Life on earth evolved in a shower
of ionizing radiation. Indeed, evolution as we currently understand
it is singularly dependent on ionizing radiation’s capacity
to alter genomic sequences and change the structure and function
of intracellular proteins. The human body experiences some 1012 collisions
daily from ionizing radiation from a wide range of cosmic and earth-bound
sources (Fig. 28–1). For obvious reasons, therefore, humans
have evolved repair mechanisms and genetic redundancies that enable
us to withstand these exposures without obvious effect. Much of
the information used to determine the health effects of radiation
on humans derives from the atomic bombing of Hiroshima and Nagasaki,
as well as industrial, research, and environmental accidents. To
better understand the health effects of ionizing radiation, and
specifically, the issue of nuclear terror, this section provides
a short introduction to radiation terminology and radiation physics.
Sources of radiation exposure.
Courtesy of WHO.
Radiation is the transfer of energy through space as electromagnetic
waves or particulate matter. The energy contained in radiation is
inversely proportional to the wavelength. There are two types of
radiation: nonionizing and ionizing. Ionizing radiation is of short
electromagnetic length and high energy and has the capability to
break covalent nuclear bonds; thereby altering genomic sequences,
destroying the structure and function of proteins, and generating
free radicals that are directly cytotoxic. Higher-frequency wavelengths
(e.g., x- or gamma rays and alpha particles) have higher energy
and are more destructive than low-frequency radiation (e.g., radiowaves
or microwaves). Nonionizing radiation (e.g., microwaves and radiowaves)
is not considered potential weapon of terror and is not discussed
further (Table 28–1).
Table 28–1 Characteristics
of the Electromagnetic Spectrum |Favorite Table|Download (.pdf)
Table 28–1 Characteristics
of the Electromagnetic Spectrum
|Region||Wavelength (Cm)||Frequency (Hz)||Energy (Joules)|
|Radio||>10||<3 × 109||<10−24|
|Microwave||10 − 0.01||3 × 109 − 3 × 1012||10−24|
|Infrared||0.01 − 7 × 10−5||3 × 1012 − 4.3 × 1014||10−23 − 10−19|
|Visible||7 × 10−5 − 4 × 10−5||4.3 × 1014 − 7.5 × 1014||10−19|
|Ultraviolet||4 × 10−5 − 10−7||7.5 × 1014 − 3 × 1017||10−19 − 10−17|
|X-rays||10−7 − 10−9||3 × 1017 − 3 × 1019||10−17 − 10−14|
|Gamma ||<10−9||>3 × 1019||10−14 − 10−10|
Multiple manmade and natural sources of radiation exist (Fig.
28–1). Background radiation from natural and artificial
sources averages about 360 millirem per year per person in the United
States. This is an order of magnitude below ...