1. Thermal comfort
Thermal comfort is a complex entity.
Much work was done to determine what
constitutes “thermal comfort”. Several
indices have been put forward from time
to time to express thermal comfort and
heat stress. These are as follows:
AIR TEMPERATURE – it was used
for a long time as an index of thermal
comfort, but it was realized that air
temperature alone was not an adequate
index.

2. AIR TEMPERATURE AND
HUMIDITY - later, air temperature and
humidity were considered together to
express thermal comfort, even this was
found to be satisfactory.
COOLING POWER – still later, air
temperature, humidity and air movement
were considered together and expressed as
“cooling power” of the air. These indices
plus mean radiant heat are used by the
Bulgarian standards to evaluate the thermal
comfort.

3. EFFECTIVE TEMPERATUTE – an
arbitrary index, which combines into a single value
the effect of temperature, humidity and air movement
on the sensation of warmth or cold felt by the human
body. The numerical value of effective temperature
is that of the temperature of still, saturated air which
will induce the same sensation of warmth or cold as
that experienced in the given conditions. For
example, if the environment has an ET value of 30
deg.C, it implies that the subjective sensation will be
the same as in a saturated atmosphere of 30 deg.C
with no air movement. A criticism of the ET is that it
ignores the effect of a radiation from the surrounding
structures.

4. CORRECTED EFFECTIVE
TEMPERATUTE – it deals with all the four
factors namely, air temperature, humidity, velocity
and mean radiant heat.
Thermal comfort is a function of many
variables, including the season of the year,
cultural practices and habits, differing from
country to country. Nevertheless, describing
comfort zones is necessary for the proper design of
heating and air conditioning systems.

5. In choosing optimal conditions for
comfort, knowledge of the energy
expended during the course of
routine physical activities is
necessary, since body heat production
increases in proportion to exercise
intensity.

6. Evaluation of the information relating
the physiology of a person to the physical
aspects of the environment is not a simple
task. Considerably more is involved than
simply taking a number of air temperature
measurements and making decisions on the
basis of this information.

7. Whenever temperature differences
exist between two bodies, heat can be
transferred. Net heat transfer is always
from the body (or object) with higher
temperature to the body with lower
temperature and occurs by one or more of
the following mechanisms.

8. Conduction – the transfer of heat from one point to
another within a body, or from one body to another when
both bodies are in physical contact. Normally, this term is
insignificant and can be disregarded except in special
cases, such as swimming.
Convection - the transfer of heat from one place to
another by moving gas or liquid. Natural convection
results from differences in density caused by temperature
differences.
Radiation – the process by which energy,
electromagnetic (visible of infrared) is transmitted through
space without the presence or movement of matter in or
through this space

9. There are two sources of heat that are
important to anyone working in a hot environment:
1) internally generated metabolic heat and 2)
externally imposed environmental heat.
The net heat exchange between a person and
the ambient environment can be expressed by:
H=M±R±C–E
Where: H = body heat storage load
M = metabolic heat gain
R = radiant or infrared heat load
C = convection heat load
E = evaporative heat loss

10. Metabolic heat gain is composed of
the basal or resting metabolism that
provides the energy necessary to keep the
body functioning, as well as the working
metabolism that provides the energy
necessary for the body to accomplish
specific tasks.
Metabolism can only add energy to the
body; therefore, M is always positive.

11. Radiant heat load is energy in the
form of wavelengths that are
transformed into heat when they strike
an object. Whether the human body
emits or receives radiant energy
depends on temperature of the body,
and the surrounding objects. Thus, R
can be either positive or negative.

12. Convective heat load is the amount
of heat energy transferred between the skin
and air. Air temperature excess of skin
temperature will warm the body; air
temperature less than skin temperature will
cause the body to be cooled.
The evaporative heat loss from the
body (perspiration) reduces body heat,
therefore its value is always negative. The
use of fans to increase E is a common
method of cooling workers.

13. The body tries to maintain a balance
between the heat gained by work, radiant
and converted heat imposed on the body,
and the heat loss by sweating (evaporation).
Ideally, the change in body heat content
should be zero. If this balance cannot be
maintained by evaporation, then heat can
build up in the body, causing a rise internal
temperature.

14. Measurement
Thermometers are the instruments used for
measuring 1. temperature. Mercury thermometers
are widely used, as mercury boils at a high
temperature, has a regular expansion and its level
can be easily seen. The essential conditions for the
use of thermometers are that 1. the air should have
access to the bulbs of the thermometers and 2. the
thermometer should be protected against radiant
heat.
Dry bulb thermometer - this is an ordinary
thermometer shielded from direct radiant energy
sources, which measures the air temperature.

