Author: Yeh-Liang Hsu, Chi-Yu Tai, Ting-Chin Chen
(1999-09-03); last updated: Yeh-Liang Hsu (2001-02-16);
recommended: Yeh-Liang Hsu (2000-04-19).
Note: This paper is published the Journal of Industrial Ergonomics,
2000, Vol. 26, pp.109-117.
Improving thermal properties of
industrial safety helmets
This paper presents
the redesign of an industrial safety helmet shell to improve its thermal
properties. First an experiment was established to simulate the conditions of a
head wearing a helmet. The average temperature beneath the helmet shell, the
speed of heat dissipation through convection, and the temperature contour
beneath the helmet shell were used to describe the thermal properties of a
helmet. Helmets of different types and makes were tested, and various design
concepts were examined. Design suggestions for improving thermal properties of
industrial safety helmets were made. According to these design suggestions, a
new design prototype was developed, tested, and further modified. The thermal properties
of our final helmet shell design have improved over the commercially available
helmets we tested. It also passed the weight, impact, and penetration tests of
the Chinese National Standards. Finally a subjective human evaluation showed
that the new helmet design has significantly better thermal properties than
existing commercial helmets.
Relevance to industry
helmets provide head protection for industrial workers. Thermal discomfort is
one of the major reasons that industrial workers do not like to wear safety
helmets. This paper presents and verifies ways on improving thermal properties
of industrial safety helmets.
Keywords: Industrial safety helmet; thermal properties.
helmets provide head protection against small falling objects striking the top
of the shell in industrial environments. Most of the tests in the standards
(CNS, 1992; ANSI, 1986; ISO, 1977; BSI, 1987) that apply to industrial safety
helmets are to measure whether a helmet can provide effective protection to the
wearers. Considerable amount of research has also been done on the performance
in impact and shock absorbing ability of the safety helmet (Mill and Gilchrist, 1987, 1993; Rowland and Keighley, 1988;
Rowland, 1987). However, many workers are not willing to wear helmets simply
because they are not comfortable. The “comfort performance,” which may often be
overlooked, is obviously another important issue for industrial safety helmet
discussed 12 factors that affect the acceptability of head protection at work:
weather protection, thermal properties, tactile properties, absorptivity/ permeability,
mass distribution, degree of fit, size and shape, retention performance and
fit, helmet volume, visual factors, speech and sound factors, and helmet
compatibility. Among these 12 factors, the one of importance for the workers in
a tropical or subtropical climate, such as that of Taiwan, are the thermal properties
of industrial safety helmets. Liu and Holmer (1995) evaluated experimentally
the evaporative heat transfer characteristics of industrial safety helmets. The
effects of ambient humidity, solar radiation, and wind on heat loss were
This paper presents
a project on how to redesign the shell of the industrial safety helmet to
improve its thermal properties. In this project, first an experiment was
established to simulate the conditions of a head wearing a helmet. Helmets of
different types and makes were then tested. The average temperature beneath the
helmet shell, the speed of heat dissipation through convection, and the
temperature contour beneath the helmet shell were used to describe the thermal
properties of a helmet. Various design concepts, e.g., adding ventilation
holes, increasing clearance between the helmet shell and the head, covering the
shell with reflective materials, were also tested. Finally some design
suggestions for improving the thermal properties of industrial safety helmet
shells were made.
According to these
design suggestions, a new design prototype was developed, tested, and further
modified. At this stage, attentions were also paid to structural safety,
appearance, manufacturability, and other practical issues. Finally we developed
our final helmet shell design. The thermal properties of this final design have
significantly improved over other commercially available helmets we tested. It
also passed the weight, impact, and penetration tests of the Chinese National
Standards (CNS). A human description evaluation showed that the new helmet
design has significantly better thermal properties than existing commercial
2. An experiment to measure the thermal properties of a
Heat may be gained
or lost from the head wearing a helmet by conduction, convection, and radiation.
Conduction to the air is typically low, while convection and radiant heat
transfer can be impeded by the presence of the helmet. There can be three heat
sources that affect the temperature beneath the helmet shell: radiation from
the sun, body heat from the wearer's head, and heat exchange with the
surrounding air through convection.
Figure 1 shows an
experiment designed to simulate the conditions of a head wearing a helmet. The
helmet being tested was resting on a headform. The headform was made of rubber
and covered with hair. A set of four halogen lamps were positioned above the
headform, so as to evenly distribute their radiant heat over the whole test
region. A heat source (a light bulb) was put inside the headform to simulate
body heat from the head. The temperature of the headform was controlled to
maintain at body temperature 37.6℃.
