Agreed, Jim. It is important for toxicologists and epidemiologists to realize that (very often, if not always): a small amount of a chemical — or in the case of this article, atom bomb radiation — can be beneficial; a larger dose will no longer be beneficial but still not toxic; and then a further increase in dose will be toxic and/or carcinogenic.
Examples that come to mind — off the top of my head — include carbon monoxide (CO), nitric oxide (NO), hydrogen sulfide (H2S), reactive oxygen species (ROS; O.), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; the lay term is ‘dioxin’). Each of the first four of these five “environmental signals” (in small amounts) are now realized/appreciated that they function as a signaling molecule in crticial life processes — such as neuromodulation in the brain, smooth muscle relaxation in the vascular system, gut motility, electrical conductance in the heart, etc.