Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

776 CHAPTER 14
that we have a nonthermal effect. Thermal effects are associated with exposures greater
than 10 mW per cm, while nonthermal effects generally are associated with exposures
less than 10 mW per cm.
Animal studies show that microwave radiation, in frequencies ranging from 200–
24,000 MHz, is lethal if the product of exposure intensity and time is sufficiently
great to increase the body temperature beyond the body’s homeostatic capabilities
(≥5 ◦ C) for a sustained period. For example, rats exposed to 3000-MHz radiation at a
power density of 300 mW cm −2 suffered a temperature increase of 8–10 ◦ C and died
after 15 minutes of exposure; at a power density of 100 mW cm −2 , rats died after 25
minutes of exposure, when their body temperature had risen by 6–7 ◦ C.
The temperature rise in any absorbing medium is related to the absorbed power
density Pa, by
Pa =
ρ kg J/kg
× c × T
m3 ◦C ◦ C
t seconds
where
ρ = density of the absorbing medium,
c = specific heat of the absorbing medium,
T = temperature increase, and
t = exposure time.
W
, (14.40)
m3 If the absorbing medium is a living tissue, the temperature increase will differ from
that predicted by Eq. (14.40) because of heat loss due to evaporative cooling, and
convective and conductive heat loss or gain. The net amount of heat stored in the
body is given by the heat balance equation
S = M ± R ± C − E , (14.41)
where
M = metabolic heat rate,
R = radiative heat gain or loss,
C = conductive and convective heat gain or loss,
E = heat loss due to evaporative cooling, and
S = rate of heat storage.
While resting, a person generates metabolic heat at a rate of about 75 W; while
engaged in moderate work or exercise, the metabolic heat output increases to about
300 W. This metabolic heat is dissipated in the environment when the temperature
and humidity do not exceed the comfort range. If the temperature and humidity are
too high to dissipate the metabolic heat, the person’s body temperature increases.
The additional heat load due to absorption of microwave energy must be dissipated
in exactly the same manner as any other heat load. An unacceptable heat stress
results when the combination of heat load and environmental conditions lead to an
increased body temperature of 1 ◦ C or more. The “comfort range” is measured by
the temperature–humidity index (THI), which is defined by
THI = 0.72(Td + Tw ) + 40.6, (14.42)