From the article by Julia Zorthian at Time Magazine:

For the human body, Kelly said space travel has “permanent effects we simply do not fully understand”

Astronaut Scott Kelly, back from his year in space on the International Space Station, said the “permanent effects we simply do not fully understand” that astronauts experience in space must be mitigated before developing space travel for people.

Kelly cited serious carbon monoxide and radiation exposure, vision problems and a loss of bone or muscle mass due to low gravity as some of the adverse effects space travel takes on astronauts. He had been aboard the ISS to learn more about the effects of space on one’s body, with his twin brother Mark Kelly on earth as the control.