completion at that time.
Table 2 shows that the reduction factor for ''Slab
Only" is 30%, for "Slab and Walls" is 42-46%, for “Slab, Walls, and Roof"
is 44-48%, and for one occupied house is 51%.
Note in Table 2 that the
average exposure levels outside the houses are less than 44 uR/hr and more
than half of the measurements were less than 20 pR/hr.
While there had
been some scraping of soil where houses were constructed, the layer of coral
gravel recommended by the Ad Hoc Committee was not yet in place.
Lower
levels are expected when this is done, and preliminary results from the
1974 survey indicate this is indeed the case.
completed houses ("Slab, Walls, and Roof"),
As for dose rates inside the
the average level in Table 2 for
10 houses in the center of the building, at a height of 3 feet is 11.2
uR/hr or 96 mR/yr.
About one third of this is from natural background.
reported for 17 houses in the New York area range from 29
to 90 mR/yr.
Values
See
"Investigations of Natural Environmental Radiation", Science 131, pages 903906,
1960 (Enclosure 12).
It should be pointed out that the source of New
York exposures is almost entirely natural radioactivity in the building
materials and environs where the levels change little with time.
At Bikini,
the source is primarily cesium-137 from testing that was deposited in the
surrounding soil with very little radioactivity in the materials of construction,
Since cesium-137 decays with a half time of 30 years, the external
radioactivity levels at Bikini from this source will continually decrease.
As for the guideline for evaluation of exposures at Bikini Atoll, the
basic numerical guide for whole body and bone marrow is 500 mrem/yr (millirem
per year).
The 500
mrem/yr standard is applicable where individual exposures