ADAPTATIONS OF HUMAN BODY TO VARYING THERMAL CONDITIONS 



305 



the ventilating engineers, the physicist's 

 gm.cal./sec./°C, and the physiologist's kg. 

 cal./hr./°C, without being dependent upon 

 these units for an approximate subjective 

 appreciation of its insulation value. With 

 these points in mind, Gagge, Burton, and 

 Bazett (16) defined a practical unit, the 

 ''clo." One clo unit of thermal insulation is 

 the clothing required to keep a resting sub- 

 ject in a comfortable state when the subject 

 is seated in an atmosphere of 21.1°C (70°F) 

 with relative humidity less than 50 per- 

 cent and air movement at 20 ft./min.(10 

 cm./sec). The standard value for the me- 

 tabolism associated with this condition is 

 50 kg.cal./m2./hr. 



The standardization of the clo unit utilized 

 the experimental work of Winslow, Gagge, 

 and Herrington (44) to determine that the 

 total insulation, which is the sum of the in- 

 sulation of the clothing, Ici , and of the 

 air, I 



A ) 



is 



Ici + Ia = 



(15) 



33-21 

 38 



= 0.32 



kg. cal./hr./m.2" 



The insulation of the air in metric units at 

 the air movement cited is 



(16) 



0.14 



=C 



kg. cal./hr./m.2 ' 



By difference, the insulation of the cloth- 

 ing, equal in the above definition to one 

 clo, is 



(17) 0.32 - 0.14 = 0.18 



kg. cal./hr./m.^' 



or 



= O.i 



=C 



BTU/hr./ft.2 



The clo definition is in terms of resistance, 

 rather than conductance, units, since the 

 former may be added directly to obtain the 

 total resistance from known sub-compo- 

 nents. Perhaps something is added to the 

 sensory appreciation of this unit by the fol- 

 lowing example. With low air movement 



and moderate humidity, the resting nude 

 subject is comfortable at 30°C (86°F). One 

 clo of insulation is required to maintain the 

 same degree of comfort when the air tem- 

 perature is dropped 8.8°C (16°F). Extend- 

 ing this analogy, one can say that two clo is 

 the clothing sufficient for comfort at 30°- 

 17.6°C or 12.4°C (54°F), three clo that suffi- 

 cient for 30°-26.4°C or 3.6°C (38°F), and so 

 on, it being understood that air movement 

 and heat production remain in agreement 

 with the formal definition. 



It is probably obvious that one clo is very 

 nearly the insulation provided by normal 

 male clothing. This is intentional, and pro- 

 vides a valuable experience correlation for 

 the unit. 



In Fig. 11 five illustrative pictograms and 

 graphs have been redra-un from an article by 

 Burton (8) on clothing in relation to human 

 heat exchange. Presentation of this type 

 has been useful in demonstrating to lay 

 authorities the practical significance of the 

 clo unit of clothing insulation in relation to 

 the met unit of human heat production under 

 varied climatic exposures. In the picto- 

 grams labeled 1,3, and 6 met, the position of 

 the mercury at +70°F (21.1°C), -1-40°F 

 (4.4°C), and -5°F (-20.6°C) illustrates the 

 approximate decrease in environmental tem- 

 perature which may be tolerated with stand- 

 ard clothing as heat production is increased 

 from a sedentary level of one met (50 Cal./ 

 m^./hr.) to six met, which represents severe 

 exertion. The fourth exercising figure indi- 

 cates that an increase of clothing insulation 

 to 1| clo with a heat production of six met 

 permits a further drop in environmental tem- 

 perature to -30°F (-34.4°C). The two 

 sleeping-bag pictograms illustrate the choice 

 that may be made between heavy or medium 

 protection in sleeping bags (11 or 8 clo in- 

 sulation value), provided that auxiliary heat- 

 ing (stove) is used to raise a shelter tem- 

 perature of -20°F (-28.9°C) to -MO°F 

 (-12.2°C). In the outline figure of a 

 clothed man, a graphic indication has been 

 given of the three components into which 



