1. Thermal comfort Factors

2. Human thermal comfort is defined as the state of mind that expresses satisfaction with the surrounding environment
Thermal comfort is affected by heat (conduction, convection, radiation, and evaporative heat loss.) Thermal comfort is maintained when the heat generated by human metabolism is allowed to dissipate, thus maintaining thermal equilibrium with the surroundings. Any heat gain or loss beyond this generates a sensation of discomfort. It has been long recognized that the sensation of feeling hot or cold is not just dependent on air temperature alone.

3. Factors determining thermal comfort
Factors determining thermal comfort include:
Air temperature
Mean radiant temperature
Air movement/velocity
Relative humidity
Personal factors
(health, psychology, sociology
& situational factors)
Insulative clothing
Activity levels.

4. Metabolism
When measuring metabolism rates, many factors have to be taken into account. Each person has a different metabolism rate, and these rates can fluctuate when a person is performing certain activities, or under certain environmental conditions. Even people who are in the same room can feel significant temperature differences due to their metabolic rates, which makes it very hard to find a optimal temperature for everyone in a given location.

5. Clothing insulation
During cold weather, layers of insulating clothing can help keep a person warm. At the same time, if the person is doing a large amount of physical activity, lots of clothing layers can prevent heat loss and possibly lead to overheating. Generally, the thicker the garment is the greater insulating abilities it has. Depending on the type of material the clothing is made out of, air movement and relative humidity can decrease the insulating ability of the material.

6. Relative humidity
The human body has sensors that are fairly efficient in sensing heat and cold, but they are not very effective in detecting relative humidity. Relative humidity creates the perception of an extremely dry or extremely damp indoor environment. This can then play a part in the perceived temperature and their thermal comfort. The recommended level of indoor humidity is in the range of 30-60%

7. Gender differences
While thermal comfort preferences between genders seems to be small, there are some differences. Females are much more likely to be sensitive to thermal conditions. Females are also more likely to be uncomfortable with the room temperature, and will find the temperature too hot or too cold before many men would. Many times, females will prefer higher temperatures. But while females were more sensitive to temperatures, males tend to be more sensitive to relative humidity levels.

9. THERMAL COMFORT INDICES
Thermal comfort indices may be used for defining comfort and its limits, assessing past exposures, and determining control strategies. It is also used for the classification of climate zones

10. EFFECTIVE TEMPERAURE(ET)
The first such scale was produced by Houghton and Yaglou in Their findings were plotted on a psychrometric chart, producing ‘equal comfort lines’.
They named the new scale as effective temperature and it can be defined as the temperature of a still, saturated atmosphere, which would, in the absence of radiation, produce the same effect as the atmosphere.

11. CORRECTIVE EFFECTIVE TEMPERARTURE (CET)
Whilst the ET scale integrates the effects of three variables-
Temperature
Humidity
Air movement
-the corrective effective temperature scale also includes radiation effects.
This scale is at present the most widely used one.

12. EQUIVALENT WARMTH (EW)
Equivalent warmth was developed by Bedford in England after conducting experiments over 2000 factory workers
Air temperature , humidity , and mean radiant temperature were measured and recorded together with subjective responses of the workers
After correlating the findings ,using statistical analysis methods EQUIVALENT WARMTH scale was constructed

13. EQUIVALENT WARMTH (EW)
Equivalent warmth scale is a derivative of equivalent temperature which underestimates the cooling effect of air movement with high humidity , and ignores both clothing and activity levels

14. OPERATIVE TEMPERATURE(OT)
Developed in the USA by winslow,herrington and gagge.
Operative temperature(OT)scale principle is very similar to the scale of equivalent warmth
It combined the effects of radiation and air temperature but ignores humidity and air movement
It is unsuitable for application above 27˚c

15. EQUATORIAL COMFORT INDEX(ECI)
Developed by CG webb in singapore during 1960
Subjective responses of acclimated subjects were recorded together with measurements of air temperature , humidity and air movement
Equatorial comfort index is suitable for warm humid climates but does not account for clothing that differs from that of the test persons
It is probably not a generally acceptable index

16. RESULTANT TEMPERATURE
Developed by misse ́nard in France
This scale is slight improvement on the ET scale
This scale is not suitable for tropical conditions since it underestimates the cooling effect of air movement at high temperature (above 35˚c) while overestimating it at the levels below

17. PREDICTED FOUR HOUR SWEAT RATE (P4SR)
Developed during experiments carried out for the British naval authorities in 1947
This scale attempts to correlate subjective sensations with climatic measurement
It is primarily concerned with the objective determination of physical stress ,as indicated by the rate of sweat secretion from the body , by the pulse and by internal temperature
Predicted four hour sweat rate measures the sweat rate and assumes comfort from that

18. HEAT STRESS INDEX (HSI)
Heat stress index (HSI) is the ratio of evaporative cooling required for maintaining heat balance (E reqd), to the maximum evaporative cooling possible under the given conditions (E max).
HIS = (E reqd/E max) x 100
It overestimates the air movement effect at low humidity, and of high humidity for medium to high temperatures

19. HEAT STRESS INDEX (HSI)
It is thought to be reliable for still air between 27˚c and 35˚c

20. INDEX OF THERMAL STRESS (ITS)
Index of thermal stress (ITS) was developed by Givoni
It is the calculated cooling rate produced by sweating which would maintain thermal balance under the given conditions
The calculation is based on refined biophysical model of the man environment thermal system
The index takes into account all the subjective and objective thermal system

21. INDEX OF THERMAL STRESS (ITS)
Its usefulness extends from comfortable to overheated conditions as far as the physiological adjustments are able to maintain thermal balance
Due to complex calculations involved it is no longer used by practitioners .