiment, the “Field Evaluation of Protective Clothing Effects on Fire Fighter Physiology: Predictive Capability of Total Heat Loss Test,” commonly referred to as the ‘Indy’ report or study.

The Indy study was prepared by International Personnel Protection Inc. of Austin Texas for the Occupational Safety and Health Section of the International Association of Fire Fighters (IAFF) in Washington D.C., and submitted about December 1998. The goal of the study was to determine how the results from the total heat loss (THL) test could predict differences in firefighter physiology and subjective responses to physical exertion. Various protective clothing systems or ensembles (consisting of an outer shell material, thermal liner and moisture barrier) were tested.

The THL test is designed to evaluate the ability of a clothing system to allow heat to transfer to the outside environment during simulated work and under a specific set of environmental conditions. The test involves placing a material composite specimen over a porous heated plate meant to represent the human skin. Heat transfer is determined by measuring the energy required (by the plate) to maintain a specific temperature as heat is transferred through the clothing system to the outside environment. Both dry and wet tests are performed. The dry tests yield heat loss associated with conductive heat transfer. The wet tests yield heat loss associated with moisture evaporation and transmission. The two tests combined yield a total heat loss figure, which represents the amount of energy that can be transferred through a given area of composite material (clothing system) under the specific conditions of the test. The results are reported in Watts per square meter (W/m²). The THL test was mentioned in the 1991 edition of NFPA 1971 (in the appendix) as a method for measuring the stress-related effects of clothing.

The Indy study was initiated by the IAFF to address concerns raised by NFPA regarding which public input and criteria to use to determine acceptable standards. It’s likely that results obtained from this study will either be included or directly influence revisions of NFPA 1971, which may require a minimum heat loss of 205 W/m2. This has, in fact, been submitted to the NFPA by the IAFF.

Previous Studies
Several previous studies, involving laboratory, field and modeling tests were summarized in table format in the Indy study for quick review. These studies were able to demonstrate that a permeable moisture barrier resulted in less internal metabolic heat buildup. One study also suggested that improved ventilation (achieved through the design of the garment and/or the wearer’s activity?) helped to dissipate heat as well. Obviously, not wearing a SCBA resulted in less heat as it requires less work and provides improved ventilation. While one study found that the advantages of less heat buildup were only significant at low work levels, another suggested that lightweight outer shell material provided better evaporative cooling and better subjective ratings.

Indy Study Objectives

1. To use work protocols simulating actual fire fighting activity (i.e. a field study);
2. To determine how well total heat loss predicted physiological responses; and
3. To provide the basis for proposing a minimum total heat loss requirement for fire fighting protective clothing composites.

A field study more closely approximates the start/stop conditions of a fire fighter’s actual working conditions (as compared to a constant treadmill test in a lab). The Indy study assumes that if a relationship exists between total heat loss and the stress-related effects of clothing material systems on fire fighter physiology, it should be confirmed by statistical analysis. The intent of the Indy study is to use results to recommend total heat loss criteria for inclusion in revisions of NFPA 1971, thereby establishing a minimum acceptable performance for turnout ensembles.

The Indy Approach
The following approach was used:

1. The total heat loss test and other measurements were used to characterize selected material composites.
2. Work protocols were devised to simulate extrication and aerial crew (ladder company) activities with the assistance of the Indianapolis Fire Department and a manikin-based model.
3. The experiment was designed to isolate independent variables of the clothing systems tested.
4. The field study used 7 subjects, 7 ensembles and two work protocols (moderate at 280 watts and hard at 400 watts) for a total of 98 experiments gathering both physiological and subjective data collected throughout each experiment.
5. Data obtained from the field study was analyzed to determine the best correlations between fire fighter physiological/subjective responses and material or garment characteristics.

Indy Study Measurements
The following physiological measurements were made:

• core (rectal) temperature
• skin temperature at two locations (upper arm and thigh)
• heart rate
• nude weight loss
• clothing weight gain

Telemetric monitoring was used to provide real-time physiological monitoring of test subjects and increase test-subject safety. Telemetric monitoring employs 4 channels for input  (core, heart rate and two for skin) with a remote transmission up to 1000′. Data was recorded from all inputs every 9 seconds and recorded on a laptop computer.  Nude weights were collected before and after each experiment, and the weight change of each ensemble was taken before and after each work activity protocol.

Indy Work Protocols
Two work protocols were selected to simulate fireground work activity. These activities were selected as representative of either moderate or hard work. Moderate work was defined as requiring an expenditure of approximately 280 Watts. Hard work was defined as an expenditure of approximately 400 Watts.

The first work protocol simulated activities typical of removing a victim during an automobile extrication utilizing heavy tools. These work activities were performed for specific time intervals, and paced to assure that identical activities were completed by each subject and within the same time frames. This protocol consisted of a 30 minute work period followed by a 30 minute idle period. During the extrication, full PPE was worn, with the flash hood in the down position and no SCBA.

The second work protocol simulated work activities of an aerial crew responding to a structural fire. These work activities were sequenced into two groups of 15 minute work periods followed by 15 minute idle periods. Full PPE was worn, with the flash hood in the raised position and including SCBA.

