根据下面资料，回答题
Imagine one day reading that you had been a subject in a risky science experiment without knowing it. At school, you had been fed cereal laced with radioactive chemicals so scientists could learn more about the digestive system. Because the food was radioactive, scientists could easily trace its path through the body.
Scientists now know that exposure to large amount of radiation can cause cancer, a group of diseases in which bodily cells grow wildly out of control. Some people who were part of the radia-tion studies later developed cancer and blamed the radiation for causing the cancer. Others are worded the radiation will some day make them sick.
Some doctors say that, in many cases, such fears are unjustified because the radiation doses used in the experiments were too small to cause harm. They also say that since dangers of radiation were less well understood then than they are now, even researchers who used high doses of radia-tion didn' t do anything wrong.
Some critics claim that no excuse is acceptable for not informing subjects about the nature of an experiment and giving them a chance to refuse to participate. Today, government rules require such informed agreement whenever a study uses people.
The passage is mainly about __________.