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  The term
microclimate describes the environmental parameters that influence heat
exchanges between the subject and the environment in the confined spaces
and that determine the so-called "thermical wellbeing".
The most important quantity that determine the thermical wellbeing
include: air temperature, relative humidity, ventilation, radiant heat,
energy expenditure, clothing's thermical resistance. The human body, in
fact, tries to preserve the equilibrium of its thermical state to
maintain its temperature at optimum values.
The heat balance of the organism can be described by the following
equation:
M ± C ± R ± E = 0
M describes the metabolic energy, i.e. the basal metabolism of a subject
at rest. The energy expenditure for each specific activity must be added
to this value.
C describes the amount of heat exchanged with the environment by
convection (solid-liquid contact) and conduction (solid-solid contact). The
conduction exchange is very little and it does not account for more than
3% of the total heat exchange.
The factors that influence the convection exchange include air
temperature, air speed and the clothing's thermical resistance. The
factors that influence the conduction exchange involve the extension of
the exchange surface, its temperature, the clothing's and the body's
thermical resistance. The bigger the difference between body's and air's
(or contact surface's) temperature, the bigger the heat exchanged. Air
speed is also very important because as it increases the thermical
exchanges increase as well. Thermical insulation of the clothing is
expressed in Clo (Clothing) and it is a function of the clothing's
thickness and neither of the fabric's quality nor of its type.
R represents the amount of heat exchanged by radiation and it is a
function of the average radiant temperature and of the subject's
cutaneous temperature. Any body in possession of a temperature other than
absolute zero gives out electromagnetic radiations called radiant energy
that can be transmitted from one body to another if between the two there
is either vacuum or a medium able to conduct such radiations, entirely or
partially. The radiant temperature is measured by means of a Vernon's
globethermometer.
E represents the evaporation, an extremely quick process for the
dissipation of energy. Evaporation leads to a lowering of the body
temperature by means of three mechanisms. The first mechanism is
sweating. This produces a remarkable loss of heat by evaporation of sweat
(600Kcal are lost for 1 litre of evaporated sweat). The second mechanism
is represented by "perspiratio insensibilis", that is the
evaporation of water from cutaneous tissue. The third mechanism is the
evaporation of water from pulmonary alveoli. In a subject at rest in a
pleasant thermical environment, evaporation takes place by means of
perspiratio insensibilis and from the alveoli. During strenuous physical
exercise in unpleasant thermical environments, the heat loss by
evaporation can be 30 times higher.
Therefore, thermical wellbeing entails the need to maintain an
equilibrium between the amount of heat produced by the body and the
amount of heat given up to the environment by means of the mentioned
mechanisms.
Whenever the heat balance becomes positive (or negative) the regulating
mechanisms are responsible to keep the temperature within acceptable
limits. An exasperated employment of such mechanisms (as it can be found
in some working environments in the presence of important heat sources)
can lead to thermical stress. This situation may prelude to real pathological
processes (for example heat stroke) if the exposure is not limited in
time. On the other hand, a moderate employment of such mechanisms can
give rise to thermical discomfort (sensation of heat/hot). In the genesis
of such sensations the subject's sensibility always plays an important
role.
For additional information
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