Figure 1. An experiment to simulate the conditions
when wearing a helmet.
environment was surrounded by electric heating coils for controlling the
temperature of the surrounding air. According to the Central Weather Bureau of
Taiwan, the average temperatures of Taiwan during summer months range from 26.8℃
to 28.3℃, and the highest temperatures of the
summer months range from 31.5℃ to 33.3℃.
Therefore the temperature of the test region was set at 30.0℃.
The halogen lamps and electric heating coils were controlled through a
temperature controller, so that the temperature of the test environment was
maintained at 30.0℃. Also according to the Central
Weather Bureau of Taiwan, the average wind speed in Taiwan is about 2.0-2.5 m/sec. An electrical fan was
adjusted to provide a constant airflow at 2.5 m/sec.
The room temperature outside the test region was kept at about 25℃.
In summary, the
standard test conditions of our experiment simulated a worker wearing a helmet
in a 30.0℃ sunny day, with wind blowing at 2.5 m/sec.
3. Describing thermal properties of a helmet
To the extent of
the authors' knowledge, there has not been a universally agreed index that
formally defines the thermal properties of industrial safety helmets.
Roszkowski (1980) compared the thermal properties of 10 industrial helmets on a
resting subject. The results obtained were expressed in terms of dry bulb
temperature and relative humidity beneath the helmet. Liu and Holmer (1995)
measured the heat loss from a wet head manikin to evaporative heat transfer
characteristics of industrial safety helmets. In their experiment, heat loss
was directly determined as the power required to maintained the target skin
temperature during the last 20 minutes in each 1 hour test for each helmet.
In our project, the
major concern is the temperature inside the helmet. We found that in sunny
weather with no wind, the temperature inside the helmet often rises over 50℃, which is simply unbearable. Therefore in our experiment, the
average temperatures beneath the helmet shell, the speed of heat dissipation
through convection, and the temperature contour beneath the helmet shell, were
used to describe the thermal properties of the helmets. The relative humidity
beneath the helmet was not used because we did not simulate sweating of the
head in our experiment. The relative humidity is directly related to the
average temperature beneath the helmet shell.
A total of 16 thermocouples
were attached to the inside of the helmet shell in a rectangular pattern, as
shown in Figure 2, to measure the temperatures beneath the helmet shell. The
average temperature was the average of temperatures taken from all 16
thermocouples when a steady state condition was reached. Repeatability of
measuring the average temperature was also examined. The standard deviation of
the average temperature measurement was 0.38℃.
Figure 2. The arrangement of the 16 thermocouples.
The distance between two neighboring thermocouples is 5 cm.
To measure the
speed of heat dissipation with convection, the average temperature was measured
with the electric fan turned off. Then the electric fan was turned on and the
time required for the helmet to reach a new steady state temperature was
recorded. This time was used to describe the speed of heat dissipation inside
Finally the 16
temperatures taken from the thermocouples were further processed to draw a
temperature contour plot. From the temperature contour plot, we can observe
whether the temperature inside helmet is evenly distributed and whether any
“hot spots” exist beneath the helmet shell.
4. Design suggestions
different types and makes were then tested. Figure 3(a) shows the test data of
a typical commercially available helmet. When all three heat sources were
turned on, the final average temperature beneath the helmet shell was 38.3℃.
When the halogen lamps were turned off (no radiant heat), but the test region
was still kept at 30.0℃ by the heat provided by electric
heating coils, the average temperature beneath the helmet shell dropped to 31.0℃.
This temperature was just slightly above the temperature of the test region. On
the other hand, when the electric heating coils were turned off, and the test
region was maintained at 30.0℃ by the heat of the halogen lamps,
the average temperature rose to 41.0℃. When the
heat source inside the headform was turned off (no body heat), the average
temperature dropped slightly to 37.2℃. Finally,
when all three heat sources were on, but the electric fan was turned off (no
convection), the final average temperature rose rapidly to 50.1℃.
From the test
results above, we can conclude that radiant heat is the major heat source when
wearing a helmet, and the “green house” effect will cause the temperature
beneath the helmet shell rise rapidly. Convection is the primary way to
dissipate heat from beneath the helmet shell. Body heat only has a marginal
effect on the temperature beneath the helmet shell. Therefore to improve
thermal properties of an industrial helmet, the helmet shell should be
redesigned to provide better insulation against solar radiation, and to provide
4.1 Improving Insulation
Adding a layer of
close-fitting helmet liner is a way to insulate the head from solar radiation
and remove the “green house” effect beneath the helmet shell. But it also adds
weight to the helmet. A commercially available helmet with such design was
tested and it showed better-than-average thermal properties. However, its
weight was 40% more than the weight allowed by Chinese National Standard (CNS).
