Interestingly, Carson began his research into environmental conservation following a letter a friend of his wrote to the Boston Herald. The letter written by Olga Owens Huckins in 1958 described the decline in bird populations and species around her property resulting from aerial spraying of DDT to kill mosquitoes; Huckins sent a copy of this letter to Carson, thus stimulating his research. Although the vast majority of Silent Spring focuses on ecosystems and the problem of pesticides, several chapters examine cases of human pesticide poisoning, cancer, and other illnesses attributed to pesticides. Carson has indeed looked at the impacts of chemicals on humans, particularly as it relates to cancer. ; Carson's research in collaboration with the National Cancer Institute concluded that there was indeed a link between pesticides and cancer. These ideas produced by Carson all relate to the central theme of Silent Spring, which is that humans