Thermal comfort Factors

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.

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.

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.

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.

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%

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.

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

EFFECTIVE TEMPERAURE(ET) The first such scale was produced by Houghton and Yaglou in 1923. 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.

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.

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

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

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

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

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

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

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

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

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

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 .