Complex sentence: However, scientists in the book caution that more evidence is needed to confirm the exact causes and effects of the phenomenon being observed to make definitive statements about if exposure to the chemicals is dangerous and at what levels cause risk to humans and wildlife.
Split into simpler sentences: However, scientists in the book caution that more evidence is needed to confirm the exact causes and effects of the phenomenon being observed. In their estimation, it is too early in the process to make definitive statements as to the dangers exposure these chemicals have and at what levels cause risk to humans and wildlife.