Civil Engineering Reference
In-Depth Information
completed in 1-10 days of exposure to a hot environment. The time required for
acclimatization is reduced when people actually perform physical work in the heat.
However, acclimatization to a hot environment can be lost over a period as short as a
weekend. People who work outdoors and spend the weekend in an air-conditioned
environment will have to acclimatize again. Recovery to the prior level will take about a
day. Acclimatization is usually completely lost after 3-4 weeks in a cool environment.
12.7
MEASUREMENT OF HEAT STRESS
In addition to the ambient temperature, there are several other factors that effect heat
exposure. In order to calculate their effect, the thermal balance of the body may be
expressed in the thermal balance equation (Barnard, 2002). A somewhat simplified
version of this equation is as follows (in W/m
2
):
M
−
W
=
C
+
R
+
E
+
S
where
M
is the metabolic power,
W
is the effective mechanical power,
C
is the heat
exchange by convection,
R
is the heat flow by radiation at the skin surface,
E
is the heat
flow by evaporation at the skin surface, and
S
is the heat storage.
As explained above, the metabolic processes are only partially effective. For the most
effective muscles only about 25% of the metabolism
(M)
can be used for work. The rest
is used to produce heat and maintain the basic metabolic processes. By expressing the
metabolic power in watts per square meter, it is possible to compensate for the body size
of individuals. Thereby individuals can be compared. For the calculation of an average
individual, one can assume a body area of 1.8 m
2
.
Heat transfer by convection
(C)
refers to the temperature exchange produced by
moving air. The amount of convection depends on the difference between skin
temperature and air temperature. The radiated heat
(R)
may be heat radiated by the
human body (in the infrared light spectrum). The human body can also absorb radiated
heat from external sources.
The evaporated heat loss
(E)
occurs primarily at the skin surface. Moisture is present
on the skin because of sweating, and when the moisture evaporates heat is taken from the
body surface. The evaporation is a function of air speed and the difference in water vapor
pressure between the sweat (at skin temperature) and the air. In hot, moist environments,
evaporated heat loss is limited, since the air cannot accept or absorb more water. If the air
has maximum water pressure (100% humidity) there can be no further evaporation of
sweat, and therefore no cooling of the body (ASHRAE, 1997). In a hot, dry environment,
however, evaporated heat loss is limited only by the amount of perspiration that can be
produced by the worker. The sweat disappears immediately from the skin and is absorbed
by the air. The maximum sweat production that can be maintained by an average man
throughout a day is about 1 1/h.
The heat storage
(S)
in the heat equation should in essence balance around zero. If
S
becomes large there is a risk of heat stroke. There are obviously many ways to reduce
S
.
Stopping work is one way. Several additional methods are mentioned below in Table
12.3. The metabolic rate for different tasks can now be classified as in Table 12.1
(International Standards Organization, 1989a).
