What are some quotes that show physical/mental dangers of books in "Fahrenheit 451"?
Your best bet for finding really good quotes that relate to the supposed "dangers" of reading books are going to be in Beatty's speech to Montag. After they burn the old lady in her house with her books, Montag is deeply disturbed, and stays home sick. Beatty shows up at his house, and gives him a pretty lengthy run-down of the history of burning books--how it started, how it came to be, and society and government's role in the matter.
One of the dangers mentioned is how reading books makes people smart, which makes other people feel dumb in comparison. No one likes
"the boy in your own class or school who was exceptionally bright."
The smart kids make the other kids feel bad about themselves; hence, as Beatty states, "Intellectual became the swear word that it deserves to be." Book are mentally dangerous because they make people who DON'T read them feel bad about themselves. This is also physically dangerous, because those smart kids are bullied and beat up after school. Then, books discuss cultures, genders, races, and other issues, and express frank opinions about those people, which end up offending others. Beatty says, "we can't have our minorities upset and stirred." People were offended by the opinions expressed in books, so they started censoring them, burning them, and it became too perilous for authors to write them anymore. Their careers were in dange because no one bought them unless they made everyone happy.
One last way that books are potentially dangerous is that they lead people to think for themselves, which means questioning the world around them. Beatty states that "that way leads melancholy." If people think too much, he says, they get upset, depressed, and worry about their world. So, it's dangerous to their happiness.
I hope that helps; I encourage you to re-read Beatty's spiel to Montag to find more dangers. Good luck!