1114 
The reflection and absorption spectra in the solar region 
are complicated and depend upon pigmentation (race, 
suntan), congestion of blood vessels (erythema), etc. 
In the yellow and red bands, noticeable fractions of the 
radiation penetrate deep layers, whereas in the region 
from 0.5 to 0.9 y, reflection is particularly strong. 
As far as its own radiation R is concerned, the skin 
acts nearly as a black body, its emission constant 
being « = 0.96 according to [16], or 0.99 according to 
[42]. We may write 
pane) ~ (2) 
= h,(8, — 3,) Calm™ hr“, 
(6) 
where, approximately, 
h, = 19.84 a |. 
and 2, is the radiation temperature of the surroundings, 
which in the case of a room is identical to the mean wall 
temperature (7, and 7 are in degrees absolute). This 
identity applies also to specularly reflecting walls, as 
long as they are sufficiently far removed. In the open, 
the following approximation is usually adequate for a 
man or a sphere just above a plane surface: 
R = a (; ary o,) ar 2 (OR = Ba) = Ri, Cal m Ine (6a) 
where #, is the mean temperature of the solid surround- 
ings, and #; is the radiation to the sky by a sphere 
having an air temperature 3. Data on R, are given 
after the calculations of Poschmann in [69]. The radi- 
ation R, depends on cloud conditions, 3 and é, and 
may rise to 100 Cal m ~ hr“. Its eieer on human heat 
loss may be considerable, for example, during clear 
winter nights. 
Total Heat Balance. The heat balance for the nude 
body is 
Q — Cd + exSFu = hoF (8: — 82) + h,F (8s — 9) 
+ lhuf =P cael a P\FaEs = €a)- (7) 
In this expression the subscript H refers to the sun 
(Helios), Q is the heat production of the body (less 
loss in breathing), C' is its heat capacity, and & is the 
change of the average body temperature with time. 
At least three different surfaces have to be considered 
in the discussion of heat balance, namely, the projected 
surface for the solar radiation (F'), the surface for the 
heat loss to the air (f.), and the surface for the heat 
loss by outgoing radiation (F’,). All surfaces are smaller 
than the geometrical surface F which has been measured 
frequently because of its dependence on the height, 
weight, age, sex, and clinical factors of the subjects. 
For aysuillis, F amounts to 1-2 m*. The values of F, and 
F, are smaller for the recumbent person because of 
mutual contact between certain skin areas and contact 
with the imsulating bed. For a standing person F, 
-& 0.8F, and for a person supine on an insulating bed it 
may be as low as Ff, & 0.5F. For the average of all 
possible positions, Fy  F,/4. Sky and reflected solar 
BIOLOGICAL AND CHEMICAL METEOROLOGY 
radiation might require special consideration. Usually, 
1, = Ji. 
Sultriness and Heat. In an ordinary room, S = 0 
and 3, = %3,. In calm air at sea level, the value of h, 
has been found to be close to that of h,. With this 
simplification in mind (which should not be generalized), 
we write ha = h, = h. In order to maintain the body 
temperature at a steady state (# = 0), equation (7) 
becomes 
Oi. hwo f + cl — f) 
Hop Oo E ¥ Qh B, | 
hof + ¢Q — f) 2 
00 
E ar ig? Bice €a 
According to experiment [17], ¢c 0.2hw. If equation 
(4) is used, the physiological quantities 3, and f, skin 
temperature and wetness factor, are thus related to the 
environmental values ?, and é,. 
We may now introduce certain standard data such 
as Q/F, = 120 Calm hr, 2ha = 8.4 Cal m= hry 
(deg C)", and 3, = 34.5C. We find from equation 
(8) that f = 1 when 
0a + 0.815e, = 63, 
in which @, is expressed in degrees centigrade and é 
in millibars. When the wetness factor f equals unity, 
thorough wetness and consequently unbearable heat con- 
ditions are indicated. This has been proved by dif- 
ferent observations. The majority of deaths during 
military maneuvers in the United States [78] or caused 
by heat waves in the United States’ have occurred with 
values greater than those approximated by the formula: 
8a + 0.75e, = 61. In general, similar climatic strain 
in the range of sultriness (7.e., high temperature and 
high humidity combined) is given by equations of the 
form 
Ba + Ala = b, (a S 0.815). (8a) 
The more comfortable the surroundings, the smaller 
the values of 3; and f, and the smaller the values of a 
and b according to equation (8). Such systems of physi- 
ologically equivalent combinations of (@, e.) corre- 
spond to the ‘‘effective temperatures” [49] of the Ameri- 
can Society of Heating and Ventilating Engineers. From 
equation (8) we can readily interpret them as lines of 
constant (J;, f). 
Evaporative cooling devices for homes and auto- 
mobiles leave the equivalent temperature of the cooled 
and moistened air, 
b+ 1.5¢ = 0 + 1.5H, (80) 
approximately constant (psychrometric equation). 
Comparison of equations (8a) and (8b) shows that re- 
lief from heat may be expected only in the case of low 
relative humidities. 
Overheating of the body (%& > 0) occurs for cor- 
responding values of S, @a, %a, etc., particularly when 
J. > 0; and 3, > %;, so that only evaporation has a 
cooling effect. This becomes inadequate either if hy 
is too small and e, too large (jungle climate), or if the 
1. T. Dill and B. Dill, personal communication. 
