2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
re-warm the worker (usually at least 20 minutes) plus the time to get to and from the rewarming
area.
Heat loss from the skin surface through the clothing and surface still air layer is a linear
function of the difference between the skin temperature and the ambient air temperature, as
controlled by the resistance to heat transfer afforded by the insulation of the clothing and
surface still air layer; the insulation unit used for clothing is the “Clo”, with 1 Clo of
insulation requiring the transfer of 5.55 kcal of heat per square meter of skin surface area per
hour per degree Centigrade difference between the skin and air temperatures; i.e.:
“Dry heat loss” (Hr+c) =5.55 x S.A. x (Tskin – Tair)/hour
This equation can be used to calculate the heat that MUST and WILL be lost in a given
ambient environment, as a function of the clothing insulation, air temperature and surface
area. Note that 1 R unit is ≈ a Clo value of 1.14. An “average” American workman has a
total body surface area of ~ 1.8 m 2 (square meters). A “comfortable” average Tskin (MWST-
mean value, weighted by temperature site surface area) is ~ 33 º C (= 91.4 ºF); body surface
cooling as a result of the initially mild, vasoconstriction to conserve deep body heat in the
cold, will reduce MWST to 32 º C (= 89.6 ºF). We used 32 º C as the Tskin value for our
calculations of body heat transfer. One can use the Clo values of individual clothing items, as
shown below, to calculate the overall clothing insulation value to use in our calculations of
body heat exchange.
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CLOTHING ITEMClo VALUE
Long Underwear 0.20
Heavy Long Sleeve Sweater 0.37
Bib-Overalls 0.41
Waist Length Heavy Jacket 0.49
Heavy, Knee length Sox 0.08
Heavy Boots 0.08
Navy type Knitted Watch Cap 0.05
Total of individual item Clo values = 1.68
80% of individual item Clo values = 1.35
Plus value of external surface “calm” air layer = 0.71
Total clothing + surface air layer Clo = 2.06
Note: Hand-wear is not included because it may well be task precluded!
RESULTS
Using this value of 2.0 Clo in our calculations of body heat exchange, we can generate the
work times until re-warming is required, as a function of the heat production and ambient
environmental conditions. These data are presented in the Figure.
DISCUSSION
These predictions, made several years after a very large construction project was completed,
showed remarkable agreement with the documented intervals between the ad lib re-warming
periods taken by the workforce.