B. L. LARSON AND K. E. EBNER
cloud of suspicion over certain dairy products, with opinions based on incomplete studies. At the very least, the conflicting data relative to the average discrimination against Sr-90 in the whole population on a milk-containing diet need
to be resolved immediately.
Danger of radioactivity in foods. The question of the extent to which ionizing
radiations are harmful to man has received considerable attention by many investigators and as to date the results are generally inconclusive (2, 9, 14, 16, 19g).
Some believe that all radiation, regardless of dosage, is harmful and cumulative; others believe that a threshold level must be reached at any given time
before there is any biological damage. Biological damage can be caused by high
levels of radiation and it is postulated that the ionization caused by radiation
leads to the formation of powerful oxidizing radicals. One of the most important
consequences of this is enzyme and other protein alteration, either by oxidation
of sulfhydryl groups or protein denaturation. The effect of ionization on riboand desoxy-ribonucleic acids is shown by a reduction in nucleic acid synthesis and
a reduction in total but an increase in abnormal mitoses depending on the dosage
received,
The debate between various authorities over the effects of low-level radiation
arises because measurements are difficult to make. Theeffects of large doses of
radiation are most apparent and they affect first the hematopoietic or blood cellforming system. The germ cells also are quite sensitive, as are the cells of the
intestinal mucosa. Bone, liver, and brain cells are less affected by large doses of
radiation. It is postulated that long-term, low-level radiation may cause increased tumors of many types, including bone cancer, leukemia, and shortening
of the life span.
Investigations with small animals have indicated that their life span is shortened when exposed to high levels of radiation. If such data are extrapolated to
humans (there is no direct evidence that such an extrapolation is valid), then
the presently accepted life-time maximum dose from all sources of 100 roentgens
(r) over a period of 70 yr. would reduce man’s life span by 500 days (19q).
Estimates have also been made by life insurance companies for the reductionof a
70-year life span by various natural habits: for example, 25% overweight, —3.6
yr.; 1 package of cigarettes per day, —7.0 yr.; driving a car in the United States,
—400 days (19g). If these data are valid, then the present accepted life-time
dose would not reduce life span any more than accepted practices.
Humans receive radiation from many sources besides Sr-90. Cosmic rays,
radium present in earth, bones, wrist watches, ete., potassium?® and cesium}3?
present in earth, foods, and tissues, x-rays, and other sources all contribute to
the total dosage received. Such radiation sources are listed (Table 6). External
radiation amounts to about 150 milliroentgens (mr) per year, with wide variations in the population. Altitude and radiation from the ground account for some
of the largest variations. A dosage of 150 mr peryear is equivalent to about 10 r
of radiation in a 70-yr. life span, or 10% of the present accepted maximum
life-time dosage.
£12]
pavyd