Activists in the anti-Bomb movement were surprised in the spring of 1958 when, after finishing a particularly dirty (fallout-heavy) series of tests, the Soviets called for an immediate halt to all nuclear testing, an obvious political move timed to generate worldwide support and to highlight -- and possibly prevent -- an upcoming US test series. It was important to Pauling that at least one nation was taking the anti-testing movement seriously. But the Soviet move had little real-world effect. Despite the continued efforts of Pauling, Russell, and many other dedicated activists, the testing and deployment of nuclear weapons by the US and USSR continued through most of the rest of the year.

The spring of 1958 also brought forward a new way in which fallout threatened the health of humanity. In April, Pauling read a speech given by Willard Libby, the head of the AEC, in which he identified a previously unreported fallout component called Carbon-14. Libby knew quite a bit about Carbon-14 because he had pioneered its use in carbon-dating artifacts. Its presence in fallout -- about 160 pounds of C-14 released in every good-sized H-bomb blast -- was particularly worrisome to Pauling, who knew that the body would treat the isotope like normal carbon and incorporate it into tissues, where it threatened to release radiation through the remaining lifespan of the organism, possibly causing cellular damage and other health effects.

Pauling called a news conference to announce the discovery of "a new threat in atomic fallout." This time, though, he moved too fast. His quick estimates of human exposure were proven to be too high by a factor of five. His mistake cost him some credibility with the press and some politicians, but he adjusted his numbers and stuck with his original, basic, and, as was later demonstrated, correct, point: Carbon 14 added significantly to the dangers of fallout.