Rachel Carson's main idea in Silent Spring is that pesticides sprayed on crops work their way up the food chain to ultimately have a negative effect on human beings. This seems like a commonplace idea now, but in the early 1960s, when the book appeared, it was a radical concept, supported by state-of-the-art scientific studies.
Carson argued that pesticides impact the entire food chain: birds eat insects that have ingested pesticides, and eventually this could result in the silent spring of the title, in which no birds sing because they have all been killed off by pesticides.
To make her argument more powerful, Carson likened pesticides to radiation: at this time the longterm effects of the atomic bombings of Hiroshima and Nagasaki were being studied. Carson noted that both radiation and pesticide poisoning are invisible and that in both cases the results would show for many years.
Carson's book was effective in launching environmental awareness in the U.S.