DDT is a bug spray [used to kill malaria-spreading mosquitoes]. It was outlawed in many countries because once you spray it, DDT breaks down very slowly in the environment. Since it stays around for a long time, it moves up the food chain and when birds eat bugs sprayed with DDT the chemical causes their egg shells to become thin… After DDT was outlawed, the numbers of birds increased.
[DDT successfully killed mosquitoes, but] the geckoes that ate the wasps, roaches and caterpillars that had DDT in their bodies also accumulated lots of DDT… The island cats walked all around inside the houses and they usually entertained themselves by hunting and eating geckoes.
To make matters even more difficult, scientists found out that when DDT was fed to laboratory mice it caused liver cancer and some studies suggested DDT might do the same thing in people.
To this very day, the World Health Organization sponsors DDT spray inside homes in places like the jungles of Indonesia where malaria is common.
This is because of the high numbers of people who die every year from malaria compared to the estimated (theoretical) numbers of people who might get cancer. So you can see the concept of “safety” is relative.