Læknablaðið - 01.10.1968, Síða 50

212
LÆKNABLAÐIÐ
than chemical, should he given the opportunity to present this
Niels Dungal Memorial Lecture, focussed on the carcinogenic
aspects of ionizing radiation.
Man has been exposed to small amounts of naturally
occurring ionizing radiation since his earliest days, and the work
of archeologists and physicists indicates that the levels of this
radiation have been relatively unchanged since the beginning of
human history. Tlie effect was prohably undetectahle, although
possibly increasing slightly the incidence of mutations. For many
thousands of years only natural sources of radiation 'were present:
cosmic radiation from outer space, minute and unrecognized
traces of radioactive elements in earth, rocks, water and air.
Man-made sources of radiation hecame availahle less than a
century ago through tlie genius of Mme. Curie and of Roentgen.
Regions of the earth vary in their natural radioactivity witli
their geologic structure and with elevation. Natural radiation
tends to be lowest at sea levcl where the thickness of atmospliere
has provided considerable protection against cosmic radiation. On
elevated plateaus such as the Colorado plateau in the United
States and the Himalayan uplands, the level of radiation is about
twice that at sea level but still insignificant. Variations in
geologic distribution also may lead to an increase in radioactivity
locally. There are two sucli jjlaces well known, one at Kerala
in southwest India, the otlier along the shore of eastern Brazii,
where, due to the presence of thorium-containing monazite sands,
the radiation levels are tliree to ten limes those naturally
occurring. Studies are now Ijeing carried on to determine whetlier
or not any deleterious effect on the populations may be found
at these levels. Thus far, there is no evidence of harmful effect.
Whether or not some congenital malformations or cases of
cancer may have been induced hy natural radiation is a matter
of speculation, with no firm supporting evidence. Assuming
validity of the hypothesis of linearity of effect with change in
radiation dose, possihly 10% of naturally occurring congenital
malformations might he referrahle to hackground radiation. It
has been assumed that an added dose of 10 R to the entire child-
bearing population, as from diagnostic use of X-rays or fallout
radiation, might double thc present mutation rate.1 However,
Russell’s work- has indicated Ihat a given dose of radiation
delivered in protracted time is less effective than an acute dose
in causing mutations.