research to improve insulation against solar heat focused on changing the
surface of the helmet shell. The color of the shell is of course an important
factor. The helmet tested in Figure 3(a) was white. The test was repeated on
the same helmet in red, and yellow. As shown in Figure 3(b), the final average
temperatures of these color helmets under standard test conditions were 4 to 7
degrees higher than that of a white helmet.
materials on the outside of the shell is also effective. When the helmet tested
in Figure 3(a) was covered with various reflective materials on the top portion
of the shell, the final average temperature dropped 4 to 6 degrees under the
standard test conditions. On the other hand, our test results showed that
helmets made of different plastic materials, PE, ABS, or PC, do not produce
significant difference in thermal properties.
Figure 3(a). The effect of all heat sources.
Figure 3(b). The effect of different color and
4.2 Improving Ventilation
design concepts for improving ventilation of the helmet shell were tested. The
most intuitive way is to put ventilation holes or slots on the helmet shell.
However, after many tests on different sizes, shapes, and locations of
ventilation holes, we concluded that the ventilation holes produced only a
marginal effect for intensifying convection. A simple two-dimensional computer
simulation revealed that, the speed of circulation of the air beneath the
helmet shell was very slow compared with the airflow outside (about 1/10 under
the assumptions of our computer simulation model). Adding ventilation holes
alone does not produce a significant change in the airflow pattern beneath the
showed that increasing the clearance between the helmet and the head is an
effective means to increase air circulation speed, thus to increase heat
exchange with the outside air. This also explains the workers’ behavior when
wearing a helmet in a hot weather: they often wear helmets in a high position
to allow more airflow between the helmet and the head. Too much clearance
between the head and the shell will affect the shock absorbing ability of
harness of the helmet. Actually the vertical clearance is specified in the test
standards for impact and penetration tests. Therefore instead of increasing the
clearance, we suggest that “wind channels” on the shell of the helmet have to
be properly designed together with the ventilation holes.
holes and wind channels should not affect the structural safety of the helmet
shell. On the contrary, if properly designed, adding the wind channels on the
helmet shell can provide local reinforcement against impact and increase
lateral rigidity of the helmet shell.
that can be observed from local outdoor workers is that, in a hot weather, the
workers often wear helmets in a reverse direction, with the beak at the back of
the head. Our test results showed that wearing a helmet this way can also
effectively decrease the average temperature beneath the helmet. Wearing the
helmet in a reverse direction actually decrease the clearance between the
helmet and the front of the head. However, without the beak, the air can flow
into the helmet much more smoothly. The beak should also be properly designed
so that it will not impede the airflow from the front.
Based on the
experiment, our design suggestions are summarized as follows:
1. Radiant heat is the major heat source when wearing a helmet. Using a
light color shell (such as a white one) and covering the top portion of the
helmet shell with reflective materials are effective for insulating the radiant
2. Convection is the primary way for heat dissipation from beneath the
helmet shell. Adding ventilation holes or slots does not produce a significant
effect for intensifying convection. “Wind channels” should be designed to
provide more clearance between the helmet shell and the head. At the same time,
those wind channels should be designed to increase the structural rigidity of
the helmet shell.
3. The beak should also be designed so that it will not impede the
airflow from the front.
5. Development of a new helmet shell design
According to these
design suggestions, a new design prototype was developed, tested, and further
modified. At this product development stage, attention was also paid to
structural safety, appearance, manufacturability of the helmet shell, and other
practical issues. Finally we developed our final helmet shell design as shown
in Figure 4. Some of the important features of this final design are list below:
1. There are ventilation holes on the front and back, as well as both
sides of the helmet shell.
2. Together with the ventilation holes, wind channels were designed on
the top of the shell and on both sides of the shell, to provide clearance
between the shell and the head.
3. The wind channels also provide local reinforcement against impact on
the top of the shell, and increase the lateral rigidity of the helmet shell.
4. The beak of the helmet has an arched front tip and a wind channel
that integrated smoothly into the front end of the helmet shell.
5. There are arched flanges on both sides and the back end of the
helmet shell, to allow more airflow between the helmet shell and the head from
6. There are flanges around the ventilation holes to prevent water from
leaking into the helmet on a rainy day.
Figure 4. Our final helmet shell design. The
arrows indicate airflow.
We also checked
with a few helmet manufactures about the manufacturability of our helmet, and
made slight modifications on the design, mostly around the ventilation slots.
With all the features list above, the estimated production cost of our helmet
is about the same as that of other helmets.