All work protocols were performed in an indoor arena with a controlled atmosphere with an average temperature of about 20 degrees C, and a relative humidity of approximately 62%.

Work continued for the tests unless; the test subjects requested to stop; the test subject’s core temperature reached 38.5 degrees C with more than 1 minute remaining until the end of the exercise, or; the test subjects heart rate exceeded 180 beats per minute for more than three consecutive readings (27 seconds). It should be noted that three of the experiments were terminated early due to excessive core temperature and one due to excessive heart rate.

Subjective Ratings
The subjective ratings utilized a 7 point scale with descriptors provided for each number. This allowed respondents to attach a number to their perception of the ensembles worn. These subjective ratings were divided into two groups, comfort and ergonomic. Comfort factors included overall wearing comfort, heat sensation, moistness, heaviness, flexibility and fatigue. Ergonomic factors included upper and lower body mobility, arm and leg mobility, donning and doffing ease and fit.

Core Temperature
A major emphasis was placed on the measurement of core temperature during the Indy study. Core temperature rise was used for making comparisons between ensemble material systems and between individuals. This method allows for a comparison of different ensembles and rank ordering based on both the maximum core temperature rise and ending core temperature rise (at the end of the complete one hour test period). Significant (statistically) differences between ending core temperature rise were recorded at the end of the idle period with a spread of 0.57 degrees C.

The highest ending core temperature rises are attributed to low total heat loss material systems. The aerial crew simulation showed higher ending core temperatures for all test subjects. However, the spread in ending core temperature was less between the poorest and best performing ensembles (0.53 degrees C). The wider discrimination in core temperature response that appeared during the idle period shows that there may be a time lag of approximately 10 minutes before the impact of work stimulus shows up as a core temperature response for these levels of work.

Statistical Analysis
The statistical analysis was performed to achieve the following objectives:

1. To determine, for each simulation, if statistically significant differences existed between the core temperature rise for each exercise and the type of ensemble.
2. To establish which variable (clothing or material system characteristics) could best explain physiological and subjective responses.
3. To determine the strength of the relationships between physiological/subjective responses and clothing/material system characteristics.

This analysis was intended to show whether the observed differences between average ensemble and final core temperature rise are statistically significant as based on the individual fire fighter responses. The final analyses suggest that the aerial crew simulation produced good discrimination between ensembles based on ending core temperature rise. The statistical analyses confirmed total heat loss as the most primary independent variable for predicting physiological responses. Other than the sensation of moisture, the subjective responses were primarily best explained by garment weight.

Physiological Bases
The range of temperature rise was limited in this study to 1.5 degrees C. This limit was chosen because the approximate average starting core temperature was 37 degrees C while the maximum core temperature permitted by the safety protocol was 38.5 degrees C. Many researchers agree that the onset of serious heat stress symptoms begin to occur at a temperature of about 39 degrees C (depending on the individual), with disorder of the human body thermoregulation at 41 degrees C.

Basis for Recommended Requirement
Several factors are considered in basing a recommended requirement, including:

1. The requirement should have some physiological basis for addressing the safety of the fire fighter.
2. The requirement should provide a physiological benefit to the fire fighter.
3. The requirement should encourage reduction of clothing stress by providing a realistic target that industry can attain.

Each of these was considered in the recommendation of a specific performance requirement based on material system total heat loss.

Performance Requirements Suggested By the Data
An analysis of the data for an inverse or hyperbolic function was able to demonstrate that optimum benefit, using the material system total heat loss, would occur at a value of 205 W/m2. This proposed minimum requirement considers the physiological predictive capability of total heat loss and is both achievable and reasonable for the protective clothing industry. However, and most importantly, the proposed requirement provides an appropriate benchmark for improving fire fighter health and safety.

Conclusions
Clearly the Indy study suggests that the fire service must begin to consider protective clothing as a total system and not just as a collection of individual components – each meeting their own specific testing requirements as listed in the applicable standards. It further suggests that total heat loss is an essential criterion that must be considered and taken into account when developing standards for protective clothing. It also concludes that its results are statistically validated and that it is sound and reasonable to recommend to NFPA that a minimum total heat loss criteria of 205 W/m2 be included in revisions of NFPA 1971.

Questions to Consider
Although the methods used and the corresponding results appear scientifically sound, as a layperson I find myself seeking additional information. The relatively small temperature differences achieved by six of the seven ensembles tested (the seventh ensemble was not NFPA compliant) makes me question how practically significant these results actually are. The section on physiological bases, discussed above, would suggest that very slight increments in core body temperature can lead to varying degrees of heat-related illnesses and must be considered as a key factor affecting fire fighter health and safety. Although I accept this, I also believe that the fire service very much needs to continue to study the effects of total metabolic heat loss and to consult with other scientists and experts.

David Ross is a member of the Editorial advisory Team and may be contacted at FSJ at 905-428-8465 of fax 905-683-9572 or email at fire@interlog.com.
© 2000 The Fire Services Journal Inc. no portion of this article may be copied without prior written permission of the publisher.