15. Wet bulb thermometer - precisely the
same as the dry bulb thermometer excepting
that the bulb is kept wet by a muslin cloth.
The evaporation of water from the muslin
cloth lowers the temperature of the mercury.
The wet bulb thermometer therefore shows a
lower temperature reading than the dry bulb
thermometer.

16. The drier the air, the lower the wet
bulb reading. It the wet and dry bulb
thermometers record the same temperature,
it means that the air is completely saturated
with moisture, which is rare.
In conclusion, the wet bulb
thermometer measures the effect of
humidity on evaporation and effect of air
movement on ambient temperature.

17. 2) Humidity, the amount of water
vapour in a given space is commonly
measured as relative humidity (RH). That is
the percentage of moisture present in the
air, complete saturation being taken as 100.
It could be determined using an Assmann
psychrometer giving accurate
measurements of the wet and dry bulb
temperature of the air.

18. In this instrument, air is drawn at a
speed higher than 5 m/s by a click-work
fan. The bulbs of the thermometers are
protected from the effects of solar
radiation. By use of suitable psychrometric
charts or tables the RH of the air may be
obtained from the readings of the
psychrometer.

19. Kata thermometer. The word “kata” is
a Greek word meaning “down”. The kata
thermometer is an alcohol thermometer with a
glass bulb. The readings on the stem are
marked from 35 to 38. Before taking the
readings, the bulbs are immersed in hot water
to warm them when the alcohol rises into a
small reservoir at the top of the instrument.
Then the instrument is suspended in air at the
point of observation.

20. The time in seconds required for the spirit
to fall from 38 to 35 is noted with a stop watch.
The length of time depends upon the “cooling
power” of the air. Each Kata has a “factor”
called Kata Factor marked on the stem. This
factor is determined for each instrument by the
manufactures. Kata Factor divided by the
cooling time gives the rate of cooling. The
instrument is used for recording low 3) air
velocity. High air velocity is measured by an
instrument, called anemometer and is expressed
in meters per second (m/s).

21. The globe thermometer is used for direct
measurement of the mean radiant temperature of
the surroundings. The instrument consists of a
hollow copper bulb 15 cm in diameter and is
coated outside with mattblack paint which absorbs
the radiant heat from the surrounding objects. A
specially calibrated mercury thermometer is
inserted, with its bulb at the center of the globe.

22. This thermometer registers a higher
temperature than the ordinary air one
because it is affected both by the air
temperature and radiant heat. The globe
thermometer is also influenced by the
velocity of air movement.

23. Location of thermal sensors
Thermal-sensing instruments should be
located at the workstation so that the actual
conditions of heat exposure are measured. In those
zones where the worker spends substantial amount
of time, measurements should be taken
periodically, three or four times for a work shift
could be adequate.

24. Where the employee moves through a
large area, several zones may be involved.
In such cases the thermal sensors could be
located in several points to collect data
from these different zones.

25. Heat stress indices
Heat stress is the load of heat that must be
dissipated if the body is to remain in thermal
comfort.
The guidelines currently used for worker
exposure to heat are based on indices developed
through subjective and objective testing of
workers or from combinations of external heat
measurements.

26. Three of the more commonly used indices
are
- EFFECTIVE TEMPERATUTE, the
- WET BULB GLOBE
THERMOMETER INDEX (WBGT) and the
- HEAT STRESS INDEX.

27. For indoor exposure, or outdoor
exposure without a solar load, the formula is:
WBGT = 0.7twb + 0.3tg
The necessary measurements require
relatively simple instrumentation. Heat stress
monitors that measure all three temperatures
and calculate WBGT index are also available.
Sensors should be at least the mean height of
the worker, or at the levels of the ankles, the
abdomen and the head.

28. A practical application of the WBGT
index is to recommend the percentage of
time that the individual is permitted to
perform the task according to the
severity of the environment and
metabolic demands of the task
For example, a task requiring light to
moderate work of 200 kcal/hr could be
performed continuously in environment up
to a WBGT of 30 C, but only 25 % of the
time at a WBGT of 32.2 C.

29. The WBGT index – (ISO 7243) is
commonly used because is easy to determine. It is
recommended by NIOSH in the United States.
The WBGT index is calculated from the
measurements of the wet bulb (WB), the black
globe (G) and the dry bulb air (A) temperatures.
For outdoor exposure with a solar heat
source, the WBGT formula is:
WBGT = 0.7twb + 0.2tg + 0.1ta
where: twb = wet bulb temperature
tg = globe temperature
tg = dry bulb air temperature

30. Heat stress index (HSI)
The determination of HSI results in
more knowledge about the environment
and possibility to perform efficient control
measures than the use of the WBGT. To
calculate the HSI, measurements of the wet
bulb (WB), the black globe (G), the dry
bulb air temperatures and the air velocity
are necessary for each jobsite, and
estimation of the metabolic rate (M) of the
workers.