Figure 5 compares
the temperature contour plots of our final design with a commercially available
helmet (Helmet B). Helmet B was used for this comparison because it is the
helmet that has the best thermal properties among the helmets we tested. Figure
5(a) is the temperature contour plot of this helmet under standard test
conditions. The average temperature was 35.1℃. It has two
“hot spots” on both sides of the helmet at the front end, with temperatures
over 38.5℃. The temperature contour plot also
shows steep temperature gradient over the entire helmet.
(a) Temperature contour plot of Helmet B.
(b) Temperature contour plot of our final design.
(c) Temperature contour plot of our final design,
with reflective material covering the top portion of the shell.
Figure 5 Temperature contour plots.
Figure 5(b) shows
the temperature contour plot of our final design under standard test
conditions. The average temperature was 32.6℃, and the
temperature was more evenly distributed over the whole helmet. There were still
two hot spots on both sides of the front end of the helmet, which indicates
that the airflow in those two areas can be further improved. Figure 5(c) shows
the temperature contour plot when the helmet was covered with reflective
material on the top portion of the shell (Figure 6). The average temperature
further dropped to 31.6℃, only 1.6℃
higher than the temperature of the testing region.
Figure 6. Covering reflective material on the top
portion of the helmet shell.
Figure 7 compares
the speed of heat dissipation of the helmet tested in Figure 3 (Helmet A), Helmet B, and our new design. When the electric fan was turned off, our
design had an initial temperature of 40.4℃(with no
reflective materials), comparing with 50.1℃ of Helmet A and 52.6℃
of Helmet B. When the electric fan
was turned on again, it took 220 seconds for our design to reach the new steady
state temperature 32.6℃, while it
took 380 seconds for Helmet B to
reach the new steady state temperature 34.6℃, and it
took 700 seconds for Helmet A to
reach the new steady state temperature 38.3℃.
Figure 7. Heat dissipation speed.
The prototype of
our final helmet design was made of ABS by vacuum molding. The mass of our
helmet with harness and other accessories is 370g (346g
without harness and other accessories), which satisfies CNS standard of 425g. It also passed the CNS impact and
6. A human description evaluation of the new helmet design
Finally a human description evaluation was performed to
compare the thermal properties of our new helmet design with those of two
commercially available helmets that are popular to the local workers. See Table
1 for a description of the helmets Helmet 1 and Helmet 2 have similar shape
shells, the primary difference between them is the color. Note that these
helmets are different from Helmet A and Helmet B tested in the previous
Table 1. The helmet settings.
Our New Design
Fourteen healthy subjects volunteered to participate in the
study, they all are male, young (average 22.3 Years old), and have good feeling
about hotness. Before the first measurement, each subject was informed about
the experiment procedure and agreed to answer the questions sincerely. All
subjects wore different helmets randomly and walk around in a sunny, outdoor
environment for 10 minutes. Then
the subjects received a 10-min break before they switched to a different
helmet. After wearing each helmet the subjects were asked to answer 5 questions
subjectively on a nine-point scale, and describe their experience about each
helmet. The 5 questions are (1)the subjects’ feeling about the temperature
within the helmet, (2)whether they feel obvious “hot spots” in the helmet,
(3)whether it’s easy to sweat wearing the helmet, (4)how well does heat
dissipate when wind blows, and (5)whether the helmet is comfortable in general.
Figure 8 shows the sum of total scores of each helmet rated
by the fourteen testers. There is a significant difference between the the new
design helmet and the two commercially
available helmets (p<0.001). Figure
8 also shows that Helmet 2 has significantly higher score than Helmet1 in thermal properties (p<0.1), which indicates that the
light color of safety helmet has a significant effect. Figure 9 shows the sum of individual scores rated by the
fourteen testers on the five thermal property evaluations. Our new industrial
safety helmet design has significantly higher scores than the two commercially
available helmets on all five questions (p<0.01).
Figure 8. Total scores of each helmet on the
thermal property evaluation.
Figure 9. Scores on the 5 questions for thermal
This paper suggests
two basic approaches that can improve thermal properties of industrial safety
helmets: (1) to provide better insulation against radiant heat, which is the
major heat source when wearing a helmet. For example, using white paint and a
reflective covering are very effective. (2) to provide better ventilation to
intensify convection, which is the primary way for heat dissipation from
beneath the helmet shell.
concepts were found to be effective for improving thermal properties of safety
helmets. Using these design concepts, we also developed a new design prototype,
which exhibited better thermal properties than the commercially available
helmets we tested. With this experience, we believe that there is still much
room to further improve industrial safety helmets to provide better thermal
comfort for the head, so that safety helmets will be more welcomed by
This project was
sponsored by Institution of Labor Safety and Health, Council of Labor Affairs, Taiwan, ROC.
This support is gratefully acknowledged.
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