Why does edward tell Bella to take her hair down in the baseball field wont taking her hair down make her scent stronger?

Answer 1

Even if it does (which will only happen if the wind blows), Edward is trying to mask the more powerful scents emitted from her human orifices. Also, it could be said that letting her hair down will make her look less suspicious to the invading vampires.

Answer 2

So the bad vampires dont smell the human scent and try to kill her.