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7
alveolihave been demonstrated by us; (2) particles between 5.0 and 20 p have a maximum
initial radioactivity at H +4 hr of about 0.5 myc, and the yh decay factor reduces this activity
by a factor of 100 during the first week and to negligible levels thereafter; (3) particles of 5.0to 20-p size in the bronchi are rapidly transported upward by 2 constantly .noving layer of
mucus propelled by ciliary action, thereby Himiting the duration of exposure at any local area
to a maximunti period of a few minutes; and (4) the largest particles which may be lodged in the
alveoli (up to 5.0 p) are rapidly engulfed by lung histiocytes.
Some additional work pertinent to the long-term inhalation hazard has been performed and
described in UCLA quarterly reports, Reports UCLA-238, 260, and 267. These controlled
studies were performed in a large inhalation chamber using both rats and rabbits, The rad‘oactive dust was obtained from an area near a crater following the Buster-Jangle series in
1952. The dust concentration employed was 2.0 x 10d, min/m’, and the particles were in the
0.1- to 5.0-p size range, Animals were given head exposures 6 hr daily, 5 days per week, for
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deposition and fate of inhaled material in all major organs, including bone. In addition, lung
clearance studies were made on alternate animals by assaying ashed organ specimens after
increasing times following completion of a given number of inhalation exposures,
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a total of 60 exposures, Following each exposure all animals were vacuumed and placed in
clean individual cages until reexposed in the chamber. Three sertes of experiments of this
type were madeto obtain data on an adequate number (90) of animals. In each series, animals
of both types were sacrificed after 10, 20, 30, and 60 days to determine accumulated pulmonary
Briefly, the major results of these studies showed that (1) the amount of pulmonary retention of radioactive particles tncreased with the numbers of daily exposures up to 20 days
and then remained nearly stationary; (2) the amounts of beta-gamma radioactivity per lung
never exceeded 17 dsec; (3) clearance from the lungs following repeated exposure was rapid
during the first few weeks and became slower thereafter; however, about 70 per cent clearance
was attained in 60 days; and (4) because of the low water solubility (1 per cent) of the material
and the minute amount retained in the lungs and intestines, values for radioactivity in other
organs, including bone, seldom exceeded twice the background, The type of material, air concentration, and particle-size range should have given maximum lung retention, and these conditions simulated in many ways the worst conditions one might expect during 2 3-month period
after a near surface detonation of a nominal wedpon in a semiarid area such as the Nevada
Test Site.
Even under these conditions the amounts of radfation exposure to lungs and other organs
did not approach levels which could cause detectable physiological changes and were less than
0.0001 of the minimum doses which have been shown to induce acute radiation pneumonitis
(2000 to 3000 rep).'?
Furthermore, these studies demonstrated the efficiency of the respiratory organs in re-
spect to their capacity to remove inhaled foreign particulate materials at a rate almost equal
to the rate of deposition from continuing inhalation exposures. Extrapolation from the very
gradual accumulation of particulate material in the lungs indicates that retention of physiologically significant quantities of such particles might be attained if similar daily exposures
were made for several years, These findings and estimates are in accord with the long periods (5 to 20 years) of expusure required to induce silicosis and other pneumoconioses in man.
In a comprehensive summaryand evaluation of the acute and chronic effects of radioactive
particles on the pulmonarytract, the conclusions” are:
i, The acute external beta-gamma radiation hazard is many times greater than that from
inhalation, and an additional safety factor for the lung (of perhaps 10) is represented by the
respiratory tract clearance mechanisms,
2. In industrial or research work with nuclear reactors and radioisotopes, situations may
occur wherein relatively small numbers of people may receive significant radiation exposure
to parts of the respiratory system from inhwing radioactive particles containing long-lived
ixotopes of high specific activity, without simultaneously exceeding tolerance levels for wholebody exposure.
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Neither particles larger than 20, in the pulmonary tree nor particles larger than 5.0 y in the
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Acarcinogenic effect to bronchopulmonary tissue probably requires a total dose of at
least 2000 rep.’ Such a dose could hardly beattained locally for the following reasons: (1)