Cornell University Ergonomics Web

DEA3500: Ambient Environment: Thermal Sensation

Thermal receptors - although attempts have failed to identify the actual receptors responsible for thermal sensations, experimental studies using heat pulses and recording electrical reactions suggest that these receptors lie at a depth of about 200 (10-6m) below the skin surface. These studies also suggest that there are separate receptors for:

cold - max. firing rate = 30C; cease to fire at 38C; fire again >45C
and warm - max. firing rate = 40C.

Some researchers propose that the sensation of "hot" arises from both warm and cold receptors firing. Skin temperature and activity of receptors also varies by body site.

RECEPTOR SITES

Skin is generally sensitive to heat and cold over most of body area, but most sensitive places are:

heat receptors: fingertips, nose, elbows (hence hold hot drink cupped in hands and close to face on cold day)

cold receptors: upper lip, nose, chin, chest, fingers (hence sip cold drinks on hot day, put fan at face level, etc.)

Fingertips are most sensitive to rate of heat conduction (hence steel at room temperature feels colder than wood at room temperature).

Mean skin temperature
Estimated by taking the weighted sum of skin temperatures over various parts of body:
Tskin = 0.12Tback + 0.12Tchest + 0.12Tabdomen + 0.14Tarm + 0.19Tthigh + 0.13Tleg + 0.05Thand + 0.07Thead + 0.06Tfoot

Mean body temperature
Derived from weighted sum of body core (rectal) temperature and mean skin temperature.
Tbody = 0.67Trectal + 0.33Tskin

Adaptation
As with all sensory receptors, thermal receptors show adaptation e.g. one hand in cold water, one hand in hot water, after a time neither feels cool or hot. Both hands put in tepid water and cold hand feels warm and warm hand feels cold. Normal range of adaptation temperatures for skin is 29C (84.2F) to 37C (98.6F) although this differs for different parts of the body.

Sensations of Warm and Cold in Rooms
Although we talk about rooms as being warm or cold, we cannot sense air temperature directly--what we sense is temperature at skin receptors. For this reason it is very difficult to predict person's sensations of warmth or cold from air temperature or skin temperature.

Relationship between skin temperature and sensation is different above and below thermal neutrality. Below neutral leads to decreased skin temperature with decreasing ambient. temperature and as temperature falls below 33.5C (92F)(comfortable level), cold sensation increases and slower increase of cold discomfort.

Above thermal neutrality, there is an increased temperature sensation until sweating starts and then there is only a slow sensation of increased temperature. Thermal discomfort doesn't follow temperature sensation directly. In part this is because of effect of skin wettedness and this seems to be a good predictor of warm discomfort.

skin wettedness = actual evaporation loss from skin / max. evaporation loss from skin if it were completely wet
e.g. if skin wettedness = 0.5 this equals 1/2 body wet and 1/2 body dry.

An increase in humidity will not affect sweat rate but will decrease maximum theoretical heat loss via evaporation and effectively decreasing skin wettedness. This increase is usually perceived as "stickiness".

Plotting discomfort ratings against skin wettedness gives good predictive relationships

Scales of Warmth Sensation
Rating scales of thermal sensation have been in use for over 50 years. Two scales commonly still in use are the Bedford scale (UK) and the ASHRAE scale (USA). The Bedford scale confounds warmth and comfort and so the ASHRAE scale is better. Note that with the ASHRAE scale a person is thermally comfortable at the neutral point i.e. the point of no thermal sensation, and so thermal comfort is here defined as the absence of thermal discomfort.


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