31. Once measurements have been made, rates of
heat exchange between the worker and the
environment by convection (C) and radiation (R)
are calculated, and with M, are used to estimate the
amount of sweating required to stay at equilibrium
(E req). Calculations of C, R, E req, and E max,
and estimation of M lead to a knowledge of the
relative contribution of each, and hence, may well
suggest possible means of solving the problem.
This can not be done with WBGT.

32. An interpretation of the HIS
values is given below:
0 No thermal stress
10-30 Moderate to mild heat
strain
40-60 Severe heat strain
Very severe heat strain
Upper limit of heat tolerance

33. Subjective and physiological method
for thermal comfort evaluation
Inquiry method of Begford – method
to collect votes from a group of people,
exposed to certain thermal environment
about their thermal sensation, estimated
according to the 7-point thermal sensation
scale.

34. Index of Fanger (PMV and PPD indices)
(ISO 7730). It could be calculated using
mathematical equations, as well as using an
integrating sensor - Bruel & Kjäer mod. 1212.
The PMV-index can be determined when the
activity (metabolic rate-1 metabolic unit = 1 met =
58 W/m2) and the clothing (thermal resistance – 1
unit of thermal resistance of clothing = 1 clo
=0,155 m2 . ºC/W) are estimated – from tables, the
following environmental parameters are measured:
air temperature, mean radiant temperature, relative
air velocity and partial water vapor pressure.

35. In moderate environment man’s
thermoregulatory system will automatically try
to modify the skin temperature and the sweat
secretion to maintain heat balance. In the PMV-
index the physiological response of the
thermoregulatory system has been related
statistically to thermal sensation votes collected
from more than 1 300 subjects.
It is recommended to use the PMV-index
only for values of PMV between –2 and + 2 and
the main parameters of the equation are inside
fixed intervals.

36. The PMV-index predicts the mean
value of the thermal votes of a large
group of people exposed to the same
environment. But individual votes are
scattered around this mean value and it is
useful to predict the number of people
likely to feel uncomfortably warm or cool.

37. The PPD-index establishes a
quantitative prediction of the number of
thermally dissatisfied persons among a
large group of people. It is recommended
that the PPD be lower than 10 %. This
corresponds to the following criteria for the
PMV:
- 0,5 < PMV < +0,5

38. Preventive measures and control of heat
stress
Modifying one or more of the following
factors can reduce heat stress: metabolic heat
production, heat exchange by convection, heat
exchange by radiation, or heat exchange by
evaporation.
Environmental heat load (C, R and E) can
be modified by engineering controls (e. g.
ventilation, air conditioning, screening, and
modification of process or operation) and
protective clothing and equipment.

39. Engineering approaches to enhancing
covective heat exchange are limited to
modifying air temperature and air
movement. When the air temperature is less
than the mean skin temperature, increasing
air movement across the skin by increasing
either general or local ventilation will
increase the body heat loss.

40. Metabolic heat production can be
modified by work practices and
application of labor - reducing devices
– mechanization of the physical
components of the job, reduction of
work time (reduce work day, increase
rest time) to reduce the duration of
exposure to a hot environment,
increased work force.

41. Work and hygienic practices and
administrative controls.
Situations in industries exist where the
complete control of heat stress by application
of engineering controls may be
technologically impossible or impractical,
where the level of heat stress can be
unpredictable. In such cases other solutions
could be applied. Preventive practices
include:

42. 1. Limiting of modifying the
duration of exposure time - when
possible, schedule hot jobs for the
cooler part of the day (early morning,
late afternoon), provide cool areas for
rest and recovery etc.
2. Enhancing the heat tolerance
by heat acclimatization.

43. 3. Adequate water supply to maintain the
electrolyte balance. It is widespread that extra
salt intake is needed to prevent the ill-effects of
heat. The normal intake of salt in some national
diets is far more than is actually needed.
Therefore there is no need to add salt to water;
only unacclimatized persons need extra salt
during the first some days of their exposure to
heat.
4. Protective clothing and equipment –
they should be light, loose and of light colors.

44. 5. Protective devices – goggles,
shields, helmets.
6. Medical screening of workers to
discern individuals with low heat tolerance
and/or physical fitness. The screening
should include a history of any previous
incident of heat illness or other pathological
conditions that could influence the health of
workers in unfavorable thermal
environment.