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2 Review of Literature: Motorcycle helmet
radiation and absorption from it, e.g. black is both the best absorber and radiator of
heat, while white is a poor absorber and radiator of heat.
2.10.4 Evaporation
Evaporation is the reverse of condensation, and it is the vaporisation of a liquid.
Evaporation is similar to heat transfer by convection but also requires an initial change
of state from liquid to vapour at the skin surface and the subsequent diffusion of
vapour across the boundary layer into the ambient air. The driving force for diffusion
is the concentration gradient, and the quantity transferred is mass. Evaporation is
caused when liquid water is converted into vapour by absorbing large amounts of
heat energy. Generally, one calorie of heat will raise the temperature of one gram of
water by one degree centigrade, and 580 calories of heat are required to evaporate
one gram of water at body temperature. When water is evaporated from the skin, the
energy required for evaporation is removed from the skin, thus cooling it [65].
Liu and Holmer [76] investigated the evaporative heat transfer characteristics
of industrial safety helmets. They analysed the effects of ambient humidity, solar
radiation and wind on heat loss. The evaporative heat loss [72] can be calculated
using the following equation:
evaporative heat loss (H e ) = k e · A · (P sk − P ab )
(2.4)
where ke is the evaporative coefficient, P sk is the saturated vapour pressure of water
at skin temperature, P ab is the ambient vapour pressure, and A is the surface area of
the body.
The basic methods of heat loss from human body are shown in Fig. 2.9 [70].
2.11 Heat Stress
The human body generates a large amount of heat during exercise and hard work.
If this heat is not quickly released, it can produce heat stress, and while resting
after exercise, less heat is produced by the body, thus leading to hypothermia. The
acceptance of a motorcycle helmet or any other helmet for continuous wear depends
on the degree of comfort it provides. In some tropical and subtropical countries, in
spite of strict laws relating to the wearing of helmets while riding a motorcycle, many
riders are unwilling to wear a helmet as they perceive it to be very uncomfortable.
However, they are wearing helmets reluctantly, because of the strict regulations [51].
For the safety and comfort of the motorcyclist, the physiological and ergonomic
properties of the helmet play a crucial part. The thermal discomfort involved in
wearing headgear by workers has motivated several researchers to work on the topic.
2 Review of Literature: Motorcycle helmet
radiation and absorption from it, e.g. black is both the best absorber and radiator of
heat, while white is a poor absorber and radiator of heat.
2.10.4 Evaporation
Evaporation is the reverse of condensation, and it is the vaporisation of a liquid.
Evaporation is similar to heat transfer by convection but also requires an initial change
of state from liquid to vapour at the skin surface and the subsequent diffusion of
vapour across the boundary layer into the ambient air. The driving force for diffusion
is the concentration gradient, and the quantity transferred is mass. Evaporation is
caused when liquid water is converted into vapour by absorbing large amounts of
heat energy. Generally, one calorie of heat will raise the temperature of one gram of
water by one degree centigrade, and 580 calories of heat are required to evaporate
one gram of water at body temperature. When water is evaporated from the skin, the
energy required for evaporation is removed from the skin, thus cooling it [65].
Liu and Holmer [76] investigated the evaporative heat transfer characteristics
of industrial safety helmets. They analysed the effects of ambient humidity, solar
radiation and wind on heat loss. The evaporative heat loss [72] can be calculated
using the following equation:
evaporative heat loss (H e ) = k e · A · (P sk − P ab )
(2.4)
where ke is the evaporative coefficient, P sk is the saturated vapour pressure of water
at skin temperature, P ab is the ambient vapour pressure, and A is the surface area of
the body.
The basic methods of heat loss from human body are shown in Fig. 2.9 [70].
2.11 Heat Stress
The human body generates a large amount of heat during exercise and hard work.
If this heat is not quickly released, it can produce heat stress, and while resting
after exercise, less heat is produced by the body, thus leading to hypothermia. The
acceptance of a motorcycle helmet or any other helmet for continuous wear depends
on the degree of comfort it provides. In some tropical and subtropical countries, in
spite of strict laws relating to the wearing of helmets while riding a motorcycle, many
riders are unwilling to wear a helmet as they perceive it to be very uncomfortable.
However, they are wearing helmets reluctantly, because of the strict regulations [51].
For the safety and comfort of the motorcyclist, the physiological and ergonomic
properties of the helmet play a crucial part. The thermal discomfort involved in
wearing headgear by workers has motivated several researchers to work on the topic.