138 Collectively, these effects are known as Hand-Arm Vibration Syndrome
HAVS. The symptoms of VWF are aggravated when the hands are exposed to cold. Workers affected by HAVS commonly report:
Attacks of whitening blanching of one or more fingers when exposed to cold.
Tingling and loss of sensation in the fingers. Loss of light touch.
Pain and cold sensations between periodic white finger attacks. Loss of grip strength.
Bone cysts in fingers and wrists.
The development of HAVS is gradual and increases in severity over time. It may take a few months to several years for the symptoms of HAVS to become
clinically noticeable. HAVS is a disorder, which affects the blood vessels, nerves, muscles and joints of the hand, wrist and arm and it can become
severely disabling if it is ignored. Vibration white finger VWF is a common complaint for workers who regularly use power tools andor drills, which can
be triggered by cold or wet weather and result in severe pain in the affected fingers.
14.4 Measurement of Vibration
Vibration is usually measured by the use of accelerometers and unlike a sound level meter, the actual probe needs to be small and light so as not to
alter the vibration pattern of the machine being measured; or if it is attached to the hand of a worker, the measured acceleration. Unlike noise
measurements, vibration ones have a subjective element to them in that the accelerometer may be held against a vibrating tool by hand; may be fixed to it
for example by plastic ties; or may be fixed to the operator‟s hand. All these
alternatives will necessarily give different results.
139
15 THERMAL ENVIRONMENT: PRINCIPLES, EVALUATION AND CONTROL
The human body can be considered as a processing plant, using complex chemical reactions to produce mechanical energy; as a consequence of the
inevitable inefficiency of these reactions heat is produced as a by-product. In order to function effectively we need to maintain our bodies at a constant
temperature within the range 36.5 - 37.5
o
C.
15.1 Human Response to the Thermal Environment
Temperature regulation centres in our brain are sensitive to small changes of blood temperature and also get feed back from sensory nerves at the skin, our
brains then use this information to adjust our bodies responses to heat.
15.1.1 Physiological responses to heat
When exposed to heat the blood vessels in our skin expand and our pulse rate increases. This increases blood flow to the surface of the body, thus
increasing the potential for heat transfer from body core to skin and surroundings. Sweating also increases heat loss due to latent heat of
evaporation. This also has the added effect that it increases our water requirements.
In very hot conditions, sweating offers the greatest potential for regulating body temperature. On going from a cool to a predominantly warmer climate it
is necessary to allow the body to acclimatise by increasing blood volume and sweat capacity while decreasing salt losses in sweat. It takes about 3 days for
this acclimatisation to be about 60 complete and about 10 days for complete acclimatisation. This increased sweat capacity is lost after a few days in a
cooler environment. Possible adverse effects of exposure to excessive heat include; fatigue,
behavioural modification, including reduced concentration, heat cramps due to salt loss, fainting heat exhaustion and heat stroke.
140
15.1.2 Physiological responses to cold
When exposed to cold the blood vessels in our skin contract and heat flow to the body surface is reduced, thus minimising heat loss from the body. Heat
production is increased by physical activity and shivering. There is no physiological acclimatisation to cold.
Possible adverse effects to excessive cold include; lassitudelistlessness, chilblains, frost bite and hypothermia.
15.1.3 Psychological responses to the thermal environment
People will often modify the way they work depending on the thermal environment. Often they will try to modify their local work environment e.g.
moving to a more comfortable area, changing clothes, increasing or decreasing ventilation etc. Performance and efficiency can also be affected by
adverse thermal conditions.
15.2 Heat Transfer from the Body
Formulae are available for calculating the heat load and balance of a person, providing enough data is available. This is a very complex area and is beyond
the scope of this course. However, an understanding of the mechanisms and factors involved in the heat balance mechanisms is useful to understand the
evaluation of thermal stress issues. In terms of assessing or evaluating thermal environments, there are six
parameters that may be taken into account. Two of these are associated with the individual and four with the environment, namely:
S = M - W + C + R + K
C
- E Where:
S =
Storage, which in the long term must equal 0 M
= Metabolism
W =
External work done C
= Convection
R =
Radiation K
c
= Conduction
E =
Evaporation
141 The heat balance mechanisms over a period of time are affected by 6
parameters, 2 associated with the individual and 4 with the environment, namely:
1 Work rate i.e. activity or metabolic rate Person
2 Clothing 3 Air temperature
Environment 4 Radiant temperature
5 Air Velocity 6 Humidity moisture conditions
15.3 Evaluating the Thermal Environment 15.3.1 Metabolic rate
