2.11 Heat Stress
27
and humidity of the hats with holes were lower, as compared to safety hats without
holes. Although the head covers only 7–10% of the total body surface, the head skin
temperature is generally higher than in other parts of the body [103]. A large number
of studies have shown that protective headgear in hot environments causes higher
levels of thermal discomfort [97, 104]. This discomfort is related to the head, as the
head is one of the most important parts of the body in determining the whole body’s
thermal comfort [105]. Many researchers have investigated the relationship between
heat transfer and thermal comfort by evaluating the heat transfer characteristics of
industrial helmets [25, 106, 107]. None of these studies were in the context of moving
vehicles and road safety.
In a recent study, Bogerd and Brühwiler [108] investigated the roles of head tilt,
hair and wind speed on forced convective heat loss through motorcycle helmets,
utilising a thermal manikin under the following conditions: (i) a 30° forward head
tilt angle (six helmets); (ii) a wig installed between the head form and helmet (six
helmets) and (iii) applied wind speeds ranging from 0 to 80 kmh
−1 (three helmets).
It was found that: (i) by tilting the head forward, a reduction in heat loss in the face
section was observed in many helmets; (ii) the heat loss was reduced by a factor of
2 by the wig; and (iii) heat loss is approximately linearly dependent on wind speed
(0–80 kmh
−1 ), with some exceptions below 30 kmh
−1 .
The flow of air over the head is reduced by wearing a helmet, and the heat loss
from the head to the environment is thereby reduced, which can lead to an increase in
heat-related stress in the case of hard physical work [109]. The effect of heat transfer
from the human head, along with other relevant environmental factors (such as air
speed and radiant heat), has been investigated by many researchers when studying
the characteristics of helmets such as effective materials [110–112], colour [113],
standoff distance [114] and general construction [115].
The human head has generally been simulated by the use of manikins for
measuring heat transfer from the head when wearing an industrial safety helmet
[13, 112]. Sometimes, human participants have been used for experiments [116].
However, there are disadvantages in using human participants because of variability
between different participants, depending on their personal circumstances and the
time of the day or the month. Hence, it might be necessary to use large numbers of
participants to obtain results that have statistical significance.
Osczevski [117] studied the effects of wind on the nude head by using manikins
he designed. In these manikins, there were four separately monitored regions for
simulation and measurement. The emphasis of solar radiation on the cooling effect
of headgear was investigated by using head forms. In this study, lamps were used to
simulate the effect of the sun, and the temperatures were measured at several places
on the head form [104]. A metallic head form with a constant power source was
placed in a wind tunnel, and a non-dimensional thermal resistance was defined by
comparing it with a bare sphere simultaneously. The temperatures were measured at
certain points on one half of a symmetrical head form [34].
Heat stress reduction when wearing a helmet has been achieved by either actively
cooling the head or by using passive vents into the helmet structure. It has been
observed that cooling the head area in order to reduce heat stress is more efficient
27
and humidity of the hats with holes were lower, as compared to safety hats without
holes. Although the head covers only 7–10% of the total body surface, the head skin
temperature is generally higher than in other parts of the body [103]. A large number
of studies have shown that protective headgear in hot environments causes higher
levels of thermal discomfort [97, 104]. This discomfort is related to the head, as the
head is one of the most important parts of the body in determining the whole body’s
thermal comfort [105]. Many researchers have investigated the relationship between
heat transfer and thermal comfort by evaluating the heat transfer characteristics of
industrial helmets [25, 106, 107]. None of these studies were in the context of moving
vehicles and road safety.
In a recent study, Bogerd and Brühwiler [108] investigated the roles of head tilt,
hair and wind speed on forced convective heat loss through motorcycle helmets,
utilising a thermal manikin under the following conditions: (i) a 30° forward head
tilt angle (six helmets); (ii) a wig installed between the head form and helmet (six
helmets) and (iii) applied wind speeds ranging from 0 to 80 kmh
−1 (three helmets).
It was found that: (i) by tilting the head forward, a reduction in heat loss in the face
section was observed in many helmets; (ii) the heat loss was reduced by a factor of
2 by the wig; and (iii) heat loss is approximately linearly dependent on wind speed
(0–80 kmh
−1 ), with some exceptions below 30 kmh
−1 .
The flow of air over the head is reduced by wearing a helmet, and the heat loss
from the head to the environment is thereby reduced, which can lead to an increase in
heat-related stress in the case of hard physical work [109]. The effect of heat transfer
from the human head, along with other relevant environmental factors (such as air
speed and radiant heat), has been investigated by many researchers when studying
the characteristics of helmets such as effective materials [110–112], colour [113],
standoff distance [114] and general construction [115].
The human head has generally been simulated by the use of manikins for
measuring heat transfer from the head when wearing an industrial safety helmet
[13, 112]. Sometimes, human participants have been used for experiments [116].
However, there are disadvantages in using human participants because of variability
between different participants, depending on their personal circumstances and the
time of the day or the month. Hence, it might be necessary to use large numbers of
participants to obtain results that have statistical significance.
Osczevski [117] studied the effects of wind on the nude head by using manikins
he designed. In these manikins, there were four separately monitored regions for
simulation and measurement. The emphasis of solar radiation on the cooling effect
of headgear was investigated by using head forms. In this study, lamps were used to
simulate the effect of the sun, and the temperatures were measured at several places
on the head form [104]. A metallic head form with a constant power source was
placed in a wind tunnel, and a non-dimensional thermal resistance was defined by
comparing it with a bare sphere simultaneously. The temperatures were measured at
certain points on one half of a symmetrical head form [34].
Heat stress reduction when wearing a helmet has been achieved by either actively
cooling the head or by using passive vents into the helmet structure. It has been
observed that cooling the head area in order to reduce heat stress is more efficient