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.
