RADIOACTIVITY IN MAN
259
get into the alveoli. That which gets into the bronchial tree is
brought up byciliary action and swallowed. Therefore, from an
acute point of view, the gastrointestinal tract and the thyroid
gland probably receive the largest dose of radiation of any organ
system. As with irradiation of the skin, the principal hazard from
internally absorbed fission products comes primarily from beta
irradiation. The short-lived radionuclides. such as I****, Sr®,
Ba*” and La™’, are important during the early period. Iodine is
probably the most important isotope in the early period sinceit is
selectively absorbed by the thyroid gland.
Initial Internal Absorption in the Marshallese. In the Marshallese
who lived for 2 days under heavily contaminated conditions, urine
analyses showed accumulation of isotopes to near maximum per-
missible levels for lifetime exposure (6, 23) in a few cases. Radioiodine producedan estimated 100 to 150 reps to the thyroid glands
of the Rongelap people—but with no apparent effect. Damage to
the gastrointestinal tract by passage throughit of 3 mc. of fission
product activity is not believed to have added materially to the
symptomatology. The remarkable thing in the Marshallese was
that. in spite of their heavy exposure to fallout without seriousat-
tempts at avoidance, urine analyses showed rapid dwindling of
activity so that by one or two years radionuclides, including long-
lived ones suchas Sr” and Cs***, were hardly detectable (see Figure
10). This leads one to the conclusion that an acute exposure to
fallout which is not lethal will probably not result in acute effects
or long-term effects from internally deposited isotopes. Even so,
simple precautions such as reasonable care in eating covered or
canned foods or drinking protected water at such a time would
greatly decrease the internal accumulation of isotopes.
Chronic Exposure Effects of Residual Contamination of the Environment on the Marshallese. Exposure to residual environmental
contamination which may persist long after fallout occurs is exemplificd by the situation at Rongelap Island. By 1957 complete
surveys of the island showed such lowlevels of persisting contamination that the island was considered quite safe lor habitation, and
at that time the people were returned to Rongelap village which
had been reconstructed for them. (Persistent background gamma
radiation as of March 1959 averaged about 0.04 mr per hour