Quoted from Living Downstream:
Based on all the data available to her in 1962, Carson laid out five lines of evidence linking cancer to environmental causes. While any one alone would be insufficient proof, when viewed all together, Carson asserted, a startling picture emerges that we ignore at our peril. First, although some cancer-producing substances—called carcinogens—are naturally occurring and have existed since life began, twentieth-century industrial activities have created countless such substances against which we have no naturally occurring means of protection.
Second, since the arrival of the atomic and chemical age that followed World War II, everyone—not just industrial workers—has been exposed to these carcinogens from the moment of conception until death. Industry manufactures carcinogens in such large quantities and in such diverse array that they are no longer confined to the workplace. They have seeped into the general environment, where we all come into intimate and daily contact with them.
Third, cancer is striking the general population with increasing frequency. At the time of Carson’s writing, the postwar chemical era was less than two decades old—less than the time required for many cancers to manifest themselves. Carson predicted that the full maturation of “whatever seeds of malignancy have been sown” by the new lethal agents of the chemical age would occur in the years to come. She also believed that the first signs of catastrophe were already visible. At the end of the 1950s, death certificates showed that a far greater proportion of people were dying of cancer than had been true at the turn of the century. Most ominously, children’s cancers, once a medical rarity, were becoming commonplace—as revealed both by vital statistics and by doctors’ observations.
Carson’s fourth line of evidence came from animals. Experimental tests were beginning to reveal that low doses of many pesticidal chemicals in common use caused cancer in laboratory mice, rats, and dogs. Moreover, many animals inhabiting contaminated environments develop malignant tumors; Silent Spring not only documents acute poisonings of songbirds but also reports on cases of sheep with nasal tumors. These incidents supported the circumstantial evidence from human populations.
Finally, Carson argued, the unseen inner workings of the cell itself corroborate the story. […] From the glimmers she was able to gather from widely scattered studies, Carson spotlighted three properties that she believed would ultimately explain why these new chemicals were associated with cancer: they were able to damage chromosomes and thereby cause genetic mutations (a property shared with radiation, which had already been shown to cause cancer); they were able to mimic and disrupt sex hormones (high estrogen levels were already correlated with high cancer rates); and they were able to alter enzyme-directed processes of metabolism (by which we break apart molecules, including foreign chemicals that are sometimes metabolically converted into carcinogens).
Steingraber, Sandra. Living Downstream: An Ecologist’s Personal Investigation of Cancer and the Environment (pp. 30-32). Da Capo Press. Kindle Edition.